Metatarsal 5. Metatarsal 5 is a stout bone. The proximal tuberosity is damaged, leaving the bone almost cylindrical in shape with flattened ventral and medial shaft surfaces. However, it is likely that P. azael had an extensive proximal tuberosity here based on its occurrence in both P. parvus AM F58870 and other diprotodontoids, and the shape of the eroded metatarsal surface. Proximally the cuboid facet is smoothly convex and meets the facet for metatarsal 4 medially. There is a deep notch on the proximoventral edge of the bone, bounded medially by a ventral projection of the facet for metatarsal 4 –this would have provided passage for digital flexor tendons. The head appears hemispherical in dorsal view, the extensive median keel only becoming visible in distal view. The medial keel is flattened and reduced, and the lateral keel is sloped and reduced almost to absence.

Metatarsal 4. The fourth metatarsal of P. azael has a smoothly convex, triangular cuboid facet, relatively more dorsoventrally elongate than in P. parvus and lacking the horizontal sulcus on its articular surface seen in the latter. The proximomedial facet for the ectocuneiform is also more dorsoventrally extensive and more sagittally oriented than in other members of its genus, instead resembling the condition in Nimbadon. There is no accompanying facet to suggest articulation with the third metatarsal, unlike in P. parvus. Proximolaterally, the facet for metatarsal 5 is slightly lunate and proportionally broader than in P. parvus, but not as dorsoventrally extensive, terminating above the volar tip of the metatarsal. The metatarsal head appears hemispherical in dorsal view, while in ventral view its large central keel is flattened proximally and projects strongly toward the plantar surface, flanked by smaller medial and lateral keels. In distal view the head is domed and symmetrical, lacking the lateral cant seen in P. parvus.

Metatarsals. The metatarsals of P. azael are thick and robust, though more elongate than the smaller P. parvus species. Both exhibit large median keels on the plantar aspect of their heads, suggesting the presence of flexor sesamoids.

Ectocuneiform. In dorsal view the ectocuneiform is much larger relative to the cuboid than in Ng. tedfordi and Zygomaturus, but not as large as in Thylacoleo. Distally, it presents a concave ovoid medial facet for metatarsal 3, two-thirds the height of its distal face, the other ventral third being made up of a well-developed plantar tuberosity. Immediately lateral and oriented 45° to this ovoid facet is a sub-equally sized flat facet for the proximomedial surface of metatarsal 4. The similarity in size between these facets may indicate that P. azael had less reduced second and third digits than P. parvus, or alternatively may reflect a greater degree of articulation of metatarsal 4 with the cuboid and thus reduction in articulation with the ectocuneiform. On the lateral edge of the ectocuneiform is a smaller surface for articulation with the cuboid, while its proximal face is almost completely occupied by a smooth, concave facet for the navicular.

Cuboid. The cuboid has a well-developed plantar tuberosity, anterior to which runs a deep, narrow sulcus for the m. peroneus longus tendon as it approaches the first digit on the plantar surface of the pes. This sulcus is the deepest we observed among our comparative taxa.

Navicular. The navicular is robust and relatively dorsoventrally expanded compared to those of P. parvus and Ng. tedfordi. The cuboid facet is relatively larger than in Ng. tedfordi and quite concave. The facet for the ectocuneiform is broad and only slightly concave. This articulation appears relatively larger in P. azael than in Nimbadon. Its plantar tuberosity is not as developed as in derived diprotodontids or Thylacoleo.

Astragalus. The astragalus is damaged posterolaterally, missing the lateral part of the tibial facet and the entire fibular facet–their contours are estimated in Fig 18B and 18C . As such, it is not possible to discern the extent of the facet for the pyramidalis sesamoid on its dorsal surface. Overall the remaining morphology strongly resembles Ngapakaldia tedfordi astragali in proportions and joint surface shape, the principal difference besides size being that it is dorsoventrally deeper as in Nimbadon. Like in Ng. tedfordi and Nimbadon, the tibial facet dorsally is very concave, much more than in Zygomaturus. In distal view, the navicular facet is ventromedially sloped relative to the tibial surface, unlike the flatter facets in Zygomaturus or Nimbadon. In ventral view, the calcaneal surface is much smaller and less anteriorly extensive than in Zygomaturus, as well as having a much better developed medial plantar tuberosity (sensu Munson [ 24 ]). The navicular surface sits separate and ventral to the calcaneal surface on the plantar aspect and has a distinct facet for articulation with the cuboid. Anterolaterally, the sustentacular facet is very convex, dipping into a deeply concave fossa (much more concave than in Zygomaturus) before the plantar tuberosity arises medially. The sulcus tali is similar to that of Zygomaturus though is more deeply excavated.

Calcaneus. The sustentaculum is better developed and more distinct than in Zygomaturus and Diprotodon, presenting a steeply sloped astragalar facet laterally. The sustentaculum has a curved articular facet on its superior edge and is dorsoventrally thickened compared to the calcaneus of Ng. tedfordi. The calcaneal tuber is comparatively straight. The posteriormost part of the tuber is eroded so the total length and extent to which it curves medially is not known. However, it is clear that the tuber is not the narrow sigmoid-shaped structure of Zygomaturus and instead appears to resemble the morphology seen in Nimbadon, Thylacoleo and Ng. tedfordi. The distolateral process is very pointed in dorsal view. The cuboid facet is a near-spherical concavity, more dorsally-facing than in most diprotodontids, bearing the closest resemblance to that of Neohelos and Ng. tedfordi. The plantar surface is pitted and rugose, especially towards the distal end.

(A) articulated partial right side pes in dorsal view; right side ectocuneiform in (B) lateral and (C) distal views; right side cuboid in (D) medial and (E) distal views; right side navicular in (F) proximal and (G) distal views; right side calcaneus in (H) dorsal and (I) medial views; mirrored left side astragalus in (J) dorsal and (K) plantar views. Hatching indicates surface damage to cortical bone, dashed lines indicate inferred bone contours. Abbreviations: af , astragalar facet; caf, calcaneoastragalar facet; cal ; calcaneus; ct , calcaneal tuber; cu , cuboid; cuf , cuboid facet; dlp , distolateral process; ect , ectocuneiform; ectf , ectocuneiform facet; entf , entocuneiform facet; ltf , lateral tibial facet; mpt , medial plantar tuberosity; Mt4 , metatarsal 4; Mt4f , metatarsal 4 facet; Mt5 , metatarsal 5; Mt5f , metatarsal 5 facet; mtf , medial tibial facet; nav , navicular; nf , navicular facet; pt , plantar tuberosity; st , sulcus tali; sut , sustentaculum tali. Scale bar 50 mm.

(A) articulated partial right side pes in dorsal view; (B) right side metatarsal 4 NMV P29620 in (left to right, top to bottom) dorsal, medial, plantar, lateral, distal and proximal views; (C) right side metatarsal 5 NMV P29619 in dorsal, medial, plantar, lateral, distal and proximal views; (D) right side ectocuneiform NMV P29622 in dorsal, medial, lateral, proximal and distal views; (E) right side cuboid NMV P29621 in dorsal, medial, lateral, proximal, distal and plantar views; (F) right side navicular NMV P29623 in dorsal, proximal, distal and plantar views; (G) left side astragalus fragment NMV P254089 in dorsal, medial, plantar, lateral and distal views; (H) right side calcaneus NMV P30723 in dorsal, medial, lateral and distal views. Scale bar 50 mm.

Distal articulations. The distal articular surface is similar to that of vombatids in basic shape, contours and orientation. The astragalar facet is highly convex and continues further anterolaterally than in vombatids to provide a large laterally-facing articular surface, larger than and totally unlike the flat facet in diprotodontids. The posterior tubercle of the malleolus is a small conical process, more distinct than in diprotodontids and vombatids while being shorter and barely extending past the distal articular surface (it is absent in Phascolonus). Behind this lies an incised groove for the ankle and digital flexor tendons.

Diaphysis. The diaphysis in midsection is flattened medially and convex laterally, creating distinct anterior and posterior borders between these surfaces. The principal difference with the Vombatus tibia is that the tibial crest in P. azael is more proximally positioned and not as convex in mediolateral view. The line for attachment of the tibiofibular interosseus ligament lies obliquely along the lower anterolateral diaphysis, while medially the scars for m. gracilis and m. semitendinosus are strongly expressed.

Proximal articulations. The medial condylar surface is suboval, much broader in all dimensions than in P. parvus and closer in extent to the Phascolonus tiba. It is concave but not as deep or regular as those seen in Diprotodon and Zygomaturus. The convex lateral condylar surface is elevated on the tibial plateau relative to the medial condyle and is a near-level surface, rather than the gently posteriorly-sloped surfaces in Phascolonus and Diprotodon or the steeply sloped equivalent in Zygomaturus. The anterior plateau and tibial tuberosity comprise almost half of the total surface of the proximal tibia. This creates a proximal tibial surface that is expanded anteroposteriorly, approaching the vombatid condition more than that of diprotodontoids. The intercondylar eminence is a quadrangular protuberance in anterior aspect, similar to that of vombatids but more robust in proximal view. The tibial tuberosity for insertion of the patellar ligament is an inverted isosceles triangle, more elongate than in vombatids and unlike the trapezoidal diprotodontid condition. The lateralmost portion of the proximal tibia is eroded so the shape and extent of the fibular articulation in P. azael is not known. A concave popliteal fossa lies beneath the tibial plateau on the posterior side.

Other than thicker transverse dimensions along the diaphysis, the tibia of P. azael resembles that of Vombatus in general proportions in that it is longer relative to the epiphyses than in diprotodontids or Phascolonus (Figs 15 and 16 ).

Patellar surface. The patellar surface in P. azael is quadrangular and only shallowly indented. In distal view the medial crest of the patellar surface only modestly protrudes from the distal femur, far less than the protrusion seen in large diprotodontids or Phascolonus, being more similar to extant wombats.

Lateral condyle. The lateral condyle in distal view is anteroposteriorly shorter, mediolaterally broader and oriented more closely along the sagittal axis than its medial counterpart. In posterior view, the lateral condyle is very short proximodistally relative to the medial condyle. This contrasts both with the anatomy of Diprotodon and Phascolonus in which the lateral is taller, and with the subequal heights of the condyles in most other diprotodontids. The Ngapakaldia femur presents a similar condition to palorchestids, however in the latter overall both condyles appear short proximodistally relative to femoral length when compared to all other vombatiforms.

