A New Brontothere (Brontotheriidae, Perissodactyla, Mammalia) from the Eocene of the Ily Basin of Kazakstan and a Phylogeny of Asian ‘‘ Horned’ ’ Brontotheres Author MIHLBACHLER, MATTHEW C. Author LUCAS, SPENCER G. Author EMRY, ROBERT J. Author BAYSHASHOV, BOLAT text American Museum Novitates 2004 2004-05-14 3439 1 1 43 http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1206%2F0003-0082(2004)439%3C0001%3AANBBPM%3E2.0.CO%3B2 journal article 3792 10.1206/0003-0082(2004)439<0001:ANBBPM>2.0.CO;2 8357bfe7-0687-42bb-a57b-ce2f63a0832b 0003-0082 4734999 Aktautitan hippopotamopus , new genus and species HOLOTYPE : KAN N2/875, a complete skull, mandible and skeleton lacking only parts of the right tarsus and pes. TYPE LOCALITY: Kyzylbulak Formation , Kyzyl Murun near Aktau Mountain , Ily Basin , Kazakstan . AGE: Middle Eocene (Irdinmanhan landmammal ‘‘age’’) ETYMOLOGY: Aktautitan , Aktau (in reference to the name of the mountain where the fossils were found); titan, giant; hippopotamopus , Greek , ‘‘ hippopotamus foot’’ (in reference to the hippopotamus­like limb proportions of this species). REFFERED SPECIMEN: KAN N2/873, a complete skull and articulated mandible, complete dentition, a fully articulated right forelimb with radius, ulna, and manus and a partial left manus. (An anterior portion of a third cranium [KAN N/2 639] was not directly examined by Mihlbachler, but is considered referable to Aktautitan hippopotamopus by Lucas and Emry.) DIAGNOSIS: Aktautitan hippopotamopus can generally be characterized as a large brontothere with a relatively elongate skull; unbowed zygomatic arches; unreduced dental formula (3/3 1/1 4/4 3/3); small elliptical frontonasal horns; relatively tall upper molar ectoloph with a very thin inner band of enamel; small third anterolingual cusp on the mesial cingulum of the upper molars; shallow central fossa in the upper molars; large pointed hypocone on the M3; i2 larger than i3; metaconid absent on p2, present but small on p3, present and molariform on p4; and m3 very elongate. A. hippopotamopus is similar to Metatitan , Protembolotherium , and Embolotherium in having frontonasal protuberances that are situated close together and are elevated on tall superorbital pillars, creating a single frontonasal process. A. hippopotamopus , like Metatitan , retains a large, elevated nasal process, with downfolded lateral margins, that extends forward from the peak of the frontonasal process. A. hippopotamopus differs from Metatitan in the following characteristics: dorsal surface of the skull forms a continuously concave surface; posterior zygomatic processes absent; orbit more anterior, positioned directly above the M2; larger incisors that arch anteriorly from the canines; upper incisors grade from subglobular (I1) to caninform (I3); lower incisors short and conical with blunt points; postcanine diastema present; and p3 metaconid smaller and less lingually positioned. The distal limb segments of A. hippopotamopus are proportionally much shorter than those of other brontotheres, resulting in remarkably shortened limbs that are more similar in proportion to those of Hippopotamus and a number of short­limbed rhinocerotoids such as Teleoceras and Metamynodon . DESCRIPTION The completeness and position of the brontothere skeletons in the sediment suggest that the animals were trapped in deep mud and preserved completely (although some of the specimens had been eroded away). All of the feet were preserved deepest in the mudstone ( 30 cm or more below the level of the thorax). The digits of the feet were hyperflexed, extending outward and upward from the distal ends of the metapodials (see figs. 12, 16). Some of the longbones, especially those of the lower limbs, were preserved vertically in the sediment, with the remainders of the skeletons collapsed directly over the limbs and feet. Compression of the mudstone bed has resulted in deformation of the bones within it, and those longbones preserved more or less vertically are substantially shorter that their counterparts that were preserved more or less horizontally. The skull of the holotype (KAN N2/875) is essentially complete, although it was lying on its side and is somewhat compressed laterally. The referred skull (KAN N2/873) is compressed obliquely (more laterally than vertically). This distortion compromises the measurement of various cranial dimensions. The postcranial material belonging to the holotype , KAN N2/875, consists of the entire skeleton, missing only some parts of the right tarsus and proximal metatarsus (the right ankle was the only part of this individual exposed at the surface). Much of the thorax and forelimbs remain articulated in a large block, preventing detailed study of some of the individual bones. Asian brontotheres are rarely known from associated skulls and postcrania. The entire skeleton of Rhinotitan mongoliensis ( Wang, 1982 ) and parts of the skeletons