Pygostyle. The fully fused pygostyle shows several features typically characteristic of enantiornithines ( Wang et al., 2017 ). The proximal end bears a craniodorsal fork formed by the prezygapophyses of the first fused vertebra. In dorsal view, these processes define a deep U-shaped concavity ( Fig. 3 ), whereas they form a V-shaped incisure in Halimornis . The cranial fork is continuous with the dorsolateral margins. The dorsal surface is gently concave and wider than the ventral surface. Ventrally the pygostyle bears a prominent pair of laminar ventrolateral processes, which extend 80% the length of the pygostyle and taper distally without the pronounced constriction present in some taxa (e.g., Halimornis , Longipteryx ; Zhang et al., 2001 ) or medial invagination present in the caudal margin of the pygostyle in pengornithids ( Wang et al., 2017 ).

Dorsal vertebrae. Two dorsal vertebrae were found; they are amphicoelous with slightly concave articular surfaces that are much larger than the vertebral foramen ( Figs. 2F – 2J ). The vertebrae are spool-shaped with deep grooves excavating the lateral surfaces. In one vertebra, a lateral groove appears to be perforated by a foramen in the cranial portion, but this may be a preservational artifact. The ventral surface is not keeled as in Elsornis and some El Brete specimens ( Chiappe et al., 2007 ). The parapophyses are centrally located, as in other enantiornithines ( Chiappe & Walker, 2002 ). The spinous process is as dorsoventrally tall as the centrum, narrowest at its base, and slightly displaced caudally (not centered on the centrum), typical of enantiornithines ( Chiappe & Walker, 2002 ). The transverse processes are not preserved.

The two post-axial cranial cervical vertebrae are substantially longer than the axis (approximately 1.5 times its length). The prezygapophyses are flat (epipophyses absent), cranioventral-caudodorsally oriented, and sub-lachriform (tapered dorsally with a straight medial margin and a convex lateral margin). A low but even neural spine extends nearly the entire length of the centrum. The ventral surface of the centrum also appears to form a low keel. The caudal articular surface of the vertebra is dorsoventrally concave and mediolaterally convex.

Cervical vertebrae. Three cervical vertebrae are preserved, including the axis ( Figs. 2A – 2E ). The peg-like dens projects dorsal to the atlantean articular facet; its craniocaudal length is 1.5 times its mediolateral width. The articular facets of the postzygapophyses are oriented ventrally with slight lateral deflection and are medially continuous with each other through a thin shelf of bone that overhangs the vertebral foramen. The epipophyses are strongly developed but do not extend caudally beyond the caudal margin of the postzygapophyses. The caudal articular surface of the axis appears weakly heterocoelic.

Scapula. The shaft of the left scapula is present, but the proximal and the caudal extremities are not preserved ( Fig. 6 ). The scapular blade is straight in mediolateral view. The cranial half of the costal surface is excavated by a shallow fossa defined by a thickening of the dorsal margin of the blade, a morphology also observed in Elsornis , Halimornis , and Neuquenornis ( Chiappe, Lamb & Ericson, 2002 ; Chiappe & Dyke 2006 ; Chiappe & Calvo, 1994 ). The caudal half of the lateral surface is also excavated by a shallow, elongate fossa, as in Halimornis . Although obfuscated by breakage, the preserved portion of the scapular blade weakly tapers distally.

Sternum. Only the xiphoid process of the sternum was recovered ( Fig. 5 ). The lateral margins are straight in dorsal and ventral view as in most enantiornithines, whereas the xiphial margin demarcates a wide V-shape (xiphoid process absent) in primitive enantiornithines (e.g., Protopteryx , Pengornithidae) ( Hu, Zhou & O’Connor, 2014 ). The dorsal surface is weakly concave. Ventrally, the narrow process bears a well-developed keel, similar to that in preserved in Neuquenornis , that decreases in height caudally. In contrast, the keel of Elsornis and Early Cretaceous enantiornithines is poorly developed along the xiphoid process ( O’Connor, 2009 ). The posterior end is weakly flexed caudodorsally and terminates in a small knob not observed in other enantiornithines.

