A few smaller sthenurines are known from the Miocene, and these forms retained the fifth toe. The middle Miocene Wanburoo hilarus ( [13] ), originally described as a bulungamayine, was the smallest known sthenurine [4] ), with an estimated body mass of 7–8 kg [14] . Unfortunately, postcranial materials from this animal are unknown. The slightly larger (9–15 kg [14] ) middle [15] and early late Miocene Rhizosthenurus flanneryi [16] is known from a partial skeleton as well as cranial material. The late late Miocene [17] Hadronomas had estimated body mass of around 30 kg [4] . Note that Prideaux [6] has determined that several species of Sthenurus are on the stem lineage of the genus Procoptodon, although lacking the specialties of this highly derived taxon, and has renamed them as “Procoptodon”. These include two smaller species (body mass of around 50 kg [2] ) included here; “P. gilli and “P”. browneorum.

Sthenurines are the kangaroos that are usually spoken of as being the “giant kangaroos”. They are distinguished not only by their size, but by many other features, such as their relatively short faces, the loss of the fifth digit in the foot in the Pleistocene forms, and their specialized arms and hands that have been interpreted as an adaptation for browsing [12] . Sthenurines have also been noted as having especially robust limb bones, but the bones of the larger species of the extinct genus Protemnodon are similarly robust (see later discussion). Sthenurines, like Protemnodon spp., were all fairly large, but not all species were larger than extant kangaroos. Figure 2 shows the difference in the skeletons of Pleistocene sthenurines and large macropodines.

The genus Macropus has species of a diversity of body sizes, the smallest today being Macropus parma (the parma wallaby) with an average body mass of around 4 kg [8] . Macropus titan (related to the extant grey kangaroo) had an estimated body mass of up to 150 kg [2] . A similar estimate has been obtained for M. ferragus (related to the extant red kangaroo) [9] , although this animal appears to have been somewhat larger in linear dimensions than M. titan. Within the genus Protemnodon, all species were fairly large, although not all were larger than extant kangaroos. Protemnodon hopei had an estimated mass of 45 kg [2] . Other smaller Protemnodon species, such as P. otibandus and P. snewini, were of a similar size (as estimated by measurements of foot bones). The largest species was P. roechus, with an estimated mass of around 166 kg [2] . Protemnodon spp. are commonly referred to as “giant wallabies”: but while they might be somewhat wallaby-like in their skull and dentition, their postcranial elements are unlike any other terrestrial macropodid; they have relatively short tibiae, short feet and (at least in P. anak) a relatively long neck. There has been speculation that at least some may have been secondarily quadrupedal in their locomotion [10] , [11] .

There are three families within the Macropodoidea (taxonomy following Prideaux and Warburton [4] , see Table S1 ): the Balbaridae (an extinct family, considered to be basal to both extant families [5] ); the Hypsiprymnodontidae (the extant musky rat-kangaroo, Hypsiprymnodon moschatus, plus a number of extinct genera); and the Macropodidae. The family Macropodidae is usually divided into four subfamilies: Potoroinae (rat-kangaroos), Lagostrophinae (containing the extant banded hare-wallaby, Lagostrophus fasciatus, and the extinct genus Troposodon), Macropodinae (containing all other extant macropodids, olus several extinct genera including Protemnodon), and Sthenurinae (containing the extinct genera Archaeosimus, Hadronomas, Metasthenurus, Procoptodon, Rhizosthenurus, Sthenurus, Simosthenurus, and Wanburoo, see Prideaux [6] ). A possible fifth subfamily is the Bulungamayinae, which is a paraphyletic assemblage of extinct taxa basal to the Macropodinae plus Sthenurinae [5] ). Molecular phylogenies (e.g., [7] ) show similar relationships among the living taxa, although the higher level taxonomic terminology differs (e.g., the rat-kangaroos are considered to be a separate family, Potoroidae). “Giant” forms can be found within the Sthenurinae (among the genera Sthenurus, Simosthenurus, and Procoptodon) [6] , and within the Macropodinae in the extant genus Macropus (extinct species M. titan and M. ferragus), and the extinct genus Protemnodon (P. brehus, P. roechus, and P. anak).