of Metatitan relictus (= Protitan khatishinus Yanovskaya, 1980 , see below), cf. Parabrontops gobiensis (PIN 3109, mistakenly referred to Metatitan relictus by Yanovskaya, 1980 ; see below for explanation), and Embolotherium andrewsi ( Yanovskaya, 1980 ) have been described. The skeletons of many North American taxa are better known. They include essentially complete skeletons of Brontops , Dolichorhinus , and Palaeosyops and partial postcranial material from several other North American taxa ( Peterson, 1924 ; Osborn, 1929a ). The postcranial remains of Aktautitan hippopotamopus are compared to Asian taxa when possible, but due to the lack of abundant comparative material, observations of the skeletons of North American species have been included as points of reference. SKULL The bony protuberances that characterize horned brontotheres are formed by a pair of triangular processes of the frontal bone that project anteriorly and overlap a pair of nasal swellings that are plesiomorphically situated superior to and somewhat anterior to the orbits ( Osborn, 1929a ; Mader, 1989 ). A ridge of bone, probably indicating the contact of the frontal and nasal bones, is clearly visible in A. hippopotamopus and forms the expected configuration with the frontal bone overlapping the nasal bone and forming the peak of the frontonasal horns (fig. 4B). There are many derived aspects of the frontonasal region in A. hippopotamopus that resemble Metatitan (sensu Granger and Gregory, 1943) . The use of Metatitan as a point of comparison in the following cranial description refers primarily to M. relictus and M. primus . ‘‘Metatitan’’ progressus shares many characteristics of the frontonasal region, but other aspects of its skull are unknown. In both A. hippopotamopus and Metatitan , the frontonasal protuberances and the free­hanging nasal processes are elevated on tall superorbital pillars formed inferiorly by the nasal bone and superiorly by the overlying frontal bone (figs. 4, 5, 19). These pillars are joined at the midline (though they have become separated in the more obliquely distorted specimens such as KAN N2/873), forming a single frontonasal process that originates above the orbits and projects superiorly and anteriorly at about a 45° angle. The frontonasal horns of other brontotheres such as Protitan , Rhinotitan , and North American horned brontotheres are positioned farther apart, remain separate, and most often project somewhat laterally. In contrast, the frontonasal process of A. hippopotamopus and Metatitan converge medially, and the bony protuberances that form the peak of the frontonasal process are placed close together near the midline of the skull. In both skulls of Aktautitan hippopotamopus the distance from the anterior rim of the orbit to the peak of the frontonasal process is 22 cm . The bony frontal protuberance at the peak of this structure is positioned over the nasal incision. The nasal incision itself extends posteriorly to the level of the P4. The nasal process extends anteriorly from the peak of the frontonasal process and is angled slightly downward. This orientation creates a sharp bend in the nasal bone underneath the frontonasal protuberances. The free­hanging nasal process of A. hippopotamopus is relatively thin, and its lateral margins are folded downward. This morphology is also essentially the same as that of Metatitan . The orbit of Aktautitan hippopotamopus is positioned anterior to the midway point between the anterior and posterior ends of the skull, a condition common to all but the most primitive brontotheres such as Lambdotherium and Eotitanops , where the orbits are more posteriorly located ( Osborn, 1929a ; Mader, 1989 ). The floor of the orbit of A. hippopotamopus is positioned directly above the M2 (fig. 4B). The posterior lateral root of M1 is situated directly below the anterior orbital rim, and the M3 is positioned completely posterior to the orbit. This orbital position is shared by the Asian taxa, Embolotherium and Rhinotitan , and is more anterior than that of Protitan , Metatitan relictus , and, M. primus , where the anterior lateral root of M3 and the posterior lateral root of M2 rest directly below the orbit, but the anterior root of M2 is anterior to the floor of the orbit. The parasagittal ridges of Aktautitan hippopotamopus (fig. 4A) originate from above the postorbital processes of the frontal bone and converge somewhat medially as they run posteriorly toward the nuchal crest. Although the posteriormost regions of both skulls, including the nuchal crests, are not well preserved, it appears as if the parasagittal ridges remained separate throughout their length from the orbits to the nuchal crest. However, it is not possible to determine the degree of constriction of the parasagittal ridges over the parietal region, but they do not appear to have converged into a single sagittal crest. The entire dorsal surface