Furcula. The furcula is nearly-complete and well preserved, with only moderate mediolateral crushing ( Fig. 4 ). It is Y-shaped with an interclavicular angle of approximately 40°, less than observed in many Early Cretaceous taxa ( Wang et al., 2014 ). The omal halves of the clavicular rami are subparallel, whereas they typically are more widely splayed in Early Cretaceous taxa. Although the narrow interclavicular angle in this specimen may be somewhat exaggerated by crushing, this morphology appears comparable with the South American avisaurid Neuquenornis . Another similarity between these two taxa is a short hypocleidium (though we note that this structure may be incomplete in Neuquenornis ), measuring less than 1/4 the length of the clavicular rami in Mirarce , compared to half the length or more in many Early Cretaceous enantiornithines ( Wang et al., 2014 ). The omal tips of the furcular rami are weakly expanded before they abruptly truncate. The omal margin is concave, presumably forming a facet for articulation with the coracoid, and oriented perpendicular to the long axis of the rami.

Manual phalanges. The only identifiable manual element is the first phalanx of the major digit ( Figs. 10E – 10H ). The cranial margin is flat and wide, and the caudal margin is keeled forming a triangular cross section. As in other enantiornithines, the phalanx lacks the caudal expansion and dorsoventral compression that is present in ornithuromorphs ( O’Connor, Chiappe & Bell, 2011 ). Breakages reveal large, pneumatic chambers in interior of the phalanx.

Carpometacarpus. A small fragment is identified as the carpal trochlea of the right carpometacarpus reveals asymmetry in the carpal trochlea, as in some living birds (e.g., Phalacrocorax , Lagopus , Gallus ), with the dorsal condyle projecting farther ( Figs. 10A – 10D ). The fragment strongly suggests the carpometacarpus was fully fused at the proximal end, as in all adult enantiornithines. Another fragment is interpreted as the distal end of metatarsal II; the distal articular surface for the first phalanx is heart shaped. This fragment shows no signs of even partial fusion to the minor metacarpal, as in Neuquenornis and all other known enantiornithines ( Chiappe & Walker, 2002 ).

Radius. Proximal and distal ends of a right radius are preserved, along with fragments of the shaft that indicate that an interosseous groove, like that observed in some other enantiornithines (e.g., Enantiornis ) was absent ( Chiappe & Walker, 2002 ). Proximally the circular humeral cotyla is concave. The distal fragment of the right radius preserves the radiocarpal and ulnar articular surfaces oriented at a 90° angle and separated by a small, distally-projecting tubercle. The radiocarpal articular face is visible, forming a bluntly triangular facet on the dorsal surface of the radius; it covers the entire distal margin and even has a small extension on the ventral surface.

Ulna. The right ulna is preserved as a mineral cast of the endosteal cavity with only small fragments of cortex present. Together, these indicate that the bone was slightly bowed, as in other enantiornithines and most basal birds. Distally, the semilunate ridge (external condylar ridge) is strongly developed. Two rugose patches preserved on the caudal margin of the shaft are interpreted as quill knobs (remige papillae) ( Fig. 9 ). These prominent rugosities are elongated in parallel to the long axis of the bone. Although breakage of the fossil makes it impossible to determine the length of each papilla, to measure the spacing between them, or to estimate the number of secondary feathers, recognition of these structures for the first time in an enantiornithine is highly significant.

The humeral shaft is bowed. The distal end is craniocaudally compressed and mediolaterally expanded, as in other Late Cretaceous enantiornithines (e.g., Martinavis , Enantiornis ) ( Chiappe, 1996 ; Chiappe & Walker, 2002 ). The condyles are slightly staggered such that the dorsal condyle is proximal to the ventral condyle and a deep intercondylar incisure cuts between them. This incisure is weakly angled proximoventral-distodorsally and is parallel to the long axis of the dorsal condyle. The ventral condyle is transversely oriented, as in other enantiornithines ( Chiappe & Walker, 2002 ). A large flexor process is present, projecting distally such that the distal margin is angled as in some other enantiornithines (e.g., Alexornis ) ( Chiappe & Walker, 2002 ). The olecranon fossa is present only as a shallow incision between the flexor process and condyles. Tricipital grooves are absent.