Kangaroos are famous for their style of locomotion – bipedal hopping (also known as ricochetal or saltatory locomotion), which is unique among relatively large mammals (i.e., over around 5 kg in body mass). While the popular notion of a kangaroo is of a fairly large animal, such as the grey kangaroo (Macropus [Macropus] giganteus) or the red kangaroo (Macropus [Osphranter] rufus), members of the superfamily Macropodoidea (“kangaroos” in the broadest sense) contain animals of a diversity of sizes and habits, including the secondarily arboreal tree-kangaroos (Dendrolagus spp.). However, the recent past diversity of kangaroos, persisting to perhaps as recently as 30,000 years ago, included several kinds of kangaroos much larger than any known at present. The largest of the extant kangaroos (red kangaroo males) can weigh up to 90 kg, although the average weight for males of this species is only around 55 kg, with females averaging around 25 kg [1] , [2] . However, Pleistocene kangaroos existed that weighed up to 240 kg [2] , a size that calls into question their biomechanical abilities for a hopping gait [3] . Three different lineages of macropodids, two of them extinct, attained masses of greater than any extant kangaroo (i.e., >90 kg); kangaroos larger than extant kangaroos are commonly referred to as “giant kangaroos” [2] . In this paper we specifically address the locomotor abilities of the extinct subfamily Sthenurinae, and propose that they employed a bipedal striding type of gait (see Figure 1 ). We propose that this gait would have been used at only at slow speeds in the smaller sthenurines, with hopping employed at faster speeds, but in the very large sthenurine species this may have been their sole mode of locomotion.

The mechanical tensile stress of tendons plays a limiting factor in body size and speed in all mammals. In placental mammals maximum locomotor performance peaks at a body mass of around 50 kg (cheetah or pronghorn size), and there is evidence that this is the size at which strain on the locomotor tendons becomes an issue. How does this relate to the condition in kangaroos? While larger species will be better able to use elastic energy recovery to assist in their locomotion, the larger the animal, the lower the tendon safety factor, and the greater the danger of tendon rupture: while smaller kangaroos have estimated tendon safety factors of around ten (typical for non-hopping mammals), the estimated safety factor of the Achilles tendon in large kangaroos approaches one. McGowan et al. [3] estimate that, at a projected body mass of around 140 kg, a fast-hopping kangaroo would have a tendon safety factor of less than one. However, a large male red kangaroo (Macropus rufus), weighing around 80–90 kg, would still be operating with a very low tendon safety factor of 1.1, and many extinct kangaroos have body mass estimates well in excess of 140 kg (although this is around the mass estimate for the largest extinct species of Macropus). Few extant kangaroo individuals can be found with a body mass of greater than around 50 kg, and most kangaroos weigh considerably less than this, see [1] . Extrapolations from the data of McGowan et al. [3] would predict that the sthenurine Procoptodon goliah, at a body mass of as much as 250 kg, would have had a tendon safety factor of around 0.89; they note that hopping would have been severely limited in this animal, especially during acceleration, if indeed possible at all. However, McGowan et al. [3] propose that relatively thicker tendons in these larger macropodids (as also seen in rock wallabies, Petrogale spp.) would have enabled them to exert higher forces at larger sizes. McGowan [32] presented evidence that the greatly enlarged site of attachment of the gastrocnemius (i.e., the Achilles tendon) on the calcaneum in sthenurines was indicative of tendons sufficiently large to withstand the forces of hopping locomotion. However, it is also true that a relatively thicker tendon has a lesser capacity for elastic energy storage and, as noted below, the moment arm for the gastrocnemius in sthenurines is much shorter than in large macropodines. Obviously large sthenurines would have had to have relatively large gastrocnemius muscles to support and propel their body mass during locomotion, but this does not necessarily mean that they could hop well, if at all.