of the skull of A. hippopotamopus , from the peaks of the frontonasal horns to the parietal, appears to have been a continuously concave surface, forming the distinctly ‘‘saddle­shaped’’ cranium that is common to many horned brontotheres ( Osborn, 1929a ). Rhinotitan and Metatitan are notable exceptions to this generality. In these taxa, the dorsal surface of the frontal is concave, but over the parietal region the dorsal surface of their crania becomes convex. In this respect, Metatitan and Rhinotitan resemble some ‘‘hornless’’ brontotheres. The condition of a convex parietal bone is more extreme in Metatitan , however, because the parietal and squamosal sinuses are greatly expanded and the occiput is widened, giving the posterior half of the skull a swollen appearance. A. hippopotamopus lacks these derived Metatitan features. Fig. 4. Holotype skull of Aktautitan hippopotamopus (KAN N2/875). ( A ) Right lateral view tilted slightly so that the dorsal surface of the skull can be seen, ( B ) right view, and ( C ) left view. Fig. 5. Skull referred to Aktautitan hippopotamopus (KAN N2/873). ( A ) Left view, ( B ) right view. The zygomatic arches of A. hippopotamopus are relatively slender (figs. 4, 6A) and lack the large lateral expansions that have been observed in Embolotherium and North American late Eocene brontotheres such as Brontops ( Osborn, 1929a , 1929b ). The zygomata of A. hippopotamopus also lack the posterior zygomatic process, a small projection rising superiorly from the posterior end of the zygomatic arch found in Metatitan relictus and Protitan (Granger and Gregory, 1943) . Unfortunately, the basicranium and occipital regions of both Aktautitan hippopotamopus skulls are badly damaged, but some phylogenetically informative details of the basicranium can be discerned in the holotype . For instance, the posterior nares appear to have been positioned between the M3s (fig. 6A). The posttympanic processes and postglenoid processes are close together, forming a constricted space for the external auditory meatus (fig. 4B). The posttympanic process is much shorter than the postglenoid process, and it curves anteriorly toward the postglenoid process, nearly making contact with it. A thin sliver of sediment can be seen in the constricted space between these two processes. Consequently, the external auditory meatus nearly forms a tube. This condition is common to all horned brontotheres with known basicrania, except Protitan (fig. 19A), where the posttympanic process and postglenoid process are spaced farther apart, creating a wide space for the external auditory meatus. UPPER DENTITION In KAN N2/875 the jaw has been separated from the skull, allowing for a complete description of the upper and lower dentition. The jaw of KAN N2/873 remains cemented to the cranium, limiting inspection of the teeth. Aktautitan hippopotamopus retains an unreduced dental formula 3/3 1/1 4/4 3/3 (fig. 6). The incisors are quite large and packed tightly together without diastemata between them, except for a narrow gap between the central incisors (I1) (fig. 6D, E). The incisor row is arched anterior to the canines. The crown morphology of the upper incisors grades mesiodistally from a blunt, subglobular I1 to a much sharper and taller caniniform I3. A short diastema separates the I3 and the canine. The size and appearance of the upper incisors strongly resemble Rhinotitan . In Metatitan the I1 and I2 are much smaller and are fully globular in appearance, whereas the I3 is of similar size and morphology as that of Aktautitan . There is some evidence of dimorphism in the canines of A. hippopotamopus . The canines of KAN N2/ 875 are larger than those of KAN N2/873 by a greater magnitude than the size differences of most other dental dimensions ( appendix 1 ). There is a short postcanine diastema with a length slightly less than the mesiodistal length of the P2. The labial enamel wall of the P1 is rounded. There is a large paracone and smaller, more lingually positioned metacone (fig. 6C). The P1 crown is expanded lingually, creating a small platform upon which there may have been a protocone or a small crest on the lingual side of the tooth, but the tooth is too worn to discern these details. The P2, P3, and P4 are about as broad as long and become progressively less oblique posteriorly. In other words, in occlusal view, P2 is somewhat rhomboidal, P3 is less rhomboidal, and P4 is essentially rectangular. The distal and mesial sides of the P2–P4 are essentially parallel. The P2, P3, and P4 possess straight ectolophs (mesostyle absent), labially oriented parastyles, and large rounded protocones. Hypocones are distinctly absent on all premolars. Vestigial paraconules and preprotocristas are evident on P2–P3, but are lacking on P4. The upper molars of A. hippopotamopus possess W­shaped ectolophs and isolated conical lingual cusps that characterize advanced brontothere molars (fig. 6B). The nearly unworn M3 indicates that the ectoloph was rather tall (paracone height ~ 65 mm ). A crown height ratio (paracone height/tooth length) yields a value of 0.67. This value falls among indexes calculated on (nearly) unworn M3s of Protitan robustus (0.64), Rhinotitan mongoliensis (0.60), and Embolotherium andrewsi (0.73). An unworn Metatitan M3 is not available. The inner (lingual) band of enamel of the upper molar ectoloph is extremely thin, much thinner than the outer (labial) band. A thinner inner band of enamel is a character common to all but the most primitive of brontotheres (e.g., Lambdotherium and Palaeosyops ), where the inner band of enamel is much thicker, particularly around the apices of the paracone and metacone. Hypocones are present on all three molars of Aktautitan and are always smaller than the protocone. All vestiges of paraconules, metaconules, protolophs, and metalophs are absent on the molars. Each molar has a small, shallow central fossa that is positioned at the lingual base of the ectoloph, directly between the paracone and metacone, and labial to the protocone. This fossa is absent among some hornless brontotheres, and is present among all large, horned brontotheres (Mihlbachler, unpubl. data). The depth of the central molar fossa in Aktautitan is similar to that of Protitan and Rhinotitan . It is much deeper in Metatitan and Embolotherium . The cingulum on the anterior (mesial) side of the molars rises to a short peak near the anterolingual corner of the crown. This shallow peak is identifiable on M1 and M 2 in KAN N2/875 (fig. 6B) by the development of small wear facets on the anterior cingulum. This anterolingual cingular cusp is a distinctive feature common to Aktautitan , Metatitan , and Embolotherium , although it is significantly taller and more distinct in Embolotherium in comparison to the other two genera. Fig. 6. Ventral view of Aktautitan hippopotamopus holotype skull (KAN N2/875). ( A ) Ventral view of skull, ( B ) molars, ( C ) premolars, ( D ) right incisors and canine, lingual view, and ( E ) right incisors and canine, labial view. MANDIBLE AND LOWER DENTITION The mandible is rather tall with a tall, slender coronoid process (fig. 7A). The proportions of the ramus are similar to those of Rhinotitan and Metatitan . The posterior extent of the symphysis is obscured by matrix in KAN N2/875 (fig. 7B). In KAN N2/873 the symphysis extends to below the trigonid of the p4 (fig. 5B). The incisors are short, conical, and recurved with blunt points and thin lingual cingulids (fig. 7D, E). They are packed tightly together. The lower incisors of Rhinotitan are similar in size but are more spatulate; those of Metatitan are globular and much smaller in size. In Aktautitan hippopotamopus , the lingual cingulid is strongest on the central incisors and progressively weakens in more distal incisors. The incisors are positioned anterior to the canines and the entire anterior toothrow forms a semicircular arch. The i2 is the largest lower incisor. This is most notable in labio­lingual width and crown height. The only diastema in the lower dentition is the postcanine diastema, which is similar in length to the mesiodistal length of p2. The p1 is mesiodistally elongate with a single cusp (fig. 7C). From p2 to p4 the premolars grade distally into a progressively molariform morphology. In p2, the talonid and trigonid are nearly the same width. In p3 and p4 the width of the talonid is notably wider than the trigonid. The paralophid of the p2 is short and angles only slightly lingually. In p4 the paralophid is longer and curves lingually a full 90°, as do the paralophids of the molars. The p3 is intermediate in its length and degree of curvature of the paralophid. The p2 lacks a metaconid. The protocristid extends posteriorly and lingually from the apex of the protoconid and joins the cristid obliqua posteriorly and slightly lingually from the protoconid. A small metaconid is present on p3 and is positioned about equally lingually and posteriorly from the protoconid. The cristid obliqua joins the metaconid of the p3. The p4 possesses a fully molariform metaconid that is positioned mostly lingually from the paraconid. Finally, the hypolophid of the p2 is relatively short and projects posterolingually from the hypoconid at about 45°from an anteroposterior orientation. In p3 and p4 the hypolophids are longer and arch a full 90° from an anteroposterior orientation from the hypoconid. As a general observation, the lower molars of brontotheres are almost morphologically static and possess few features of taxonomic interest. However, a few noteworthy observations can be made about those of Aktautitan hippopotamopus (fig. 7B). Despite the rather tall ectolophs of the upper molars, the lophids of the lower molars are remarkably low, with shallow talonid and trigonid valleys. A thin, beaded cingulid traces entirely around