Medial condyle. In distal view the anteroposterior extent of the medial condyle is ~30% greater than that of the lateral (as is typical of large vombatiforms). It is mediolaterally narrower and more oblique to the sagittal axis of the bone than the lateral condyle. The medial surface of the condyle is deeply pitted for the attachment of the medial collateral ligament. In posterior view, the medial condyle presents a narrower articular surface than the lateral, with a hooked process curling in toward the bone midline as in vombatids and Thylacoleo, though more pronounced than both. In anterior view the distal surfaces of the medial and lateral condyles sit approximately level to a plane perpendicular to the femoral shaft, unlike the marked distal offset of the lateral condyle in large diprotodontids and Phascolonus.

Diaphysis. The femoral diaphysis in P. azael is smooth overall. Like in other diprotodontoids there is no linea aspera as such, however the femoral shaft shows elliptical pits, grooves and muscle scarring in the area immediately distal to the lesser trochanter on the medial and posteromedial midshaft. These mark insertion areas for adductor musculature and are much more pronounced than in other vombatiforms studied. On the distolateral diaphysis a raised area bordered by a crest marks the origin for lateral head of m. gastrocnemius, similarly situated to that in Zygomaturus but sharper and more distinct than in the latter.

The proximal femur as a whole resembles that of Phascolonus more than any other vombatiform, though with key differences in P. azael including; a more acute angle between the higher greater trochanter and shorter femoral neck, more distally-extensive articular surface around all edges of the head, and a mediolaterally narrower proximal femur overall.

Lesser trochanter. The lesser trochanter is an extensive plate-like flange on the proximomedial femoral shaft, arising at the femoral neck and extending distally to become a rugose insertion scar that extends to a third of the way down the entire bone length. Some damage to the proximal portion of this crest obscures its full extent, but overall the lesser trochanter appears to be similar to those of Diprotodon and Zygomaturus in that it is a broad crest rather than a distinct tubercle, especially appreciated in posterior view.

Greater trochanter. In anterior view the greater trochanter is tapered proximally as it projects strongly above the femoral neck unlike in diprotodontids. The anterolateral surface of the greater trochanteric epiphysis projects anteriorly from the shaft and curls slightly medially. In lateral view the trochanter appears expanded anteroposteriorly to give a rounded appearance, and the anterior projection of its distal portion is clearly appreciated. Posteriorly, the trochanteric fossa is very deep and relatively longer superoinferiorly than any other taxon studied. There is no discernible intertrochanteric crest or gluteal tuberosity.

Femoral head. The apex of the hemispherical head lies slightly proximal to the greater trochanter when the femoral condyles are placed on a flat surface. The neck is stout and very short relative to femora of similarly-sized diprotodontids, and projects anteromedially, positioning the head such that ~30% of its bulk overhangs the anterior shaft when viewed medially. In proximal view the head is significantly deeper cranio-caudally than the diaphysis or neighbouring greater trochanter, being very similar in this respect to the anatomy of Vombatus and differing from other large diprotodontoids.

The Palorchestes azael femur is elongate and gracile for its size, with a slender sub-cylindrical shaft and narrow epiphyses. In proportions and overall shape it strongly resembles the femur of extant Vombatus (see S3 Fig ), but is of course significantly larger, with a weaker lesser trochanter and no gluteal tuberosity (Figs 13 and 14 ).

Detached symphysial epiphysis. Associated with the NMV P159792 os coxae of P. azael is the detached symphyseal epiphysis from the ventral pubis. This provides the subpubic angle and angle of the anterior pelvic brim, indicating P. azael had a more V-shaped girdle as in Ngapakaldia than the U-shape in vombatids or broad, open girdle of large diprotodontids. The reconstruction in Fig 12D shows that the ilia are less laterally extensive than in vombatids or large diprotodontids, possible evidence for a slenderer body form in P. azael.

Auricular surface. The auricular surface is rugose, with two distinct articular facets; a small one facing medially and a longer, deeper one facing more anteriorly. The sacral surface is craniocaudally short compared to that of Zygomaturus and much shorter than in Phascolonus, being more similar to the Ngapakaldia form. Inferior to the auricular surface, a small greater sciatic notch is visible in posterior view.

Inferior/ischiopubic ramus. The ischiopubic ramus is long relative to the ischial body, indicating a deep pelvic cavity as in diprotodontids. It is dorsoventrally shallow at its pubic end. It deepens dorsoventrally as it extends posteriorly, though not as much as in Phascolonus, upturning but not increasing in transverse thickness as it gives rise to the ischial tuberosity.

Obturator foramen. The obturator foramen is ovotriangular and slightly elongate, with the apex of its shape directly under the pectineal origin, similar to that seen in Ngapakaldia and Neohelos, though relatively broader than both.

Iliopubic ramus. The iliopubic ramus in P. azael is slender overall, with a large ovoid fossa for articulation with the epipubic bone. This ramus is relatively longer and more slender than in Ngapakaldia, Phascolonus or Diprotodon and more inferiorly directed than in Neohelos, with more circular section profile.

Anterior inferior iliac spine. The anterior inferior iliac spine for the origin of m. rectus femoris is located inferiorly and close to the acetabular margin as in Zygomaturus, though it is a narrower muscle scar in P. azael.

Acetabulum. The shape of the acetabular margin in lateral view is arched, with a slightly tapered point at its superior margin greatly overhanging the acetabular fossa. Inside the acetabulum the notch is narrow, leaving a large articular surface area within the socket resembling the condition in Zygomaturus in extent and shape, though less buttressed around the perimeter than the latter. There is a deeply excavated fossa for the ligamentum teres. The acetabulum overall is shallower and less circular than in diprotodontids and vombatids, and more posterolaterally oriented than in the latter.

Ischial tuberosity. The ischial tuberosity is a gracile, elongate, weakly rugose structure in posterior view and lacking the distinct triangular shape with posterolateral projections seen in wombats. It appears narrower and shorter than in all other diprotodontoids studied.

Ischial body. The body of the ischium in P. azael is long, with a flattened ovoid profile. It is relatively longer than in Diprotodon or Zygomaturus, with a similarly upturned distal end for the ischial tuberosity. Unlike extant wombat ischia, there is no ischial spine present.

Iliac blades. In P. azael the iliac blades are sickle-shaped, with mediolaterally narrow dimensions, quite unlike the laterally flared ilia of Zygomaturus and Diprotodon. In relative length the ilia are intermediate between those taxa and the elongate ilia of extant wombats. They resemble Neohelos ilia but have a more deeply concave lateral margin and are slightly broader. The best-preserved specimen is missing the lateral tip of the blade and the epiphyseal rim along its proximal margin–their contours are estimated in Fig 12 . In lateral view the ilium is oriented to create a 140° angle with the body of the ischium, a much more dorsally-rotated position than in Zygomaturus or the almost aligned Diprotodon. The ilium is slightly twisted anteriorly causing the posterior (gluteal) blade surface to be visible in lateral aspect.

(A) right side os coxa, lateral view; (B) right side os coxa, dorsal view; (C) right side os coxa, medial view; (D) reconstructed pelvic girdle, superoventral view (this view not to scale). Hatching indicates surface damage to cortical bone, dashed lines indicate inferred bone contours. Abbreviations: acf , acetabular fossa; acn , acetabular notch; aiis , anterior inferior iliac spine; aur , auricular surface; epi , facet for articulation with epipubic bone; glf , gluteal fossa; gsn , greater sciatic notch; ilb , iliac blade; ilc , iliac crest; ilf , iliacus fossa; ilpr , iliopubic ramus; ipe , iliopectineal eminence; isb , ischial body; ispr , ischiopubic ramus; ist , ischial tuberosity; obf , obturator foramen; psy , pubic symphysis. Scale bar 50 mm.

Proximal end. The articular facets for the intermediate phalangeal condyles are transversely narrow, deep semilunate notches with a low median crest. In proximal view the facets are slightly tapered dorsally and broader ventrally, giving an elongate trapezoidal shape overall in this aspect. The dorsal extensor process is expanded and is more dorsally deflected than in any other vombatiform, overhanging the joint while allowing a degree of hyperextension. Dilated and robust flexor tubercles on the plantar surface are larger relative to the rest of the ungual than other large diprotodontoids. These tubercles are as mediolaterally broad as the articular facets, giving a more quadrate shape to this region of the ungual in proximal view unlike the ventrally tapered appearance in Ngapakaldia and Neohelos.

Ungual crests. When preserved, these crests are very well-developed plates of bone, projecting distally to an even greater extent than in Nimbadon or extant Phascolarctos. The resulting sulcus would have provided a deep and secure attachment for the proximal rim of the keratinous claw sheath.

Palorchestid unguals are morphologically distinct from all other vombatiforms in their deep, laterally-compressed, lunate shape (Figs 9 and 10 ). Overall, they most closely resemble those of Phascolarctos or Nimbadon, but in palorchestids they are dorsoventrally deeper and larger relative to the proximal and intermediate phalanges.

Distal end. The distal section profile of the P. azael radius is trapezoidal, with a flattened dorsal surface parallel to the slightly concave ventral surface. This contrasts with the dorsoventrally broad, triangular distal profile of diprotodontid and Vombatus radii, the flattened palorchestid state being most similar to that of Lasiorhinus and Ngapakaldia.

Diaphysis. The interosseous crest begins immediately distal to the radial tuberosity and is less sharply defined than in extant wombats, and dramatically less so than the broad flange present in Phascolonus. The crest continues along the lateral border of the diaphysis, forming the ‘apex’ of the triangular midshaft section profile and terminating in a slightly rugose area ¾ of the way down the shaft. This rugosity differs strongly from the Vombatus and Phascolonus forms in which it is deeply pitted. The shaft becomes extremely thick and robust distally, more so than any other taxon studied. The medial diaphyseal border is dominated by the insertion scar for m. pronator quadratus, which is very similar in positioning and extent to that seen in extant wombats.

Radial tuberosity. The radial tuberosity for insertion of m. biceps brachii is more distally situated on the shaft than in Vombatus, closely resembling Lasiorhinus and Ngapakaldia spp. in relative positioning. However, the radial tuberosity in P. azael is not prominent and does not project from the shaft anything like the marked tuberosities seen in the radii of Phascolonus, extant wombats or Ngapakaldia spp., suggesting reduced size or contractile force in m. biceps brachii.