Certain allometric scaling relationships differ between kangaroos and placental mammals, which likely relates to the fact that large kangaroos maintain the crouched posture of smaller ones, rather than altering their posture with increasing body size, see [30] . The overall hind limb length of macropodoids scales with positive allometry, largely due to strong positive allometry of tibia length [3] . The size of the hind limb, the limb muscles, and the cross sectional area of the foot extensors all scale with isometry in placentals. All of the extensor muscles of the hind limb in macropodines, with the exception of the sartorius, scale with strong positive allometry, while fascicle length tends to scale with negative allometry, resulting in extremely strong positive allometry for hind limb muscle physiological cross sectional area [3] . However, note that in potoroines, elastic energy saving is primarily in the plantaris muscle, with the gastrocnemius and flexor digitorum longus being more involved in joint control and acceleration capacity [3] . This greater muscle power in kangaroos is required to offset the relatively greater torques around the ankle joint in the absence of changing posture with increasing size. In contrast, the tendon cross sectional areas of the gastrocnemius and flexor digitorum longus scale with negative allometry, although the cross sectional areas of the plantaris tendon scales with positive allometry [3] . While this anatomy allows for greater proportional muscle power and elastic energy storage in larger kangaroos, accounting in part for their superior locomotor performance, it appears that larger macropodids operate with unusually high musculoskeletal stresses, and tendon safety factor (estimated from the ratio of the tendon cross sectional area to the physiological cross sectional area of the attached muscle (see [3] : at safety factors below one tendon failure becomes likely [31] ) might be a limiting factor for body mass and/or locomotor performance.

However, larger kangaroos pay a price for their locomotor efficiency in terms of bone and tendon stress. Placental mammalian cursors change their posture with increasing body size, thereby reducing the torque of the ground reaction force around their limbs [30] , but large kangaroos hop with the same flexed limb posture as smaller ones. There is no evidence that there is any difference in the properties of the ankle extensor muscles and their tendons that power hopping in kangaroos (gastrocnemius, flexor digitorum longus [ = profundus], and plantaris [ = flexor digitorum superficialis]) in comparison with similar placentals, although enzyme levels in these muscles in kangaroos may indicate higher levels of aerobic work, and 86% of the total body mitochondrial volume in M. rufus is in the upper hind limb musculature [28] . In addition, bone stresses on the tibia appear to be greater in large kangaroos (but not in smaller kangaroos) than in similar-sized placentals, although there is some evidence that tibial bone cortical thickness increases with increasing body size in kangaroos [3] .

The pentapedal walk, which is employed at low speeds in kangaroos, is energetically very expensive, more so than hopping at higher speeds [28] . Kangaroos have been shown to progress from a pentapedal gait to hopping at a Froude number of 0.5 [25] , which is similar to the transition to the gallop in quadrupeds (the Froude number relates size, speed, and stride length, and is used in the analysis of vertebrate gaits). There must thus be some biomechanical or energetic reason why hopping cannot be employed at low speeds: Dawson [29] proposed (p. 68) that, due to the specialized limb morphology of kangaroos, hopping would likely be even more expensive than pentapedal locomotion at slow speeds. The role of the tail is also important in hopping in kangaroos: in bipedal or quadrupedal running there is no net torsion on the body that would cause the head to pitch, as the angular momentum of the legs cancel each other out. But in bipedal hopping, where the legs act together in phase, their action creates a moment around the center of mass, such that the body would tend to pitch with each hop. Motion of the tail reduces this tendency: the tail swings forwards as the hind legs swing backwards, thus cancelling out the moment produced by the limbs, and reducing the effective pitch of the head to around ten degrees [27] .

It is well known that, in larger kangaroos (e.g., Macropus rufus), hopping is an extremely efficient gait: unlike the situation in placentals, where the energy costs of locomotion increase linearly with speed, in the larger kangaroos energy costs and speed become decoupled, and thus the daily expenditure of locomotor foraging is much less for the large kangaroos than for similarly-sized cursorial placentals. (Note: although the term “cursorial” usually refers to quadrupedal locomotion, we use the term here in relation to kangaroo locomotion, where “more cursorial” equals “more specialized for fast hopping”.) Even in a medium-sized wallaby, the energy savings from elastic energy storage during hopping have been estimated at around 40% [27] . However, while much of the research focus has been on the spectacular performance of the large kangaroos, this cannot explain the initial reason for adopting a hopping gait. This decoupling of energy costs and speeds is only true in kangaroos above around 6 kg, and the smaller potoroines (e.g., Potorous, Bettongia) show no such advantage, although hopping in Bettongia (at least at relatively high speeds, above 3 m/sec) is nevertheless less expensive than locomotion in a quadruped of similar size [28] . However, one distinct advantage of hopping, over and above any storage of elastic energy, is that speed can be attained simply by increasing stride length without concomitant increase in stride frequency, which reduces the energetic costs of limb recycling [28] .