Only the fused portions of the pelvic elements contributing to the left acetabulum were recovered ( Fig. 11 ). The antitrochanter is small, triangular, and laterally oriented. It is positioned on the caudodorsal margin of the acetabulum as in other enantiornithines ( Chiappe & Walker, 2002 ). The preserved portion of the ilium indicates a dorsal antitrochanter was present along the dorsal margin forming a tubercle-like expansion of the laterodorsal iliac crest located just over the antitrochanter, as in some other enantiornithines (e.g., Sinornis ; Sereno & Chenggang, 1992 ; PVL4042; Chiappe & Walker, 2002 ). Only the ventral half of the proximal pubis is present; at least along this portion, the pubic shaft was not mediolaterally compressed. The large, circular acetabulum appears to be fully perforated, whereas it is partially occluded in the Early Cretaceous Qiliania graffini ( Ji et al., 2011 ).

Pelvic limb

Femur. The right femur is complete and well preserved (Fig. 12). Fragments of the left femur were also found, comprising the proximal end (minus the head) and the medial half of the distal end. The femur is long, almost equal to the humerus in length and nearly twice the length of the tarsometatarsus. The femoral shaft is bowed cranially, as in most Early Cretaceous enantiornithines and Martinavis; this curvature is more pronounced in the distal half of the element, as in Martinavis (Chiappe & Walker, 2002; Walker & Dyke, 2009). The femoral neck is relatively distinct and elongate, similar to other Late Cretaceous enantiornithines (e.g., Martinavis), and projects slightly dorsally at a proximomedial angle (whereas it projects laterally in PVL 4037; Chiappe & Walker, 2002). A fossa for the capital ligament is not present on the femoral head (though present in femora from El Brete, for example, PVL 4060, PVL 4037), but there is a distinct flattening and rugosity where the ligament would have presumably attached. The trochanteric crest is robust and thick, although it thins caudally. It projects proximally slightly less than the femoral head and is angled craniolaterally—caudomedially. On the cranial surface, it is laterally angled to form the craniolateral margin of the femur; as it diminishes along this surface, it becomes continuous with an intermuscular line (potentially of the mm. femorotibialis lateralis and femorotibialis intermedius) (Baumel et al., 1993). This line angles medially and extends midway down the bone before splitting near the center of the cranial surface and diminishing away a quarter length from the distal margin. In two Hungarian enantiornithine femora (MTM PAL 2011.20, -.21; Ösi, 2008) this intermuscular line is not split. The lateral surface of the femur is excavated caudolaterally by a deep posterior trochanter, as in other enantiornithines. Inside the excavation lies a delicate, cranially convex, semilunate muscle scar. The excavation of the posterior trochanter forms a laterally oriented boney shelf, as in other enantiornithines. This shelf formed by the posterior trochanter is craniocaudally convex (similar to PVL-4060) and opens on the caudal surface (Chiappe & Walker, 2002). A unique feature present on both femora is a small, deep, circular pit located just craniolateral to the posterior trochanter (Fig. 12C). This may represent the point of insertion for major hip flexors, such as the mm. iliotrochantericus cranialis and medius (Mosto, Carril & Picasso, 2013).

Figure 12: Right femur. (A) Cranial view. (B) Caudal view. (C) Medial view. (D) Lateral view. (E) Proximal view. (F) Distal view. Abbreviations: cil, cranial intermuscular line intermuscular line; lc, lateral condyle; lgt, lateral gastrocnemial tubercle; lil, lateral intermuscular line; iicm, insertion of m. iliotrochantericus cranialis and medius; mc, medial condyle; ofm, origin of m. femorotibialis medialis; pt, posterior trochanter. Scale bar equals one cm. Photos: David Strauss. Illustrations: Gregory C. Arena.