Head. In proximal view, the perimeter of the capitular fossa is suboval, with the slight transverse elongation seen in wombats and Ng. tedfordi, rather than the more truly circular fossae of Diprotodon and Phascolonus radial heads. The hemispherical fossa is deeply cupped, closest in relative depth to that of Ng. tedfordi, though lacking the distinct craniolateral tilt and sharp rim edge of the latter. The articular surface for the ulna occupies two-fifths of the capitular circumference, roughly equivalent to the vombatid condition and less than in Ng. tedfordi.

The palorchestid radius is most like that of Vombatus in overall proportions (Figs 7 and 8 ). The angle of the head and neck, and the degree to which the diaphysis curves medially down toward the styloid process is very vombatid. Like wombats, the radial midshaft is triangular in section. It becomes progressively more mediolaterally expanded to appear flatter and bulkier in distal section than the wombat though still distinctly trapezoidal, unlike the triangular distal profile of Diprotodon and Phascolonus radii.

Styloid process. The styloid process is thick and distinctly anteromedially deflected like in vombatids, tapering from a wide, dorsolaterally-elongate base to a small bulb featuring a distally-facing convex articular facet for the triquetrum. The styloid is ovoid in distal view and much smaller than the surrounding epiphysis, similar to that of Ng. tedfordi in proportions and shape and unlike the inflated, bulbous styloid in Ng. bonythoni, Neohelos, Diprotodon and Zygomaturus.

Diaphysis. The diaphysis varies in section profile along its length from ovo-quadrangular proximally, lachrymiform at midshaft due to the thickened posterior border, to subcircular at the distal epiphysis. This circular distal section contrasts with the mediolaterally-flattened epiphyses of vombatid and Ng. tedfordi ulnae, being more similar to those of Diprotodon and Zygomaturus, likely corresponding to changes in directional stresses associated with increased body mass. The P. azael ulnar shaft also varies in anteroposterior depth along its length, being deepest at the ulnar tuberosity and gently tapering distally. The lateral diaphyseal surface is largely smooth and lacks the marked longitudinal scarring seen in diprotodontid ulnae. The most obvious muscle attachments are two large ovoid scars–one arising laterally at the approximate midshaft (mm. abductor pollicis longus and extensor digitorum profundus), the other occupying the distal third of the lateral diaphysis (m. pronator quadratus). The latter scar strongly resembles the condition in Ngapakaldia.

Ulnar tuberosity. The ulnar tuberosity for insertion of mm. brachialis and biceps brachii is approximately 15 mm in diameter. It is raised, bulging and is situated well distal to the coronoid process and radial notch on the lateral aspect of the ventral shaft margin. In relative displacement of the tuberosity from the coronoid, the P. azael ulna is surpassed only by that of Phascolonus.

Radial notch. The radial notch projects laterally and is similar to the anconeal process in its extent. The radial notch is more posteriorly offset than any other taxon studied, positioned very near the dorsal border of the shaft. A deep, inset ligamentous pit lies directly mediad to it for attachment of the annular ligament. Like the notch, this pit is more dorsally positioned than in other vombatomorphs.

Anconeal process. The anconeal process is strongly laterally deflected, its axis in the transverse plane offset by ~90° to that of the shaft so that in medial view it is not visible.

Coronoid process. In lateral view the coronoid process is robust with a triangular profile, owing to the flattened trochlear surface on the proximal side and the gradual slope of diaphyseal bone on the distal side. It is less dorsoventrally extensive relative to the diaphyseal thickness than in all other comparative taxa observed.

Trochlear notch. The trochlear notch and trochlear surface are flattened (in agreement with the corresponding articular surface of the humerus), with only the slightest concavity as the trochlear surface ramps distally to form the coronoid process. The orientation of the trochlear surface is slightly less steep than in vombatids, being approximately 45° to the axis of the shaft. This contrasts with the near-horizontal equivalent in diprotodontids and is not nearly as concave as the latter. In P. azael this flat trochlear surface lies on a medially-projecting platform, creating a deep elongate fossa beneath it on the medial shaft surface as in vombatids and Ng. bonythoni. The floor of this fossa represents the narrowest lateral dimension of the ulna overall and provided origin for m. flexor digitorum profundus. The lateral semilunar facet for the humeral capitulum is concave and less than half the proximodistal length of the trochlear surface.

Olecranon process. The olecranon is very enlarged and elongate, representing nearly a quarter of the total ulnar length, and in lateral view appears quadrangular with a slight bulbous projection posteriorly. Unlike in the diprotodontid ulna, the olecranon lies along the axis of the shaft with no posterior deflection, and medial deflection is only very slight–less than in extant wombat or Ng. bonythoni ulnae and much less than the medially-curled olecranon in Phascolonus. The olecranon is deep in the sagittal plane, slightly more so than at the midshaft of the bone, and does not taper proximally as extant wombats do. The transverse width at the dorsal edge of the olecranon is around half its dorso-ventral depth.

Generally, the ulnar shape and proportions of P. azael resembles that of extant wombats, Ngapakaldia and Thylacoleo much more than other large diprotodontoids, albeit with some significant alterations (see S2 Fig ). Overall the ulna is straight when viewed anteriorly, while in lateral view it is slightly convex posteriorly, giving a bowed appearance (Figs 5 and 6 ).

Medial epicondyle. The medial epicondyle in anterior view is rounded and projects strongly medially, its axis perpendicular to the vertical axis of the humeral shaft in both dorsal and transverse planes. The medial epicondyle constitutes approximately 32% of the total distal humeral width, making it the largest, both relatively and absolutely, among the vombatiforms.

Supracondylar foramen. The bridge defining the ceiling of the supracondylar foramen is broad proximally and tapers somewhat at its mediodistal end. This bridge lies almost horizontally along the corresponding contour of the medial epicondyle, giving the underlying foramen a vertical orientation unlike the more oblique foramina of other vombatiforms (where present). The foramen is approximately four times longer than its breadth.

Capitulum. In anterior view the capitulum is near hemi-spherical and lacks an obvious radial fossa proximally. In distal view it is ~20% smaller than the trochlea. Posteriorly, the joint surface for articulation with the anconeal process of the ulna extends slightly higher than on the anterior side of the capitulum and is slightly elevated from the posterior shaft surface.

Trochlea. The trochlea is another notably unique feature of the P. azael humerus. Its articular area is completely flattened and would have allowed almost no parasagittal movement of the ulna. Rather than a pulley-shaped or domed articulation as in other vombatiforms, it is a simple, distolaterally-facing ovoid facet oriented ~115° to the neighbouring capitulum. In distal view the trochlea is offset anteriorly from the dorsal (coronal) plane of the humerus, projecting cranially much more than the smaller capitulum. In the larger P. azael specimens the posterior trochlear surface curves superiorly to provide slightly more anconeal articulation than in the smaller humeri.

Lateral epicondyle. The lateral epicondyle projects only slightly beyond the lateral margin of the capitulum. In lateral view, the lateral epicondyle is thick inferiorly before curving proximally up and tapering into the lateral supracondylar crest, a sheet of bone that presumably terminated in a hooked process as in most other vombatiform marsupials. Almost the entire edge of this crest is damaged in all specimens observed. However, the concave, arced profile that remains at the top of the crest, curling superiorly from the lateral shaft, suggests such a hooked process was once present. This crest would have provided origin for mm. brachioradialis and extensor carpi radialis.

Diaphysis. The humeral diaphysis is highly irregular in section and in profile due to the many large muscle attachment crests along its relatively short length. It is stout and broad in anterior view with an almost ‘hourglass’ silhouette due to the flared proximal and distal ends and narrow constriction in line with the lateral supracondylar crest. In lateral view the posterior contour of the shaft is near-flat (as is typical for diprotodontoids), while the anterior contour is dominated by the hooked pectoral crest. The posterior diaphyseal surface features obvious scars for the origins of the humeral heads of m. triceps brachii. At the base of the humeral neck is a distinct fossa for the origin of m. brachialis, so expansive as to nearly reach the shaft midline as in Phascolonus. This fossa curves laterally beneath the deltoid tuberosity and anterior to the lateral supracondylar crest. There is no olecranon fossa on the posterior aspect of the distal humeral shaft, which is flat and smooth but for a marked rugose patch immediately proximal to the trochlea. This forms the origin for m. epitrochleoanconeus.

Tuberosity for mm. teres major and latissimus dorsi. An elongate, ovoid muscle scar lays halfway down the medial aspect of the humeral shaft for the insertion of mm. teres major and latissimus dorsi. Its posterior edge overhangs and sharply demarcates the posterior from the anterior shaft surface. This insertion is relatively larger, better developed and more distal than in any other vombatiform studied.

Pectoral crest. The pectoral crest is a distinctive feature of the P. azael humerus, originating from the greater tubercle just lateral to the humeral midline. It follows an oblique inferomedial path along the anterior shaft, becoming a laminar crest with a rectangular sectional profile which increases in height and thickness as it curls around medially. At its distal tip it swells to a bulbous terminus in the largest specimens, which then re-joins the shaft via a recurved, sickle-shaped ‘pulley’, presumably for the passage of m. biceps brachii descending along its medial plane. At its zenith the anterior projection of this pectoral crest from the shaft is greater than in any other vombatiform studied, in both relative and absolute extent.

Deltoid tuberosity. Viewed anteriorly the deltoid tuberosity is extensive and approximately triangular in shape, projecting strongly from the lateral diaphysis and contour of the greater tubercle above. It tapers to a rugose, bulbous terminus at its apex which lies a third of the way down the humeral shaft. The posterolateral lip of this deltoid tuberosity curls posteriorly to form the lateral margin of the fossa for m. brachialis. The clear separation of this large tuberosity from the pectoral insertion crest, contrasting with morphology of other palorchestids and vombatids, suggests a strongly developed scapular deltoid in P. azael and a reduction in the clavicular deltoid, with no distinct attachment site visible for this muscle. This arrangement resembles that seen in Diprotodon, although in P. azael the deltoid tuberosity differs strongly from that taxon in its triangular form and far more proximal position relative to the pectoral crest.