The adaptive reasons for adopting the hopping gait are not entirely clear. Hopping is clearly a very efficient form of locomotion in larger kangaroos (see discussion below), but it would have first evolved in relatively small forms, likely of a mass of less than 5 kg [24] . Baudinette [25] , Bennett [26] , and McGowan et al. [3] summarize much recent information about kangaroo hopping, and the discussion below is derived from their papers. Hopping has evolved several times in rodents, but most of these are very small, with the largest being the springhare (Pedetes capensis) with a body mass of around 4 kg (the size of a small wallaby). These rodents, like small macropodids, use quadrupedal gaits at slow speeds [18] . However, the ankle extensor tendons of hopping rodents are more robust for their size than in kangaroos, which may reflect the need to withstand relatively high forces during acceleration. Hopping can also be seen in some lemuriform primates (e.g., sifakas moving on the ground) and occasionally in Arctic hares (Lagomorpha): again, these animals are no more than around 5 kg in weight, below the weight where hopping becomes efficient in terms of storage of elastic energy in the extensor tendons (see below).

Hopping is the quintessential kangaroo gait, seen in all extant macropodoids with the exception of the musky rat-kangaroo (Hypsiprymnodon moschatus), where it is considered to be primarily absent (although this is conjectural because there are inadequate data on this animal locomoting at high speeds [18] . Some earlier, extinct macropodoids (Balbaridae) may have been quadrupedal bounders rather than hoppers [11] . All extant kangaroos have a digitigrade hind foot posture while locomoting, although they may rest with the hind feet in a plantigrade stance. Among the potoroines (rat-kangaroos), potoroos (Potorous spp.) habitually bound and only hop at high speed when alarmed [19] , while bettongs (Bettongia spp.) habitually hop like macropodines [20] . Within the macropodines, all species use the form of slow, pentapedal progression while foraging on the ground [21] , a gait that is actually used more frequently than hopping during the course of the day [22] . In the pentapedal “walk”, the forefeet are placed together on the ground, the hind feet are lifted, and the tail is used as a “fifth leg” to propel the animal through the space between the fore feet [22] . A quadrupedal bound is seen in the quokka (Setonix brachyurus) and tree-kangaroos (Dendrolagus spp.), and tree-kangaroos uniquely perform (along tree branches) a quadrupedal walk, with alternate movement of the limbs [21] . Tree-kangaroos have also been observed walking bipedally along branches [23] .

Descriptive anatomy of sthenurine kangaroos

While sthenurine locomotor performance has rarely been considered as markedly different from that of modern large kangaroos (genus Macropus), there are many anatomical differences that reflect the fact that these animals had a rather different functional biology. The classic work on sthenurine anatomy is that of Wells and Tedford [12], in their comparison of species of Sthenurus (mainly S. stirlingi and S. tindalai) with the exant grey kangaroo, Macropus giganteus): they note that, while the overall postcranial proportions of Sthenurus resembles those of Macropus, there are some key differences.

While the forelimbs of Macropus and Sthenurus are of similar overall length, the manus of sthenurines is longer, and the radius and ulna are shorter. Modifications of the sthenurine forelimb allow for specialized grasping, and the scapular morphology may allow for elevation of the forelimb over the head, as in humans. Somewhat similar modifications of the forelimb, especially in the scapula, are seen in tree- kangaroos, which do extensive reaching with the forelimbs [33]. The hind limbs are of similar proportions in both, but Wells and Tedford [12] claim that the limbs are proportionally longer in relation to the vertebral column in Sthenurus. However, this is not apparent in their figures (see Figure 2): Sthenurus appears to have a somewhat shorter trunk than Macropus, which would mean that this apparent difference in proportions is due to a shorter trunk rather than to longer hind limbs. (See [34], which concludes that sthenurines have a relatively shorter vertebral column than macropodines, despite the same number of vertebrae, due to a shorter length of the lumbar vertebrae.) Pleistocene sthenurines have reduced the fifth digit in the hind limb to a vestigial nubbin of the metatarsal. There is little evidence that they retained the syndactylous small second and third digits in the hind limb typical of other kangaroos (although a remnant of metatarsal III is known in a couple of specimens [12]). The tail of Sthenurus is slightly shorter than that of Macropus. In addition, the anterior caudal vertebrae have shorter (although robust) centra, but with shorter diapophyses and metapophyses, and vestigial anapophyses. This may indicate a reduced capacity for lateral extension of the tail [12]. The lumbar vertebrae of Sthenurus are massive, with huge metapophyses, but the transverse processes have been reduced or lost, and the backbone appears to have been relatively rigid with limited flexion in the dorsoventral plane. And, finally, in general the limb elements of Sthenurus, especially in the hind limbs, are much more robust than those of Macropus: Wells and Tedford [12] note that the cross-sectional area of the femur of Sthenurus is almost double that of a Macropus of similar linear dimensions.