the hypoconulid heel of the m3. The molars of A. hippopotamopus are notably elongate. Elongation of the molars is generally thought to correlate to the relative degree of cranial elongation among brontotheres ( Osborn, 1929a ). Molar elongation in brontotheres is most evident in the m3, where the length/width ratio varies from a minimum of 1.59 ( Eotitanops borealis ) to a maximum of 2.94 ( Gnathotitan berkeyi ) (Mihlbachler, unpubl. data). The m3 of A. hippopotamopus is among the most elongate among brontotheres with a length/width ratio of 2.93. This ratio is similar in magnitude to a number of middle and late Eocene Asian brontotheres, including Protitan , Microtitan , Gnathotitan , Rhinotitan , Metatitan , and Embolotherium . Fig. 7. Mandible and lower dentition of Aktautitan hippopotamopus holotype (KAN N2/875). ( A ) Lateral view of right ramus, ( B ) anterior view or mandible, ( C ) premolars, ( D ) incisors and canines, lingual view, and ( E ) incisors and canines, labial view. Fig. 8. Atlas of Aktautitan hippopotamopus holotype (KAN N2/875). ( A ) Anterior view, ( B ) anterior view, and ( C ) posterior view. VERTEBRAE The atlas (fig. 8) has been laterally compressed and the transverse processes are partial, preventing measurement of transverse width. The dorsal and ventral arches support tall, massive medial tubercles, although the height of these tubercles seems exaggerated by the lateral distortion. The intervertebral foramen is visible on the dorsal surface of the left lateral mass and is similar in size to that of other brontotheres. The transverse foramen is large in early brontotheres such as Telmatherium and Palaeosyops , but is lost in many late Eocene North American brontotheres (e.g., Brontops ) ( Osborn, 1929a ). The transverse foramen is visible in the atlantes of Rhinotitan mongoliensis and cf. Parabrontops gobiensis . In Aktautitan hippopotamopus , damage to the specimen complicates description of the transverse foramen, but it is evident that it was either very small or absent. A small dimple can be seen on the ventral surface of the right lateral body in the position where the vertebral artery would be expected to pierce the atlas, but there is no sign of the transverse foramen on the posterior side of the lateral body where the vertebral artery would be expected to exit the atlas. There is an additional cervical vertebra with a relatively short neural spine, a transversely wide but anteroposteriorly short centrum, and steeply angled prezygophophyses (fig. 9A, B). Two thoracic vertebrae have been completely removed from the matrix block, one with a very tall, posteriorly angled neural spine (fig. 9C, D) and one with a much shorter neural spine (fig. 9E, F). Preservation of these elements is poor, but the sizes and proportions are similar to those of other large brontotheres, including Rhinotitan mongoliensis and cf. Parabrontops gobiensis . FORELIMB Both scapulae (fig. 10) of the holotype are nearly complete. Both are figured because, together, they allow a complete description of its shape. The scapular neck is short and broad. The supraspinous fossa is narrower than the infraspinous fossa. The anterior border is slightly curved. The posterior border is triangular in outline. The infraspinous fossa widens proximally from the glenoid cavity and reaches its widest point about two­thirds of the length of the scapula from the glenoid cavity. Proximal to this point, the infraspinous process narrows. The anterior margin of the scapula is rounded. These characteristics are typical of brontothere scapulae ( Osborn, 1929a ). The shape and proportions, particularly the very wide, triangular infraspinous fossa, are similar to the scapulae of Brontops robustus and Metatitan relictus . The scapula of Rhinotitan is somewhat narrower and resembles more closely those of hornless brontotheres, particularly that of Dolichorhinus . Fig. 9. Vertebrae of Aktautitan hippopotamopus holotype (KAN N2/875). ( A ) Cervical, anterior view, ( B ) cervical, posterior view, ( C ) thoracic, anterior view, ( D ) thoracic, posterior view, ( E ) thoracic, anterior view, and ( F ) thoracic, posterior view. The humerus (fig. 11) is similar in appearance to that of Rhinotitan and Metatitan relictus . It is relatively short and highly constricted at the midshaft. The lateral tuberosity is anteroposteriorly wide, but transversely it is slender and rises well above the humeral head. The deltoid tuberosity does not rise higher in relief than the deltoid crest that connects it with the lateral tuberosity. This differs markedly from that of Brontops , where the deltoid tuberosity stands much higher than the deltoid crest, forming a distinct tubercle. On the distal end of the humerus, the lateral epicondyle is large, rugose, and expanded laterally. The olecranon fossa is very wide. The trochlea itself is very shallow and is markedly asymmetrical, with the medial condyle