Lesser tubercle. The lesser tubercle in P. azael is comparatively poorly developed among diprotodontoids. In medial view, it extends obliquely from its uppermost tip anteriorly to its distal edge posteriorly, terminating proximal to the tip of the beaked humeral head. A medially-oriented facet for insertion of m. subscapularis is, like in other palorchestids, narrow and steeply oblique, though less steep than in Nimbadon.

Greater tubercle. The greater tubercle is rugose and projects above the humeral head, narrowing to a proximally rounded tip resembling that of Lasiorhinus in shape and proportion. Discrete proximal (for m. supraspinatus) and lateral (for m. infraspinatus) facets are present. The tubercle continues inferomedially to give rise to the pectoral crest. Viewed laterally, the greater tubercle is broad and occupies two thirds of the total anteroposterior depth of the proximal humerus.

Head. The domed head extends its articular surface posteroinferiorly to provide increased posterior surface area, similar to that of other large-bodied diprotodontoids, rather than the superiorly-oriented head in vombatids. Viewed medially, this articular surface ‘beaks’ out from the humeral neck at its posteroinferior tip. The head is flanked anteriorly by greater and lesser tubercles separated by a broad and shallow bicipital sulcus.

The humerus of Palorchestes azael is remarkable in its blade-like pectoral crest and its curiously flat trochlear surface (Figs 3 and 4 ). Of the two complete humeri figured, the larger (NHMUK PV OR 46914) shows marked increase in relative size and rugosity of muscle attachments, while the smaller humerus with its unfused metaphyses likely represents an osteologically-immature animal. Descriptions below are based on the former.

Infraglenoid tubercle. The infraglenoid tubercle, though damaged, is very extensive mediolaterally and superoinferiorly, and deeply pitted. Roughly triangular in shape, it would have provided large origin for the long head of m. triceps brachii. It does not appear to have projected away from the glenoid to alter the posterior scapular border as it does in derived diprotodontids like Diprotodon and Zygomaturus, instead resembling the vombatid condition, though being thicker mediolaterally.

Scapular spine. The base of the distal scapular spine is preserved, showing it to curl anteriorly over the supraspinous fossa as in zygomaturines. The dorsal margin and acromion are missing.

Supraspinous and infraspinous fossae. Little of the supra- and infraspinous fossae are preserved, although the scapular notch is intact and appears more deeply curved than in vombatids and Diprotodon but less than in zygomaturines.

Glenoid fossa. The glenoid fossa is shallow, anteroposteriorly elongate and mediolaterally narrow as is typical of vombatiforms, although none of the outer circumference is preserved so the details of its shape are uncertain. The coracoid process is damaged and not clearly discernible.

The only known scapula for P. azael is part of the associated skeleton NMV P157144 (Figs 1 and 2 ). It is very eroded and encrusted with matrix which could not be safely removed. Preserving only the glenoid and distal portions, its proximodistal length, anteroposterior width and overall shape are not known. What little morphology is retained appears wombat-like, with the notable exception of the enlarged infraglenoid tubercle.

All following specimens were collected as part of the 1970s Gallus excavations at Keilor, VIC, found associated in the same ‘D Clay’ layer, and assigned to Zygomaturus trilobus by Marshall [ 50 ]. While no palorchestid material is yet reported from Keilor, these elements strongly differ from the morphology seen in Zygomaturus, and some were identified as palorchestid by Szalay [ 71 ], so we refer them to Palorchestes azael on the basis of their very large size and morphological similarity to elements of AM F58870.

NMV P252196. Ungual phalanx (some erosion to edges of ungual crests). This element was tentatively attributed to Thylacoleo and photographed by Wakefield [ 70 ], pg. 79. Collected by N. A. Wakefield from Mount Hamilton Caves, VIC, in 1963.

NMV P26534. Right femur (superficial damage to rim of head, lateral margin of lateral condyle). Associated with NMV P159792 but given own registration when figured by Scott ([ 23 ], Plate 21) as a ‘phascolomyform’ femur. Collected by F. Spry from Buchan Caves (probably Foul Air Cave), VIC, in 1907.

NMV P159792. Associated partial skeleton including: left humerus (superficial damage to upper third of pectoral crest, lateral supracondylar ridge); right humerus (middle third of shaft with partial pectoral crest, both epiphyses missing); left ulna (proximal two-thirds only); right ulna; right radius (open distal metaphysis with missing distal epiphysis); right os coxa; left os coxa (fragment with acetabulum, superior iliopubic ramus and ischial tuberosity only) with detached symphyseal epiphysis; left femur (shaft fragment only); right tibia (lateral proximal epiphysis damaged, hole in cortex of superior posterior diaphysis); ungual phalanx (dorsal process missing). Unfused metaphyses indicate bones were not yet fully mature. Humeral and ulnar morphology consistent with NMV P157144 but is smaller ( S1 Table ), other elements are here referred to the species by association with this humerus and ulna. Collected by F. Spry from Buchan Caves (probably Foul Air Cave), VIC, in 1907. Identified by Flannery and Archer in 1980, articulated humerus and ulna illustrated in Flannery and Archer [ 10 ].

(A) anterior; (B) lateral; (C) posterior; (D) medial views. Hatching indicates surface damage to cortical bone, dashed lines indicate inferred bone contours. Abbreviations: bg , bicipital groove; brf , fossa for m. brachialis origin; ca , capitulum; del , deltoid insertion; eta , origin for m. epitrochleoanconeus; gt , greater tubercle; hh , humeral head; inf , fossa for insertion of m. infraspinatus; ldtm , insertion for mm. latissimus dorsi and teres major; le , lateral epicondyle; lsc , lateral supracondylar crest; lt , lesser tubercle; me , medial epicondyle; pec , pectoral crest; subf , fossa for insertion of m. subscapularis; supf , fossa for insertion of m. supraspinatus; sf , supracondylar foramen; tr , trochlea; tri , origin for humeral heads of m. triceps brachii. Scale bar 50 mm.

Left side humerus NMV P159792 (A-D, G-H) and right side humerus NHMUK PV OR 46914 (E-F, I-J, mirrored for comparison). (A) anterior; (B) lateral; (C) posterior; (D) medial; (G) proximal and (H) distal views. Right humerus NHMUK PV OR 46914 in (E) anterior; (F) medial; (I) proximal and (J) distal views. Scale bar 50 mm.

NHMUK PV OR 46914. Right humerus (diaphyseal cortex fractured and repaired). This element was figured and attributed to Nototherium mitchelli by Owen ([ 69 ], plate CXXVII Figs 1 – 6 ). Collected by W. L. R. Gipps from Castlereagh River, Mendooran, NSW, in 1875.

NMV P157144. Associated partial skeleton including: mandible with diagnostic molars; left scapula fragment (glenoid cavity and base of spine with partial supraspinous/infraspinous/subscapular fossae only); right humerus fragment (distal two-thirds of shaft and epiphysis, missing capitulum and lateral epicondyle); left ulna fragment (proximal two-fifths, olecranon and anconeal surface damaged); left and right os coxae fragments (acetabular regions and partial ischia only); and femur fragment (diaphysis only). Unknown collector from unknown locality.

Palorchestes parvus De Vis 1895

Referred material. Measurements for all referred material below are provided in S1 Table. AM F58870. Associated partial skeleton including: premaxilla fragment with incisor alveoli matches syntype QMF789 (Woods [3], Fig 4; Trusler [36], Fig 5.19K); left humerus; right os coxa (acetabulum and partial ilium); left femur (two fragments; proximal two-fifths, some damage to posterior femoral neck and greater trochanter; distal epiphysis); left tibia (proximal two-fifths fused in flexion to distal femur); partial left manus (missing phalanges 5 and all carpals except for trapezium); partial left pes (missing calcaneus and astragalus). Collected by G. Hope from ‘cave at Wee Jasper, Punch Bowl Hill, below and left of Signature Cave’, NSW in 1977. NMV P159792. Associated partial skeleton including: left tibia (shaft broken and repaired above distal epiphysis, some cortical bone missing, referred based on morphological match with tibia from AM F58870); left ulna (proximal fragment containing humeral and radial articulations with some damage to articular surfaces); right radius (proximal two-thirds with damage to the capitular rim); left radius (distal fragment with well-preserved epiphysis). These are smaller, yet mature elements registered as NMV P159792, leading us to conclude that P. azael and P. parvus species are both represented, with a total MNI of 3 across all NMV P159792 specimens (two P. azael and one P. parvus). Collected by F. Spry from Buchan Caves (probably Foul Air Cave), VIC in 1907.