We present here a description of the hindimb locomotor anatomy of sthenurine kangaroos, which is essential for an understanding of the analyses performed. Much of this anatomical description is drawn from Wells and Tedford [12], but some novel features are also included. Note that the anatomical illustrations here are designed to show features not emphasized in Wells and Tedford [12], and the reader is referred to this publication for additional details. The supplementary information contains a number of bivariate plots (Figures S1–S3), which are separate from the multivariate analyses discussed later, and provide a visual impression of some of the anatomical differences between sthenurines and other kangaroos.

Vertebral column. The number of precaudal vertebrae is similar in sthenurines and macropodines (7 cervical, 13 thoracic and 6 lumber), but the overall appearance of the vertebral column is very different. The overall appearance in sthenurines is for massive vertebrae in comparison to macropodines, especially in the lumbar region, where large and laterally expanded metapophyses give the impression of rigidity. Wells and Tedford [12] note that the anticlinal vertebra in Sthenurus is more posterior (L1 or L2, versus T10 or T11 in Macropus), with no modification of the nature of the zygapophyses (as in Macropus). Large metapophyses appear on T11, and increase in size dramatically in the more posterior portion of the trunk. While metapophyses are also present in the posterior trunk of Macropus, they are smaller, and less deflected medially. The neural arches are shorter in Sthenurus, although broader in the anteroposterior direction, and are directed caudally (while those of Macropus are directed cranially). The lumbar vertebrae of Sthenurus are also markedly different in having greatly reduced transverse processes, and in the vestigial nature or absence of diapophyses and anapophyses (that are prominent on the lateral surfaces of the lumbar vertebrae in macropodines), and the almost platycoelus centra indicate limited mobility. Wells and Tedford [12] interpret this anatomy as indicating rigidity to resist rotational stress on the backbone. In Sthenurus resistance would have been effected not only by the zygapophyses, but also by by the epaxial muscles (multifidi) and ligaments attached to the greatly enlarged metapophyses. They [12] note that the loss of diapophyses and anapophyses indicate the reduction of the longissimus dorsi component of the epaxial musculature. They [12] interpret the massive dorsal epaxial components (the multifidi) as being used to elevate the front half of the body for the proposed browsing posture. But this interpretation is problematical. The notion of the “elevation of the body” seems to be derived from the function proposed by Elftman [35] in kangaroos for the erector spinae muscles (this being the lumbar region merging of multifidus and longissimus dorsi muscles); but this proposed function was in the context of preventing the front end of the body collapsing under the force of gravity during locomotion. It would be impossible for the erector spinae to raise the body up over the hip, as they insert cranially to the hip joint (along the dorsomedial border of the ilium). The muscles that would be capable of exerting this action of raising the body would be ones that span the hip joint dorsally: the gluteus superficialis and the cranial head of the caudofemoralis ( = gluteobiceps) muscles (see [36]), that originate from the tuber coxae of the ilium (and/or the thoracolumbar fascia, see below) and run dorsally over the hip socket to insert on the femur.

Sacrum. The sacrum of sthenurines is broader and shorter than in macropodines, with more pronounced sacral wings (alae sacrales) for the articulation with the ilium. Wells and Tedford [12] note only two sacral vertebrae in Sthenurus, as in Macropus (and most other marsupials), but the senior author has observed sthenurine sacra that are composed of three vertebrae, involving the incorporation of the first caudal vertebra (e.g., Simosthenurus occidentalis SAM P18308). Wells and Tedford [12] note a more rigid sacrolumbar connection in Sthenurus stirlingi than in Macropus (whereas the smaller S. tindalei is more like Macropus). The sthenurine sacral anatomy is again indicative of a resistance to rotational torsion. All macropoidoids exhibit the derived anatomy of a very distally-positioned attachment of the ilium to the sacrum, with the result that the ilia project dorsally above the sacrum. Personal observation by the senior author shows that this is also true for the musky rat-kangaroo (Hypsiprymnodon moschatus, the only extant macropodoid considered to be primarily non-hopping), but to a slightly lesser extent than in other macropodoids, and it is also a feature of koalas and wombats. Elftman [35] interprets this morphology as allowing for a greater area for the insertion of the erector spinae muscles, which insert along the dorsomedial border of the ilium anterior to the sacral attachment, and notes that it also allows for an increased sacroiliac angle.