The medial margin of the shaft just distal to the head bears a small, triangular muscle scar followed distally by a much larger proximodistally elongate oval not observed in any other known enantiornithine (Fig. 12). In modern birds, the m. femorotibialis medialis has an elongate point of origin along the medial surface of the femur (Mosto, Carril & Picasso, 2013). The more prominent muscle scar present in Mirarce may possibly be analogous, suggesting a short and wide origin for the m. femorotibialis medialis.

In caudal view, three distinct intermuscular lines are present (Fig. 12B). The first originates on the medial surface near the proximal end of the large muscle scar, curving around to the caudal surface at an angle, ending in a large oval rugosity (forming the more proximal rugosity). Just distal to this landmark, another intermuscular line runs parallel to the long axis of the femoral shaft, almost on the caudomedial margin of the shaft; distally it is truncated by a distinct, rugose, ridge-like muscle attachment located a quarter length from the distal end (forming the more distal rugosity). The third line, located on the caudolateral margin of the bone, extends from the posterior trochanter distally. Near its distal end, this intermuscular line is interrupted by a small, raised, lachriform muscle scar, possibly the lateral gastrocnemial tubercle (Baumel et al., 1993). A fourth, faint intermuscular line parallels the middle section of the laterocaudal muscular line; these two intermuscular lines define a thick strip of rugose bone that extends from the proximal rugosity to the distal rugosity, likely representing a major site of muscle attachment. A lateral intermuscular line is also reported in an indeterminant enantiornithine from the Late Cretaceous of Madagascar (FMNH PA 752; O’Connor & Forster, 2010) and MTM V.2002.05 from Hungary, although in these specimens the lines do not terminate in a scar for muscle attachment.

Distally, the region between the condyles has been moderately crushed, but an intercondylar sulcus does not appear to have extended onto the cranial surface of the femur. On the caudal surface, a shallow popliteal fossa is present. A weak circular impression possibly for the origin of the cranial cruciate ligament is observed between the two condyles (Baumel et al., 1993). The medial condyle is much larger than the lateral condyle, as in living birds and other enantiornithines (Chiappe & Walker, 2002). The medial condyle is broad, rounded, and tapered medially, while the lateral condyle is more ridge-like and mediolaterally compressed. A prominent lateral epicondyle is present, as well as a small impression for the attachment of the collateral ligament, but the fibular trochlea is poorly developed as in other enantiornithines (O’Connor, 2009). A caudally-projecting lateral flange like that present in some enantiornithines (e.g., Enantiornis, Neuquenornis, Concornis) (Chiappe & Walker, 2002) is also absent, although the lateral condyle does project further caudally and distally relative to the medial condyle. The medial surface of the medial condyle bears a deep circular excavation also present in Martinavis (PVL 4036).

Tibiotarsus. Only the proximal and distal ends of the right tibiotarsus were recovered (Fig. 13). The proximal articular surface is weakly convex with a low tubercle developed in one area, similar to observations of Soroavisaurus (Chiappe, 1993). Distally the medial condyle is much larger than the lateral condyle, following the plesiomorphic state for enantiornithines (Chiappe & Walker, 2002). As in other enantiornithines, the two condyles contact, whereas they are separated by an intercondylar incisure in most ornithuromorphs. The medial and lateral surfaces of the condyles are both excavated by a deep circular pit present in some other well-preserved enantiornithines (e.g., Qiliania; Ji et al., 2011). On the lateral side, the pit is closed caudodorsally by a small tubercle, similar to PVL 4021, 4027 (Chiappe & Walker, 2002).

Figure 13: Preserved fragments of right tibiotarsus. (A) Proximal view. (B) Lateral view. (C) Cranial views. Abbreviations: dt, distal tubercle; lc, lateral condyle; lte, lateral trochlear excavation; mc, medial condyle; pt, proximal tubercle. Scale bar equals one cm. Photos: David Strauss. Illustrations: Gregory C. Arena.