Humerus (Figs 19 and 20). The humerus of P. parvus is wombat-like overall, with stout and robust proportions and thick muscle attachment crests (Figs 19 and 20). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 19. Left humerus of Palorchestes parvus AM F58870. (A) anterior; (B) lateral; (C) posterior; (D) medial; (E) proximal; (F) distal views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g019 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 20. Labelled illustrations of the Palorchestes parvus left humerus AM F58870. (A) anterior; (B) lateral; (C) posterior; (D) lateral views. Hatching indicates surface damage to cortical bone, dashed lines indicate inferred bone contours. Abbreviations: bg, bicipital groove; brf, fossa for m. brachialis origin; ca, capitulum; del, deltoid insertion; eta, origin for m. epitrochleoanconeus; gt, greater tubercle; hh, humeral head; inf, fossa for insertion of m. infraspinatus; ldtm, insertion for mm. latissimus dorsi and teres major; le, lateral epicondyle; lsc, lateral supracondylar crest; lt, lesser tubercle; me, medial epicondyle; of, olecranon fossa; pec, pectoral crest; subf, fossa for insertion of m. subscapularis; supf, fossa for insertion of m. supraspinatus; sf, supracondylar foramen; tr, trochlea; tri, origin for humeral heads of m. triceps brachii. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g020 Head. The humeral head is sub-hemispherical and larger relative to its surrounding proximal humeral structures than in other palorchestids. The posteroinferior tip of the head appears to ‘beak’ less from the posterior diaphysis in lateral view than in P. azael. Greater tubercle. The greater tubercle in P. parvus projects slightly proximally to the humeral head and has two major muscle attachment fossae; the fossa for insertion of m. supraspinatus, which is a flattened ovoid shape on the proximal surface of the tubercle, and the broad, posterolaterally-oriented fossa for m. infraspinatus. These fossae resemble those of P. azael in relative proportions to the tubercle and to each other. In proximal view, the greater tubercle is slightly more anteriorly positioned than in P. azael, but not as anterior as in Propalorchestes. In lateral view the greater tubercle is extensive anteroposteriorly, to the same degree as in P. azael. Lesser tubercle. In anterior view, the apex of the lesser tubercle sits just inferior to the humeral head, the highest relative position of the palorchestids. In this view it has a rounded medial margin which appears more inflated than any other taxon studied. In medial view, the elongate attachment scar for m. subscapularis lies obliquely along the posterior margin of the tubercle, similar to that of Propalorchestes. Viewed proximally, the lesser tubercle resembles that of P. azael in all respects except in the contours of the bicipital groove, which in P. parvus is more concave/less flattened and positioned nearer to the midline of the bone. Deltopectoral crest. The deltopectoral crest in P. parvus is thick and well developed, being similarly shaped to vombatids overall but differing in some key respects. The pectoral insertion crest runs subvertically from the greater tubercle down the approximate midline of the humeral shaft, and along its entire length the pectoral crest sharply overhangs the bicipital groove medially. This is unlike vombatids where the entire deltopectoral crest is offset laterally from the humeral shaft, the posterior part overhanging the brachialis fossa rather than the anteromedial part overhanging the bicipital groove. The medial overhang of this crest begins at the proximal metaphysis and is clearly visible in proximal view–this overhang is more proximal than in P. azael, though the crest becomes less developed than the latter as it passes distally. Like vombatids, there is an oblique crest coursing inferoanteriorly from the lateral shaft to converge medially with the pectoral crest at its distal tip. The lateral edge of this oblique crest is likely to have been the attachment site for the scapular part of m. deltoideus. Just inferior to the greater tubercle, a faint ridge runs vertically for a short distance on the anterolateral aspect of the pectoral crest–this probably marks the insertion point for the clavicular deltoid. This insertion ridge in P. parvus is weaker than the vombatid condition but is totally absent in P. azael, suggesting the clavicular deltoid played a reduced role in Palorchestes species (and especially in P. azael) relative to wombats. In P. parvus the terminal point on the deltopectoral crest is swollen into a distinct tuberosity. Vombatids lack a tuberosity here, and the shared distal end of these crests is more laterally positioned. Propalorchestes has a similar tuberosity in this position, although it is not associated with the scapular deltoid insertion which is situated proximally. Tuberosity for mm. teres major and latissimus dorsi. The insertion scar for the combined tendon of mm. teres major and latissimus dorsi is a lachrymiform fossa situated on the medial humeral shaft overhanging its posterior border slightly more than halfway down its length. In relative terms it is both the largest and most distally-positioned of these tuberosities among all the palorchestids. Diaphysis. In anterior and lateral views, the P. parvus humeral shaft appears straight overall though somewhat distorted by the crests and tubercles along its length. Like P. azael, in P. parvus attachment scars for m. triceps brachii lie on the posterior shaft surface, though the fossa for the origin of m. brachialis below the lateral lip of the humeral head is less pronounced. Inferiorly, a deep triangular olecranon fossa lies above the posterior articular surface of the capitulum, facilitating at least some extension of the elbow. The posteroinferior diaphysis lacks the marked rugosity for m. epitrochleoanconeus seen in P. azael. Lateral epicondyle. The lateral epicondyle projects slightly from the upper rim of the capitulum when viewed anteriorly. In lateral view, it appears pitted and rugose, resembling that of Propalorchestes in its shape and extent. From this epicondyle, the lateral supracondylar crest extends proximally as a thin sheet of bone, its lateral margin damaged but the remnant characteristic vombatiform hook still visible. Trochlea. In P. parvus the trochlea faces inferiorly but due to slight convexity both mediolaterally and anteroposteriorly its articular surface is just visible in anterior view. This represents an intermediate morphology between the curved, wombat-like condition in Propalorchestes and the flat trochlea of P. azael. Like the latter species, in anterior view the trochlea and capitulum project equally distally, though the angle between the two processes is less acute in P. parvus. In distal view the trochlea is ovoid and a little larger than the capitulum in dorsoventral depth. It is aligned with the capitulum in the dorsal (coronal) plane of the humerus. Supracondylar foramen. The supracondylar foramen in P. parvus is deep and ovoid, spanned by a bridge broader and more robust than in other palorchestids. Medial epicondyle. The medial epicondyle projects strongly medially from the vertical axis of the humeral shaft. In anterior view, the overall shape and relative extent the P. parvus medial epicondyle resembles that of Propalorchestes, being less wide and rounded than in P. azael. In medial view the P. parvus epicondyle is bulbous and thick anteroposteriorly.

Ulna (Figs 21 and 22). The only ulna specimens are fragments preserving the proximal articulations and short adjacent portion of the olecranon (Figs 21 and 22). The preserved morphology suggests that the olecranon would have been consistent with other palorchestids, aligned with the diaphysis and not posteriorly deflected as in diprotodontids, and not medially deflected as in Propalorchestes, instead resembling the morphology in P. azael. The trochlear surface is broad and subcircular like that of Vombatus and Ng. bonythoni, not narrowed and elongate as in P. azael. The tallest point of the anconeal process is much more anteriorly oriented than the laterally-deflected equivalent in P. azael, and creates a C-shaped trochlear notch in medial view similar to those of Propalorchestes and extant wombats. The coronoid process is relatively low and proximodistally thick like in P. azael, not thin and tall as in Vombatus. The radial notch and facet for the humeral capitulum lie in the same dorsal plane, more in line with one another than in P. azael where the radial notch is more dorsally offset than the capitular facet. The radial notch is large, approximately half the proximodistal length of the adjacent trochlear surface. In P. azael the radial notch is relatively shorter. The pit for attachment of the annular ligament between the trochlear and radial notches is shallower in P. parvus than in P. azael. The ulnar tuberosity is small and less rugose than in P. azael, and in a more proximolateral position. The posterior border of the ulnar shaft immediately dorsal to the coronoid process is narrower mediolaterally than in P. azael, indicating a less robust bone overall. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 21. Left ulna fragment of Palorchestes parvus NMV P159792. (A) anterior; (B) lateral; (C) posterior; (D) medial views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g021 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 22. Labelled illustrations of Palorchestes parvus left ulna fragment NMV P159792. (A) anterior; (B) lateral views. Hatching indicates surface damage to cortical bone. Abbreviations: ap, anconeal process; cf, capitular facet; cp, coronoid process; rn, radial notch; tn, trochlear notch; ut, ulnar tuberosity. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g022

Manus (Figs 25 and 26). The P. parvus manus is represented by an associated set of stout metacarpals and phalanges (Figs 25 and 26). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 25. Associated partial left manus of Palorchestes parvus AM F58870. (A) articulated manus in dorsal view; (B) metacarpal 4 in (left to right) lateral view; (C) metacarpals 4–2 (cemented together) in dorsal view; (D) metacarpal 2 in medial view; (E) metacarpals 2–4 in palmar view; (F) metacarpals 4–2 in proximal view; (G) metacarpals 4–2 in distal view; (H) metacarpal 5 in (left to right, top to bottom) dorsal, lateral, plantar, medial, distal and proximal views; (I) trapezium in dorsal, proximal, lateral, distal and medial views; (J-K) digit 1/pollex ungual and proximal phalanges in dorsal, lateral, plantar, medial, proximal and distal views; (L-N) digit 2 ungual, intermediate and proximal phalanges in dorsal, lateral, plantar, medial, proximal and distal views; (O-Q) digit 3 ungual, intermediate and proximal phalanges in dorsal, lateral, plantar, medial, proximal and distal views; (R-T) digit 4 ungual, intermediate and proximal phalanges in dorsal, lateral, plantar, medial, proximal and distal views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g025 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 26. Labelled illustration of articulated Palorchestes parvus left manus AM F58870. (A) articulated manus in dorsal view; (B) digit 4 in lateral view. Hatching indicates surface damage to cortical bone, dashed lines indicate inferred bone contours. Abbreviations: dij, distal interphalangeal joint; Ip1-4, intermediate phalanges 1–4; Mc2-5, metacarpals 2–5; mk, median keel; mpj, metacarpophalangeal joint; pij, proximal interphalangeal joint; Pp1-4, proximal phalanges 1–4; Tra, trapezium; Up1-4, ungual phalanges 1–4. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g026 Trapezium. The trapezium is very similar to that of Ngapakaldia bonythoni overall, being only a little larger and more proximodistally compressed. It has a saddle-shaped facet for the first metacarpal on its palmar-distal face with slightly flatter contours than the former species. A shallow concave facet for the trapezoid lies on its lateral face (though this is somewhat eroded), and a well-defined facet for the palmar process of the scaphoid sits on its proximal surface. The orientation of its articulations indicates the pollex would have been abducted to a similar degree to that seen in Ngapakaldia and much more than in Zygomaturus or extant wombats. Metacarpals. The metacarpals overall are stouter and more distally expanded than in Vombatus, zygomaturines and Ngapakaldia. They are compact, with contoured margins for adjacent contact over most of their length. Distal condyles are sub-hemispherical in lateral view, with pronounced keels on the palmar surface whose contours are visible even in dorsal view. The metacarpals are deeply pitted distolaterally and distomedially for attachment of collateral ligaments. Metacarpal 2. The second metacarpal is considerably smaller than its neighbours, with a sharply tapering proximal end resembling that of Ngapakaldia. The metacarpal presents a short, oblique dorsolateral facet for the magnum/capitatum and convex medial facet where the trapezium cups its medial edge. Metacarpal 3. The third metacarpal has a triangular proximal facet for the magnum/capitatum, which in dorsal view has a chevron-shaped notch as in Ngapakaldia, and a broad, flat facet laterally for metacarpal 4. Metacarpal 4. The fourth metacarpal is the longest and most robust as is common in marsupials. Proximally, the facet for the unciform/hamatum is triangular, with an oblique median groove. On the proximolateral surface, two ovoid tubercles separated by a sulcus are present for articulation with metacarpal 5. Metacarpal 5. Metacarpal 5 has a strongly developed tuberosity bulging proximolaterally from the proximal quarter of the bone and extending beyond the unciform/hamatum facet. Due to the overall shape of the metacarpal, this tuberosity is more exaggerated in relation to the shaft than in Zygomaturus or Phascolonus but does not approach the enormous equivalent in Diprotodon. There is a lunate dorsomedial facet for articulation with the fourth metacarpal. As in Ngapakaldia, the facet for the lateral process of the unciform does not extend as far laterally as it does in Vombatus. Proximal phalanges. There are well-developed pits and crests for collateral ligaments of the digital joints throughout the hand. The proximal phalanges are short, broad and flattened dorsoventrally with dished-in dorsal surfaces. Their bases are markedly asymmetrical in dorsal view, with deeply concave metacarpal sockets buttressed strongly on one side (4 and 1 laterally, 2 and 3 medially). This asymmetry is presumably to resist different prevailing forces acting on each metacarpophalangeal joint. This buttressing pattern was also noted in Nimbadon and may be indicative of similar range of motion (ROM) and loading regimes at these joints; however, the P. parvus proximal phalanges lack the palmar tuberosities of Nimbadon, so similar overall use of the manus seems unlikely. The heads are wide, each with two flared distal condyles tilted ventrally. These create joint surfaces necessitating a flexed posture for the proximal interphalangeal joints. Between these articular surfaces there are deep median incisurae to accommodate the corresponding dorsomedian processes of the intermediate phalanges. The proximal phalanx of the pollex is slightly different to the other digits, being more gracile overall, with a more compact and bulbous distal end allowing greater ROM in extension. Intermediate phalanges. The intermediate phalanges are short and squat, similar in overall proportions to those of vombatids and Diprotodon, with broader proximal than distal ends in dorsal view. In proximal view the joint surfaces approach isosceles trapezoids in shape. Their proximal articular surfaces are divided by dorsomedian processes into two deeply excavated fossae to accommodate the condyles of the preceding phalanges. These fossae are subequal in size, with 3 and 4 having slightly larger lateral surfaces. There is slight axial torsion in the orientation of the proximal and distal articular ends, especially in the fourth digit where the distal end is medially rotated relative to the proximal. The bicondylar distal articulations of these intermediate phalanges are most strongly congruent with their unguals in a flexed distal interphalangeal joint position. These condyles have a more reduced dorsal articular surface than in Nimbadon or Neohelos, indicating a reduced extension ROM. The intermediate phalanges of the manus can be distinguished from those of the pes by the shape of the proximoventral border; manual phalanges have a distinct median excavation between the condylar fossae, making a “W” shaped ventral contour. Pedal intermediate phalanges 4 and 5 are much flatter, with little to no concavity. Ungual phalanges. No individual ungual from the manus of AM F58870 is complete and undamaged, but the intact morphology can be understood in composite across the specimens available. In shape and proportion they agree with unguals of P. azael, being only slightly smaller than the smallest example from that species (see S1 Table). Across the digits the unguals differ greatly in size, with the pollex being smallest and fourth ungual the largest.