Pelvis (Figure 3). The ilium of both Sthenurus and Macropus is long, but in Sthenurus the ilia flare more laterally, and have a greatly expanded blade. Elftman [35] interprets flared ilia as allowing for a greater volume of erector spinae musculature. Wells and Tedford [12] note that the areas of insertion for the gluteal muscles (on the lateral side of the blade) and the iliacus muscle (on the dorsomedial side of the blade) in Sthenurus are, respectively, 1.8 and 1.6 times the amount of insertion area in Macropus. Flared iliac blades, indicating enlarged gluteal muscles, are common among large mammals that engage in bipedal browsing (see [37]). They [12] also note that the iliopectineal tuberosity (at the base of the iliac spine), the area of origin of the rectus femoris, is larger in Sthenurus: the acetabulum is also larger in Sthenurus, and the acetabulae are placed further apart, resulting in a more wide-legged stance. Macropodines in general also have a fairly narrow tip of the ilium (tuber coxa), while this is broad in sthenurines, and narrower in species of Macropus than in most other kangaroos. The sthenurine condition is approached only by tree-kangaroos (Dendrolagus spp.) among macropodines, while other more open-habitat, fast hopping kangaroos such as the nail-tail wallabies (Onychogalea spp.) and hare-wallabies (Lagorchestes spp.) parallel the Macropus species in having narrow tuber coxae (see Figure S1A). While there is debate about the systematic position of the nail-tail wallabies, the molecular data [7] show that both Onychogalea and Lagorchestes are independent radiations to Macropus within the Macropodinae: they can thus serve as a comparison to Macropus species for considerations of morphological features related to greater cursoriality. The tuber coxae serve as the area of origin of the gluteus superficialis and the cranial head of the caudofemoralis in Macropus [36], but not in Setonix and Dendrolagus, where these muscles originate from the thoracolumbar fascia [38]. The gluteal and caudofemoralis muscles act to extend the hip: they may be important in elevating the body over the hip when the feet are on the ground, and would also enable supporting the body over a single hind leg. In all kangaroos the sartorious muscle originates from the tuber coxa, and the medial and deep gluteals originate from the iliac blade [38]. The larger species in the genus Macropus have elongated ischia (dorsal length of ischium around 65–70% of the length of the ilium), with a pronounced posteroventral projection to the bone. This long ischium provides an elongated moment arm for the muscles that retract the femur, both the hamstring complex and the adductor complex. Elftman [35] notes that the elongation of the ischia effectively turns the action of the adductors into femoral retractors (i.e., hip extenders). This is obviously advantageous for powerful hip extension during rapid locomotion, paralleled among placentals by the extension of the ischia in the cheetah (Acinonyx jubatus) in comparison with less cursorial felids [39]. However, this condition is derived in Macropus among the other macropodoids, which have a relatively shorter ischium (less than 60% the length of the ilium), although the nail-tail wallabies (Onychogalea spp.) have also elongated their ischia, to an even greater percentage of the ilium (>80%) than in Macropus (see Figure S1B). This increased length of the pelvis caudal to the acetabulum can be seen even more clearly in a comparison of the length of the puboischiatic symphysis (Figure S1C). However, sthenurines have ischia that are markedly different to those of other macropodids, approached (convergently) only among the tree-kangaroos. The ischia are only somewhat shorter than in the regular macropodid condition, but are tipped dorsally: Wells and Tedford [12] note that the angle between the ilium and the ischium is 170o in Macropus, but 145o in Sthenurus. This difference in anatomy results in a markedly different shape of the obturator foramen, which is elongated and ovoid in Macropus, moderately oval in most other macropodoids, and circular/triangular in sthenurines (and also in Dendrolagus spp.) (see Figure 3). This dorsal tipping of the ischium markedly repositions the moment arm for the hamstring muscles, especially for the biceps femoris, which originates from the ischial tuberosities [36]. Wells and Tedford [12] note that the ischial anatomy in sthenurines would increase the area of origin of the quadratus femoris, which acts as a femoral adductor, and this could be important in preventing the legs from spreading when standing upright. PPT PowerPoint slide