Tarsometatarsus. The left tarsometatarsus is complete (Fig. 14); the right tarsometatarsus is missing the distal portion. The tarsometatarsus is proportionately short and wide, similar to other avisaurids (Fig. 15). The proximal and distal ends of the metatarsals are approximately the same width, and show no distal expansion, as in Yungavolucris brevipedalis, or distal narrowing, as in Lectavis bretincola (Chiappe, 1993). The metatarsals are proximally fused to the distal tarsals and to each other but are otherwise unfused throughout their lengths. In proximal view, the slightly concave medial cotyla is much wider than the flatter lateral cotyla, consistent with the widths of the tibiotarsal condyles. The two cotylae are separated by a weak convexity that continues onto the surface of the lateral cotyle—like other enantiornithines an intercotylar eminence is absent. The entire proximal articular surface is weakly angled, similar to A. archibaldi but lacking the extreme tilt observed in “A. gloriae” (Fig. 15). The proximal articular surface is expanded to form a circumferential labum that overhangs the shaft of the tarsometatarsus. The labum is thickest on the plantar surface in the location of the neornithine hypotarsus, similar to the condition in basal ornithuromorphs. The plantar surface of the labum may represent the origin of the m. extensor hallucis longus, or may have supported a cartilaginous hypotarsus (Jiang et al., 2017). The proximocranial margin of the medial cotyla slopes mediodistally. The center of the proximocaudal margin is slightly concave, also as in A. archibaldi, and bears a small, dorsally-directed tubercle level with the intercotylar contact. In proximal view, the lateral cotyle tapers laterally and bears a minute, proximally-directed tubercle on the lateral margin, again also present in A. archibaldi.

Figure 14: Left tarsometatarsus. (A) Dorsal view. (B) Plantar view. (C) Medial view. (D) Lateral. (E) Proximal view. (F) Distal view. Abbreviations: lc, lateral cotyle; mc, medial cotyle; mtIa?, metatarsal I articulation?; mtII, metatarsal II; mtIII, metatarsal III; mtIV, metatarsal IV; pl, proximal labum; std, supratrochlear depression; tct, tibialis cranialis tubercle. Scale bar equals one cm. Photos: David Strauss. Illustrations: Gregory C. Arena.

Figure 15: Comparison of avisaurid tarsometatarsi showing variation in size of the element and location of the tibialis cranialis tubercle, among other morphological variations. (A) Avisaurus archibaldi. (B) Mirarce eatoni. (C) Gettyia gloriae. (D) Bauxitornis mindszentyae. (E) Sauroavisaurus australis. Abbreviations: tct, tibialis cranialis tubercle. Scale bar equals one cm. Illustrations: Gregory C. Arena.

As in most other enantiornithines, the metatarsals are aligned in a single dorsoplantar plane (Fig. 14). Metatarsals II and IV are straight throughout their lengths as in A. archibaldi, but differing from the more curved metatarsals of “A. gloriae.” No vascular foramina are observed. As in other avisaurids, metatarsals II–IV are subequal in width; metatarsal IV is only marginally thinner than metatarsals II and III in dorsal view, although it is much more delicate in lateral view where a dorsoplantar compression is observable (especially at the midshaft). The long axis of the cross-section is dorsomedial-lateroplantarly oriented such that the lateral margin of metatarsal IV forms a weak plantar crest, laterally defining the excavated plantar surface of the tarsometatarsus. The dorsal surface of metatarsal III is strongly convex, as in other avisaurids; the dorsal surfaces of metatarsals II and IV are nearly flat. The well-developed tubercle for attachment of the m. tibialis cranialis on metatarsal II is located nearly at the mid-point of the element, intermediate between the position in A. archibaldi and “A. gloriae.” As in all avisaurids, the tubercle is located on the dorsomedial margin of metatarsal II, whereas it is more laterally located in some enantiornithines and most ornithuromorphs (O’Connor, 2009). An elongate, slightly raised, flat, oval surface is present on the medial edge of the plantar surface of metatarsal II, continuous with a weak medial plantar crest; a similar feature is present is present in the holotype of A. archibaldi, but in this taxon is more elongate and proximally located. This feature probably represents the attachment site of a muscle, such as the m. abductor digiti II that originates on this region of metatarsal II in modern birds (Venden Berge & Zweers, 1993), or a strong ligament, such as the medial plantar ligament of the tarsometatarsus, which also occupies a similar position in modern taxa. This surface may alternatively represent the articular surface for metatarsal I, which is not demarcated in other enantiornithines (O’Connor, 2009). However, in other avisaurids (Nequenornis and Soroavisaurus) metatarsal I articulates on the medial surface of metatarsal II, and no enantiornithine preserves a distinct surface on metatarsal II to indicate the articulation with metatarsal I. This fossa in living birds is slightly concave whereas the facet in A. archibaldi and UCMP 139500 is flat. The length of the oval facet is also inconsistent with its identification as the metatarsal I fossa; metatarsal I is proportionately shorter in most enantiornithines (except the basal Pengornithidae). A similar scar is also present in some dromaeosaurids; although early interpretations considered this to be the metatarsal I fossa, this feature has since been reinterpreted as the possible origin of the digits I and II flexor and abductor tendons (Norell, Makovicky & Mongolian-American Museum Paleontological Project, 1999). Therefore, we favor interpretation of this facet as the attachment point of a ligament potentially associated with the hallucal joint.