Os coxa (Figs 27 and 28). The single os coxa specimen for P. parvus is missing the iliac blade and most of the pubis and ischium (Figs 27 and 28). The remaining element appears similar to that of P. azael, with the same positioning of the ilium, ischium and pubis relative to the acetabulum in lateral view, though the acetabulum may be slightly smaller relative to its surrounding morphology in P. parvus. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 27. Left os coxa fragment of Palorchestes parvus AM F58870. (A) lateral; (B) dorsal; (C) medial; (D) ventral views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g027 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 28. Labelled illustrations of Palorchestes parvus left os coxa fragment AM F58870. (A) lateral; (B) dorsal; (C) medial views. Hatching indicates surface damage to cortical bone. Abbreviations: acf, acetabular fossa; acn, acetabular notch; aiis, anterior inferior iliac spine; aur, auricular surface; gsn, greater sciatic notch; ipe, iliopectineal eminence; obf, obturator foramen. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g028 Ischial body. The proximal-most part of the ischial body is preserved in P. parvus. It is thicker and more triangular in section than in P. azael, but its full extent and shape of the ischial tuberosity is not known. Acetabulum. The acetabulum in P. parvus is slightly more circular than in P. azael, with relatively less overhang and narrower articular surface area posterior to the acetabular notch. Anterior inferior iliac spine. The anterior inferior iliac spine is a broad, triangular scar for origin of m. rectus femoris. It is relatively wider and more deeply pitted than in P. azael and lies higher on the iliac body relative to the acetabular rim, similar to that of Neohelos though more rugose. Iliopectineal eminence. The iliopectineal eminence is more diffuse and flattened compared to that of P. azael. Auricular surface. The auricular surface in P. parvus is relatively more superoinferiorly extensive and more irregular than in P. azael, lacking the distinct facets seen in the latter. It is bordered posteriorly by a more open greater sciatic notch than in the larger species.

Femur (Figs 29, 30 and 31). The femur of P. parvus is represented by associated proximal and distal ends, missing the central femoral diaphysis (Figs 29 and 30). For this reason, the length of the intact bone can only be estimated (378 mm predicted based on ratio of proximal breadth to length in P. azael, see Fig 31). In shape and proportions it strongly resembles the P. azael femur, albeit with a broader distal epiphysis relative to proximal breadth (0.86 in P. parvus, 0.80 in P. azael). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 29. Proximal left femur of Palorchestes parvus AM F58870. (A) anterior view; (B) posterior view; (C) proximal view. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g029 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 30. Cemented fragments of distal left femur and proximal left tibia of Palorchestes parvus AM F58870. Distal left femur fragment photographed in orthogonal views with tibia fragment greyed out (A-D, I) and proximal left tibia fragment photographed in orthogonal views with femur fragment greyed out (E-H) in (A, E) anterior; (B, F) medial; (C, G) posterior; (D, H) lateral; (I) distal views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g030 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 31. Labelled illustrations of estimated reconstruction of Palorchestes parvus left femur AM F58870. (A) anterior; (B) posterior; (C) distal views. Length estimated based on Palorchestes azael femur (Fig 13). Hatching indicates surface damage to cortical bone, dashed lines indicate inferred bone contours. Abbreviations: fh, femoral head; gas, origin for m. gastrocnemius lateral head; gt, greater trochanter; lc, lateral condyle; lt, lesser trochanter; mc, medial condyle; ps, patellar surface; tf, trochanteric fossa. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g031 Femoral head. The femoral head is hemispherical, with approximately equal articular surface available anteriorly, medially and posteriorly. The head is anteriorly offset from the diaphyseal axis as in the larger P. azael, but to a slightly lesser extent. The neck is short and does not project superomedially from the proximal shaft to the degree seen in the long-necked femora of Zygomaturus or Diprotodon, or to a lesser extent Neohelos. Greater trochanter. The greater trochanter is proximally tapered and anteroposteriorly deep. In anterior view its rugose muscle attachments are superolaterally located as in P. azael, more so than the lower and more anterior trochanter of Phascolonus. Unlike the femur of P. azael, the P. parvus greater trochanter lacks the distinct swelling at the distal end where the epiphysis merges with the anterolateral femoral shaft and extends a shorter distance down the shaft. Posteriorly, the trochanteric fossa is elongate and deep as in P. azael. Lesser trochanter. The main tuber of the lesser trochanter is eroded so its full medial extent is unknown, however it appears to have been more robust and its flange more developed than P. azael, while proximodistally a little shorter. At the base of the lesser trochanter on the medial diaphysis there is a rugose area immediately proximal to the break in the shaft. Diaphysis. The bulk of the femoral diaphysis is missing, leaving only the proximal and distal portions, however from the section profiles it appears similar in shape to that of P. azael. Notable on the distolateral diaphysis is a deep and narrow, proximodistally elongate muscle scar for the lateral head of m. gastrocnemius. In contrast to P. azael, this scar is deeply inset, being similar to that of Phascolarctos in shape and proximal extent. Medial condyle. In anterior view, as in P. azael, the distal surfaces of the medial and lateral condyles sit approximately level. Viewed distally the medial condyle is more anteroposteriorly extensive than the lateral, but they are more alike here in P. parvus than in any of the other diprotodontoid femora studied. The medial condyle in this view also appears less bulbous and presents a flattened surface posteriorly, unlike the inflated and more laterally-canted condyle in P. azael. Lateral condyle. The lateral condyle is short, broad and quadrangular in distal and posterior views, very closely resembling the morphology of P. azael in all respects. Patellar surface. The patellar surface of the distal femur is anteroposteriorly and proximodistally short. The medial condyle of P. parvus projects anteriorly even less than in P. azael. Overall this creates a distal femur in stark contrast to those of other diprotodontoids and Phascolonus, again resembling extant wombat morphology most closely.

Tibia (Figs 30, 32 and 33). The tibia of P. parvus is very similar morphologically to that of P. azael, the principal difference being that for approximately the same length it is more gracile, an allometric difference expected for the smaller species (Figs 30, 32 and 33). Differences to the P. azael morphology described above are provided below. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 32. Right tibia of Palorchestes parvus NMV P159792. (A) Anterior; (B) lateral; (C) posterior; (D) medial; (E) proximal; (F) distal views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g032 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 33. Labelled illustrations of the Palorchestes parvus right tibia NMV P159792. (A) Anterior; (B) lateral; (C) posterior; (D) medial; (E) proximal views. Hatching indicates surface damage to cortical bone, dashed lines indicate inferred bone contours. Abbreviations: af, astragalar facet; ff, fibular facet; gr, insertion of m. gracilis; ie, intercondylar eminence; lc, lateral condylar surface; mc, medial condylar surface; mm, medial malleolus; pf, popliteal fossa; ptm, posterior tubercle of medial malleolus; st, insertion of m. semitendinosus; iol, interosseus line; tc, tibial crest; tt, tibial tuberosity. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g033 Proximal articulations. The medial condyle, though its posteromedial border is eroded, appears much smaller and less posteriorly extensive than in P. azael, probably representing a weightbearing allometric difference. The intercondylar eminence is substantially narrower mediolaterally in P. parvus than in P. azael. The fibular facet is preserved in both P. parvus specimens. It is distinct and expanded proximally to form a more laterally-facing circular shape than the flattened, inferiorly-directed oval facet of Vombatus, being more similar to that of Phascolarctos. This distinct facet does not resemble the more diffuse depression for the fibula in Diprotodon or Zygomaturus tibiae. This may indicate a more mobile and less weightbearing proximal tibiofibular articulation in P. parvus. The popliteal fossa on the posterior surface is more deeply furrowed than in P. azael, similar to that of vombatids. Diaphysis. In P. parvus the diaphysis is more slender overall. In mediolateral view the tibial crest has a concave profile immediately below the tibial tuberosity before flaring out again at the point of insertion for m. gracilis roughly a third of the way down the diaphysis, strongly resembling the Vombatus condition. The interosseous border is more rounded than in vombatids and diprotodontids, but not quite to the extent seen in P. azael. Distal articulations. The medial malleolus is eroded in the only specimen preserving the distal tibia, but from the available material it appears similar in proportion and extent to that of P. azael. The remaining astragalar surface on the distal tibia is much more steeply inclined and less laterally extensive than in P. azael, though similarly convex.