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larger image TIFF original image Download: Figure 3. Pelvis. (A) Simosthenurus occidentalis (SAM: P17358). (B) Dendrolagus dorianus (SAM: M9190) (C). Macropus robustus (SAM: M3695). Left lateral view. Scale bar = 5 cm. https://doi.org/10.1371/journal.pone.0109888.g003 A notable difference in the pelvic area is in the size of the epipubic bones (see Figure 3). The epipubics of Sthenurus are almost as long as the ilium, but are no more than half the length of the ilium in Macropus [12]. The epipubics of Macropus are possibly somewhat reduced over the primitive macropodoid condition, but in Dendrolagus spp. the epipubics are of similar relative size to those of sthenurines (see Figure S1D). The prime function of the marsupial epipubic bones, to which the pectineus, pyramidalis and hypaxial muscles attach, is to stiffen the trunk during locomotion: the epipubics act part of a kinetic linkage between the femur and the hypaxial muscles, resisting torsion and diagonal stress across the trunk [40].

Femur (Figure 4). The head of the femur is proportionally large in sthenurines (matching the enlarged acetabulum, see above): tree-kangaroos (Dendrolagus spp.) also have relatively large femoral heads, as do Protemnodon spp. (see Figure S2A). The shape of the femoral head is round in sthenurines, rather than ovoid as in Macropus (see Figure 4). The more ovoid morphology of Macropus is the derived one among macropodoids, and is likely related to restricting femoral motion to a parasaggital plane, as also seen among cursorial bovids [41]. The neck of the femur is also elongated in Macropus, which may increase the moment arm for the gluteal muscles, again reflecting cursorial adaptations. PPT PowerPoint slide

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larger image TIFF original image Download: Figure 4. Femur. (A) Simosthenurus occidentalis (SAM: P17259). (B) Macropus sp. (SAM: P17270). All left side: upper = proximal articular view; lower = lateral distal view. Scale bar = 5 cm. https://doi.org/10.1371/journal.pone.0109888.g004 The femoral shaft is curved in both Macropus and sthenurines, but the orientation of the femur is slightly different, so that the knee points medially in Macropus and laterally in sthenurines [42]. The greater trochanter, the major point of insertion of the gluteal muscles, is large in both Macropus and the larger sthenurines: this may relate partly to body size, as smaller sthenurines (e.g., S. andersoni) have a proportionally smaller greater trochanter [12]. However, Wells and Tedford [12] also note that the greater trochanter is relatively longer, and more closely aligned with the axis of the femoral shaft, in Sthenurus. This echos the point made above, that a larger volume of gluteal musculature would enable sthenurines to balance their body weight over one leg: in humans, larger superficial gluteal muscles are important in preventing collapse of the body medially when the weight is borne on one leg [43]. Wells and Tedford [12] note that the lesser trochanter is “weaker” in Sthenurus, but it is placed more distally on the femoral shaft in sthenurines than in other macropodoids (although tree-kangaroos are more like sthenurines), increasing the moment arm for the iliopsoas (see Figure S2B). Note that the area for the origin of the iliacus on the medial surface of the iliac blade is also greater in sthenurines. It is not completely clear what the function might be of a more powerful iliopsoas in sthenurines: it may relate to different mechanics of femur protraction if the femur is being held in a more vertical position (i.e., with an upright trunk), because the moment arm of the iliopsoas would be less favorable with a limb in this position. As noted by Wells and Tedford [12] the adductor scar on the posterior part of the femur in Sthenurus is placed more distally than in Macropus. This marks the insertion point of the quadratus femoris ( = ischiofemoralis), which originates from the ischium and is essentially the most proximal component of the adductor musculature complex. Hopwood and Butterworth [36] note that this muscle is tendinous in Macropus, and suggest that it acts as a form of check ligament to prevent overflexion of the hip during jumping (i.e., to limit the forward motion of the femur). Without knowing the more general condition for this muscle in all macropodids it is impossible to even guess whether or not this muscle was tendinous in sthenurines. The longer moment arm afforded by the more distal insertion on the femur might be indicative of more powerful limb retraction and abduction, or it might simply be the case that the different morphology of the sthenurine ischium (see above) changed the previous nature of the moment arm, and the more distal placement of the insertion is merely compensating for this. Tree-kangaroos (Dendrolagus spp.) also have a relatively distally placed adductor scar (see Figure S2C), and a correspondingly large quadratus femoris muscle [38]. Wells and Tedford [12] suggested that the articular facets on the Sthenurus distal femoral condyles would allow for a greater range of knee motion than in Macropus. They also noted that the lateral femoral condyle is markedly larger than the medial one in both Macropus and Sthenurus. However, both condyles are elongated in the anteroposterior direction in sthenurines, giving them a more elliptical shape than in Macropus (see Figure 4). More importantly, sthenurines have a greater width across the distal femoral condyles than do macropodines: that is, they have relatively bigger knees, as well as relatively bigger hip joints (see Figure S2D). Tree-kangaroos also have relatively broader knees than other macropodines (see Figure S2D).