A deep and narrow medial intertrochlear incisure extends the distal third of the tarsometatarsus, along with a much smaller lateral intertrochlear incisure (Fig. 14). In distal view, the metatarsal trochleae are nearly coplanar, with II and IV slightly angled toward metatarsal III. The distal end of metatarsal II is slightly deflected and expanded medially as in A. archibaldi and “A. gloriae.” The trochlea of metatarsal II is ginglymous; there is no collateral ligament fovea on the medial surface of the trochlea, although there is a slight tubercle in this region where the ligament could have inserted. This fovea is well developed on the lateral surface of the metatarsal II trochlea and the lateral and medial surfaces of metatarsal III, and weakly developed on the lateral surface of metatarsal IV (is absent in A. archibaldi). The trochlea of metatarsal II is subequal in width to that of metatarsal III but the distal margin is slightly angled such that the distal margin of the medial condyle is proximal to the lateral condyle. The trochlea of metatarsal III is ginglymous; the dorsal surface bears a small, pit-like dorsal trochlear depression continuous with an intercondylar sulcus, a morphology not developed in A. archibaldi. In dorsal view, the medial condyle is slightly larger than the lateral condyle. In distal view, the medial condyle projects farther plantarly and is 86% the thickness of the lateral condyle. In comparison, the medial condyle is 73% the thickness of the lateral condyle in A. archibaldi. In plantar view the medial condyle tapers sharply and extends proximally farther than the lateral condyle. The trochlea of metatarsal IV is reduced to a single condyle as in other enantiornithines (O’Connor, Averianov & Zelenkov, 2014); in dorsal view, the proximomedial margin of the trochlea bears a very small, medially-directed tubercle that suggests an enclosed vascular foramen was present between metatarsals III and IV. This indicates that the metatarsals are slightly disarticulated, and that the intertrochlear incisures may be exaggerated.

Pedal phalanges. A number of isolated phalanges are preserved representing parts of both the left and right feet; 23 of the 28 pedal phalanges were collected, including nearly the entire left foot (Figs. 16 and 17). The hallucal claw is the largest in the foot. The second digit has two phalanges that are subequal in length although the second is more delicate, followed by a large claw. The first phalanx of the third digit is the longest in the foot; the following two phalanges decrease in slightly in length, followed by a claw. The phalanges of the fourth digit are all short and robust with a small claw that appears to be more recurved than that of the other digits; the first phalanx of this digit is slightly longer than the phalanges distal to it. All non-ungual phalanges have well-developed medial and lateral fovea for the attachment of the collateral ligaments. All ungual phalanges have a deep neurovascular groove.

Figure 16: Non-ungual pedal phalanges of the left foot. (A) Proximal view. (B) Distal view. (C) Dorsal view. (D) Medial view. Elements are identified on the left by digit number and phalanx number (D#, P#). Scale bar equals 0.5 cm. Photos: David Strauss.