Pes (Figs 34 and 35). The digital posture of the pes in P. parvus appears clearly plantigrade, with the shape of the proximal interphalangeal joints indicating a habitually straightened position unlike the flexed posture in the manual equivalent (Figs 34 and 35). The second and third digits are extremely reduced relative to their robust lateral counterparts, more so than in Nimbadon, Ng. tedfordi or Thylacoleo. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 34. Associated left pes of Palorchestes parvus AM F58870. Articulated pes in (A) dorsal and (B) mediodorsal views; (C-E) digit IV ungual, intermediate and proximal phalanges in (left to right) dorsal, lateral, plantar, medial, proximal and distal views; (F) metatarsal 4 in (left to right, top to bottom) dorsal, lateral, plantar, medial, distal and proximal views; (G-H) digit 5 ungual and intermediate phalanges in dorsal, lateral, plantar, medial, proximal and distal views; (I) metatarsal 5 in (left to right, top to bottom) dorsal, lateral, plantar, medial, distal and proximal views; (J) metatarsals 2 and 3 (cemented together) in dorsal, plantar and proximal views; (K) navicular in dorsal, proximal and distal views; (L) ectocuneiform in dorsal, lateral, medial, proximal and distal views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g034 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 35. Labelled illustration of the articulated Palorchestes parvus left pes AM F58870 in dorsal view. Hatching indicates surface damage to cortical bone. Abbreviations: ect, ectocuneiform; ent, entocuneiform; Ip4-5, intermediate phalanges 4–5; mes?, possible mesocuneiform; Mt2-5, metatarsals 2–5; nav, navicular; Pp2-4, proximal phalanges 2–4; Pp5?, possible proximal phalanx 5; Up3-5, ungual phalanges 3–5. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g035 Ectocuneiform. The ectocuneiform is intermediate in form between those of Ng. tedfordi and P. azael, being dorsoventrally longer and proximodistally thicker than the former, but with relatively smaller, dorsally positioned and more circular metatarsal facets than the latter. The facet for metatarsal 3 in particular is small, commensurate with the small proximal facet of this highly reduced syndactylous digit. Proximally, the facet for the navicular is flattened and dorsoventrally elongate as in P. azael. Entocuneiform. The entocuneiform is highly eroded, with most facet edges and bony margins incomplete. Overall it is elongate, slightly inflected, and appears larger relative to the navicular than in other diprotodontoids. Distomedially the preserved surface of the facet for metatarsal 1 is quite flat, being less saddle-shaped than in Nimbadon. Navicular. The navicular is similar in mediolateral length to that of Ng. tedfordi, but is anteroposteriorly much thicker and dorsoventrally compressed, with a less concave facet for the astragalus proximolaterally. Due to its increased thickness the P. parvus navicular has a much larger convex facet surface for the ectocuneiform and mesocuneiform distally, but a similarly elongate articular surface for the entocuneiform medially. The navicular is smaller relative to its neighbouring entocuneiform than in Ng. tedfordi or Nimbadon pedes. Metatarsals 2 and 3. As is typical of diprotodontian marsupials, these elements are syndactylous and strongly reduced compared to the other highly robust pedal digits, with the third being slightly larger than the second. At their base they are triangular, with convex articular surfaces for the ecto- and mesocuneiforms. The third metatarsal has a dorsolateral facet at the base for articulation with metatarsal 4 as in Diprotodon. The diaphyses of both bones appear similar in dimensions. The distal ends are missing so lengths are unknown. Metatarsal 4. The fourth metatarsal is shorter and relatively more robust than in P. azael. The concave, triangular cuboid facet is transected horizontally by a shallow sulcus. A facet for the ectocuneiform extends mediodistally from the medial edge of the cuboid facet and is smaller and more dorsally positioned than in P. azael. The mediodistal tip of this ectocuneiform facet projects from the medial shaft and on the ventral surface of this projection lies a small triangular facet for articulation with the dorsolateral facet of metatarsal 3. Viewed distally, the dorsalmost articular surface head is laterally canted, with slight reduction of the lateral keel as in the fifth metatarsal. Metatarsal 5. The fifth metatarsal is slightly shorter than the fourth when measured between the base and head. But overall the fifth is longer owing to the very large lateral tuberosity that tapers proximally beyond the articular surface for the cuboid. This lateral tuberosity is similar in shape and proportion to that of Phascolonus, being a continual subtriangular flange originating at the distal end of the metatarsal. This is distinct from the more bulbous equivalent in diprotodontids and extant wombats. The lateral tuberosity lacks the dorsal uptick of the flattened Ngapakaldia equivalent. This tuberosity would have provided attachment for the m. peroneus brevis proximally, with another smaller tubercle distolaterally for the m. abductor digiti V. The smooth triangular cuboid facet on the base curves medially to provide articulation with the matching contour of the neighbouring fourth metatarsal. In ventral aspect the central keel of the head is laterally inclined with the lateral keel reduced, in contrast to the enlarged lateral keel described for Nimbadon, or subequal lateral keel in Ngapakaldia. Though we lack a proximal phalanx for this digit this morphology may indicate an abducted posture for the P. parvus fifth digit, and along with its lateral tuberosity shows high lateral loading of the pes. Phalanges. The proximal interphalangeal joints of the pes are less restricted in extension due to slightly smaller dorsomedian crests. They were able to extend to a straightened position unlike the flexed manual equivalent. When viewed mediolaterally, their proximal articular fossae are less tightly concave and more open than the intermediate phalanges of the manus. In distal view, the condyles are more flared ventrally to create a wider sub-articular angle than the steep, more vertically-oriented distal condyles in the manus. In proximal view, the intermediate phalanges are slightly asymmetrical, with broader lateral facets. Unguals. The non-syndactylous pedal digits 4 and 5 have unguals of very similar morphology to those of the manus. The dorsoventral extent of the condylar fossae and flattened plantar areas on the flexor tubercles indicate these pedal unguals were habitually held in maximal extension with their straight dorsal border parallel to the ground. Presumably this was to accommodate an even more ventrally recurved keratin claw sheath. The much smaller ungual for digit 3 is poorly preserved but has the same twin lunate condylar fossae which are slightly broader ventrally than dorsally and separated by a median keel, as those seen in other palorchestid unguals. Ventrally, this ungual lacks the plantar flexor tubercle seen in the manual and large pedal unguals, likely reflecting its reduced weightbearing role and weaker flexor tendon. The ungual process is broken off and not preserved. Based on this associated specimen, the pedal unguals of digits 4 and 5 in P. parvus appear to have a dorsoventrally deeper flexor tubercle, with more deeply concave articular facets lying more dorsally on the bone than their manual equivalents. Propalorchestes sp. Murray 1986

Referred material. Measurements for all referred material below are provided in S1 Table. All specimens were collected by T. H. Rich from Top Site, Bullock Creek, NT. As two Propalorchestes species are known from the Bullock Creek local fauna, and no postcrania are yet assigned to either, we refer the following elements to Propalorchestes sp. on the basis of their morphological distinctiveness from the other diprotodontoids which are well known from this locality, Neohelos spp. The articular surfaces of the unassociated humerus and ulna agree in shape but are from individuals of slightly different sizes. NTM P87115-6. Left humerus (intact but for damage to the lateral margin of the lateral supracondylar crest). Photographed in Vickers-Rich et al. [72], pg. 170, Fig 232. NMV P253947. Right ulna (proximal half, some damage to radial facet). NMV P179370. Ungual phalanx.