Tibia and Fibula (Figure 5). All macropodids have relatively long tibia, up to twice the length of the femur. Tibial length scales with positive allometry in macropodids in general [3], [11]. Tibia lengths are shorter in Dendrolagus spp. and taxa that rely on more quadrupedal (pentapedal) locomotion, such as the New Guinea forest-wallabies (Dorcopsis and Dorcopsulus spp.). Sthenurines have tibiae of comparable lengths to generalized macropodids, but the extinct Protemnodon spp. have relatively short tibiae [11]. On the proximal tibia, Wells and Tedford [12] note that the lateral and medial condyles are of approximately equal size in Sthenurus and Macropus, but did not comment on the elongation of the tibial tuberosity in Macropus (which is derived relative to other macropodoids) (see Figure S3A). The enlarged tibial tuberosity of Macropus goes along with the greater size of the proximal portion of the tibial (cnemial) crest. Murray [42] notes that the macropodines and sthenurines differ in tibia diaphysis: macropodines have a sharply defined tibial crest that is limited to the proximal quarter of the bone, terminating in a distinct notch, and the anterior profile of the tibia is straight; sthenurines have a elongated crest that is convex in profile, and blends into the more distal shaft, and the anterior profile of the tibia tends to be sinuous (especially in Procoptodon). By comparison with the potoroine condition, Murray [42] concluded that it is the macropodine condition that is the derived one. The tibial crest serves as the insertion point both for the tibalis anterior (which flexes the foot) and for the gracilis, which abducts the leg [36]. A shorter, more prominent tibial crest would concentrate the origin of the tibialis anterior proximally, and may relate to the ability for more rapid foot flexion. On the distal tibia, Murray [42] notes that the tibioastragalar joint is rotated in an anteriomedial direction in sthenurines, which he ascribes to a compensation for the outwardly-rotated knees, and a morphology which would rotate the feet to be in a more medial position. Wells and Tedford [12] note a longer and more robust medial malleolus in Sthenurus than in Macropus, and an articular groove that is more of an “oblate cup” in shape than the “shallow, arcuate” form in Macropus. They comment that this morphology would mean a more constrained tibioarticular articulation in Sthenurus, but they do not specifically note a unique morphology of the sthenurine distal tibia: that is, of a plantar process that fits in a tongue-in-groove linkage into the astragalar trochlea (see Figure 5). This morphology can also be observed in the Miocene sthenurine Hadronomas (NT 2469: personal observation of senior author). PPT PowerPoint slide

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larger image TIFF original image Download: Figure 5. Tibia. (A) Procoptodon goliah (NMV2010). (B) Macropus giganteus (AMNH 2390). All left side: upper = distal articular view (plantar side upwards); lower = posterior (plantar) view, showing articulation with tarsus. Scale bar = 5 cm. https://doi.org/10.1371/journal.pone.0109888.g005 With regards to the fibula, Wells and Tedford [12] note that Sthenurus lacks the distinctive posterior process of the head of the fibula seen in Macropus, which apparently allows for greater flexion of the knee. They conclude that this morphology may relate a longer groove in the proximal tibia for the insertion of the fibula, and interpret this as allowing for a greater internal rotation of the lower limb about the knee. They relate this to the notion of to sthenurines needing to achieve greater limb rotation to position their feet medial when landing while hopping, as their wider pelvis would otherwise result in more lateral placement of the feet. However, this could also relate to a rotation of the body around the knee when the foot was placed on the ground, as would be experienced with bipedal striding.