Humerus (Figs 36 and 37). The Propalorchestes humerus is a robust bone with a straight diaphysis and broad epiphyses, particularly the distal portion which is both laterally and medially expanded. The shaft has pronounced flanges anteriorly and laterally, for deltopectoral and wrist extensor muscles respectively (Figs 36 and 37). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 36. Left humerus of Propalorchestes sp. NTM P87115-6. (A) anterior; (B) lateral; (C) posterior; (D) medial; (E) proximal; (F) distal views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g036 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 37. Labelled illustrations of the Propalorchestes sp. left humerus NTM P87115-6. (A) anterior; (B) lateral; (C) posterior; (D) medial views. Hatching indicates surface damage to cortical bone, dashed lines indicate inferred bone contours. Abbreviations: bg, bicipital groove; brf, fossa for m. brachialis origin; ca, capitulum; del, deltoid insertion; gt, greater tubercle; hh, humeral head; inf, fossa for insertion of m. infraspinatus; ldtm, insertion for mm. latissimus dorsi and teres major; le, lateral epicondyle; lsc, lateral supracondylar crest; lt, lesser tubercle; me, medial epicondyle; of, olecranon fossa; pec, pectoral crest; rf, radial fossa; subf, fossa for insertion of m. subscapularis; supf, fossa for insertion of m. supraspinatus; sf, supracondylar foramen; tr, trochlea; tri, origin for humeral heads of m. triceps brachii. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g037 Overall the humerus of Propalorchestes bears the strongest resemblance to that of Ngapakaldia, though it is larger than that of the latter (see S1 Table and S1 Fig). Head. The humeral head is sub-hemispherical, with a posteriorly offset position creating the distinct ‘beak’ in medial view present in all vombatiforms. The head is slightly broader mediolaterally than anteroposteriorly, but not to the same degree seen in P. azael or the vombatid species and totally unlike the flattened, laterally-expanded head of Neohelos. Greater tubercle. The greater tubercle in Propalorchestes projects proximally beyond the humeral head as in other palorchestids, unlike the in zygomaturine or diprotodontine species where it is subequal in height to the humeral head. The tubercle has two distinct attachment scars; a small fossa for m. supraspinatus presenting a flat surface superoanteriorly, and a larger fossa for m. infraspinatus with a directly lateral orientation, similar to that seen in other palorchestids. In lateral view the greater tubercle occupies two thirds of the anteroposterior extent of the proximal humerus, like other palorchestids. In proximal view the greater tubercle in Propalorchestes lies more anteriorly on the epiphysis than the laterally-positioned equivalent in P. azael. The tubercle overhangs medially to create a deep notch at the proximal part of the intertubercular sulcus, as seen in P. parvus. This contrasts with the shallow groove here seen in diprotodontines and vombatids. Lesser tubercle. In Propalorchestes the lesser tubercle presents a flattened anterior surface, with margins slightly pointed proximally and rounded medially. This medial margin projects slightly more in Propalorchestes than in the other diprotodontoids, though not as much as in vombatids. In proximal view the tubercle is larger relative to the humeral head and more anteriorly situated than in other palorchestids, most closely resembling the extant wombat species in this respect. In medial view the well-defined insertion scan for m. subscapularis is obliquely oriented, descending posteriorly at an angle ~25° from vertical. This is similar to the orientation seen in P. parvus, steeper than in P. azael (~40°) and vombatids (~30°) and much steeper than in diprotodontids (> 55°). Pectoral crest. Descending from the broad greater tubercle, the pectoral attachment in Propalorchestes narrows to a tall crest which runs sub-vertically down the approximate midline of the anterior humeral shaft. In medial view the crest is a broad-based isosceles triangle, reaching its apex slightly proximal to the midpoint of the humeral shaft. This makes it the shortest pectoral crest among the vombatiforms relative to total humeral length. Viewed anteriorly, its medial edge curls slightly over the bicipital sulcus, but to a far lesser extent than in other palorchestids. The apex of the crest terminates in a bulbous protuberance resembling that of P. parvus. However, the deltoid insertion does not converge directly on this protuberance as in the latter species. Instead, the insertion for the deltoid muscle is a poorly-defined curved scar. This scar is bent anteromedially from a site beneath the greater tubercle to merge with the pectoral crest approximately halfway down its length (a quarter of the way down the shaft). This is distinct from the morphology in P. parvus and vombatids where the deltoid insertion ridge is discrete and runs parallel lateral to the pectoral crest before converging on it distally. It also differs from small diprotodontids like Neohelos in which the deltoid insertion scar is a weakly defined vertical facet that merges with the distal apex of the pectoral crest. Tuberosity for mm. teres major and latissimus dorsi. In Propalorchestes the attachment scar for the mm. latissimus dorsi and teres major is a sharply defined ovoid on the lateral shaft just distal to its midpoint. Like other palorchestids, its posterior margin is recurved and overhangs the posterior surface of the diaphysis. Two distinct scars of subequal size are discernible within this ovoid in the Propalorchestes humerus; an anteroproximal, slightly bulging scar, and a posterodistal rugose portion. Diaphysis. The shaft of the Propalorchestes humerus appears straight in anterior and medial views. The posterior surface of the shaft is not strongly scarred, though fossae for the humeral heads of m. triceps brachii and m. brachialis are discernible, and a deeply excavated olecranon fossa is present distally. This olecranon fossa is much deeper than in P. parvus and instead resembles the ulnae of Ngapakaldia species in form and positioning. As in P. parvus, the Propalorchestes ulna lacks the strong origin scar for m. epitrochleoanconeus seen in P. azael. In overall section the shaft is a laterally-expanded oval, distorted anteriorly by the large pectoral crest and distally by the sheet-like lateral epicondylar crest arising on its distolateral margin. Unlike in Palorchestes species, a deep radial fossa lies proximal to the capitulum on the anterodistal shaft surface. Lateral epicondyle. The lateral epicondyle only barely projects from the lateral edge of the capitulum in Propalorchestes, making it the least developed of the palorchestids in this respect. In lateral view it tapers proximally into the supinator crest almost immediately. Its surface is strongly rugose, with sharply defined scarring present on all sides and a distinct fossa for the attachment of the lateral collateral ligament. In distal view, the lateral epicondyle is slightly posteriorly deflected from the transverse axis of the distal epiphysis, unlike the aligned, undeflected condition in Palorchestes. Trochlea. In Propalorchestes the humeral trochlea is a domed shape presenting slightly more articular surface on the anterior side than posteriorly. It is more convex than in P. parvus and much more so than the flattened trochlea of P. azael. The trochlea and capitulum, though subequal in size, are distinct and separate from one another. A ~115° angle separates the two in anterior view, and there is a narrow constriction dividing their dilated surfaces when viewed inferiorly. This is similar for most vombatiform humeri except for Neohelos in which this constriction is much thicker and a much shallower angle (> 150°) separates the structures in anterior view. Viewed inferiorly the Propalorchestes trochlea is ovoid, with its long axis slightly anteriorly deflected to lie offset from the transverse axis of the medial epicondyle as in other palorchestids. This is distinct from vombatid and Diprotodon humeri, where the medial epicondyle is posteriorly deflected from the trochlear axis, and from Ngapakaldia, where the trochlea is aligned with the medial epicondyle. On the posterior aspect the trochlear surface is barely visible, while a deeply excavated triangular olecranon fossa. Capitulum. The Propalorchestes capitulum is large and hemispherical in anterior view, unusual amongst vombatiforms in its marked distal offset from the transverse plane of the distal articular surface, giving the humerus a slightly lopsided appearance. In lateral view the anteroposterior extent of the capitulum is shallower (as in all palorchestids) than the proportionally deep capitula of other vombatiforms. Viewed distally, the Propalorchestes capitulum tapers slightly to form a small capitular tail, but not to the extent of the pronounced tail seen in Ngapakaldia where it forms a lateral buttress for the olecranon process when the elbow is fully extended. Such buttressing is weak in the Propalorchestes humerus, which in posterior view shows only a small amount of capitular surface compared with other palorchestids and vombatiforms generally. Supracondylar foramen. The supracondylar foramen is a well-defined ovoid canal. In orientation it more closely resembles the arrangement in P. parvus than the near-vertical foramen in P. azael. Medial epicondyle. The medial epicondyle in Propalorchestes is a rounded, medially-projecting rugose process. It is greatly expanded to create a broad distal epiphysis. In distal view the medial epicondyle accounts for 29% of the total distal epiphyseal width, comparing closely to P. parvus (29%), Ngapakaldia (25%) and extant wombat (27%) humeri, but not quite approaching the proportions in P. azael (32%). The medial epicondyle lies in the same dorsal (coronal) plane as the lateral supracondylar crest.

Ulna (Figs 38 and 39). The Propalorchestes ulna strongly resembles that of Vombatus in proportions and morphology but is approximately 50% larger, similar in overall size to those of Thylacoleo and Ng. bonythoni, albeit with a much longer olecranon relative to the humeral articular surface than either of the latter (Figs 38 and 39). The ulna NMV P253947 is missing its distal half, so the total length and distal epiphyseal morphology are not known (though we approximated the length to be 276 mm based on Vombatus proportions). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 38. Right ulna fragment of Propalorchestes sp. NMV P253947. (A) anterior; (B) medial; (C) posterior; (D) lateral views. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g038 PPT PowerPoint slide

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larger image TIFF original image Download: Fig 39. Labelled illustrations of the Propalorchestes sp. right ulna fragment NMV P253947. (A) anterior; (B) medial; (C) lateral views. Hatching indicates surface damage to cortical bone. Abbreviations: ap, anconeal process; apl, origin for m. abductor pollicis longus; cf, capitular facet; cp, coronoid process; edp, origin for m. extensor digitorum profundus; fdp, fossa for origin of m. flexor digitorum profundus; op, olecranon process; rn, radial notch; tn, trochlear notch; ut, ulnar tuberosity. Scale bar 50 mm. https://doi.org/10.1371/journal.pone.0221824.g039 Olecranon. The olecranon process is elongate, medially deflected and proximally enlarged, nearly identical to Vombatus in all respects bar absolute size. It is less mediolaterally thick and proximally bulbous than in Ng. bonythoni. In medial view, the ventral border of the olecranon tapers slightly towards the bulbous proximal tip, again similar to that of Vombatus and quite unlike the more regular, quadrangular olecranon in P. azael. Trochlear surface. The Propalorchestes ulna has a scooped, subcircular trochlear surface. It resembles Vombatus, Ng. bonythoni and P. parvus trochleae more than the elongate, narrow and flat surface in P. azael, but is relatively broader than in any of the former taxa. Coronoid process. The coronoid process appears relatively shorter dorsoventrally than in Vombatus; however this is due to the relative increase in dorsoventral depth of the Propalorchestes shaft immediately distal to it, likely for increased load bearing. Anconeal process. The anconeal process is tall and in lateral view rises roughly perpendicular to the longitudinal axis of the ulnar shaft, though not as tall or proximally curled as in wombats. This curled anconeal process in wombats articulates with their pronounced capitular tail on the humerus, a feature lacking in palorchestid humeri. The anconeal process is less laterally deflected in the transverse plane than in P. azael or Ng. bonythoni, instead resembling the P. parvus condition. Radial notch. The radial notch sits in a ventral position on the lateral shaft, resembling P. parvus, Ng. bonythoni and Vombatus ulnae in this regard rather than the dorsal positioning of the notch in P. azael. Its perimeter is eroded in NMV P253947, but the preserved morphology indicates a wide concave platform for the head of the radius, unlike the proximo-distally narrow, curved notch in extant wombats. Ulnar tuberosity. In Propalorchestes, the ulnar tuberosity takes the form of a raised subtriangular area of rugosity as in wombats and Ng. bonythoni, rather than a distinct tubercle as seen in Palorchestes species. However, it does resemble the morphology of Palorchestes in its position on the ventrolateral border of the shaft, well distal of the coronoid process and radial notch unlike the condition in wombats. Diaphysis. In lateral view, the ulnar shaft has a posteriorly convex dorsal border. Based on the uptick in the ventral contour immediately proximal to the break, the bone may have had the gently sinuating shaft contour of extant wombats rather than the gradual ventral concavity seen in P. azael. The attachment scar for m. abductor pollicis longus is strongly expressed and lies both more dorsally on the lateral shaft and arises more distally than in Vombatus (but not as distal as in P. azael). The attachment scar for m. extensor digitorum profundus is less rugose than in Vombatus and is more dorsal on the lateral shaft, as in P. azael.