being larger than the lateral condyle. The articulated lower elements of the forelimb, distal to the humerus, of the holotype (KAN N2/875) remain in the large block of matrix and are not readily described. However, the articulated lower forelimbs of KAN N2/873 (fig. 12) are readily described. The radius and ulna appear to have been relatively straight and are not notably different from those of other brontotheres. The olecranon process of the ulna is missing. The fully articulated forefoot indicates a digitigrade foot of graviportal proportions, with four digits. The articulated and heavily cemented state of the carpals prevents a detailed description of individual articular facets, but from what can be discerned, the articular relationships of the individual carpals are essentially the same as those of other brontotheres. The carpus is relatively broad, but not as flattened as those of Embolotherium andrewsi or Brontops robustus . The metacarpals are remarkably short and stout, more so than any other brontothere for which a relatively complete manus is known, although this effect has been exaggerated by vertical crushing of the left manus. The proportions of the metacarpals of the right manus are more nearly intact. HINDLIMB The innominate (fig. 13) is relatively slender and somewhat intermediate in its length. The superior border of the iliac crest is rugose, but the iliac blade is relatively narrow and slender. The iliac crests of Brontops robustus , Metatitan relictus , and Rhinotitan mongoliensis are broader, although it is not known how much of this difference is a result of distortion. The proportions of the iliac shaft and the remainder of the innominate are more intact. They are similar in proportion to those of Metatitan relictus and Rhinotitan mongoliensis , which in turn are similar to those of hornless brontotheres such as Telmatherium validus and Dolichorhinus hyognathus . The iliac shaft of Brontops robustus is shorter and broader, whereas those of very primitive brontotheres, such as Palaeosyops , are longer and more slender. Fig. 10. Scapulae of Aktautitan hippopotamopus holotype (KAN N2/875). ( A ) Left, ( B ) right. Both femora (fig. 14) are preserved in the holotype but are distorted in different ways; the left femur is crushed vertically, and the right side is crushed anteroposteriorly. Due to the distortion, the two bones are of different lengths. The left specimen is artificially shortened due to the vertical collapse of the head of the femur, but both femora are nearly proportionate in length from the distal end to the third trochanter. The right femur seems to more closely approximate the true length. Notable aspects of femoral morphology of Aktautitan hippopotamopus are the nearly flat femoral head, the curved shaft, and the very small, indistinct second and third trochanters. The third trochanter is much lower on the shaft than the second trochanter. Fig. 11. Right humerus of Aktautitan hippopotamopus holotype (KAN 2/875). ( A ) posterior view, ( B ) anterior view. Fig. 12. Lower forelimb and manus referred to Aktautitan hippopotamopus (KAN N2/873). ( A ) Left manus, medial view, ( B ) left manus, anterior view, ( C ) left manus, lateral view, ( D ) right manus, anterior view, ( E ) and right lower forelimb, medial view. More primitive, hornless brontotheres differ from A. hippopotamopus in having a more spherical head and more prominent second and third trochanters. Rhinotitan mongoliensis and Metatitan relictus differ from A. hippopotamopus in similar respects, although the trochanters of these species are intermediate in size. (Note that the femoral head of Metatitan relictus is unknown). The femur of Brontops robustus is straighter than that of A. hippopotamopus , and the third and second trochanters are more nearly opposite each other, although the sphericity of the femoral head of Brontops robustus is similar to that of A. hippopotamopus . The distal end of the left femur of A. hippopotamopus is intact. The medial side of the distal extremity of the femur is significantly wider anteroposteriorly than the lateral side. This is partly due to a medial trochlear ridge and a medial condyle that are larger than the lateral counterparts. Although there is some variability in the degree of asymmetry of the trochlea of the distal femur among brontotheres, the degree of asymmetry never approaches the extreme asymmetry found in the distal femoral trochlea among horses and rhinos (Hermanson and MacFadden, 1996). Fig. 13. Left innominate of Aktautitan hippopotamopus holotype (KAN N2/875). ( A ) posterior view, ( B ) lateral view. Fig. 14. Femora of Aktautitan hippopotamopus holotype (KAN N2/875). ( A ) Left, anterior view, ( B ) right, anterior view, ( C ) right, medial view, and ( D ) left, distal view. The patella (fig. 15) is rather narrow, much more so than that of extant horses and rhinos, as are those of other brontotheres. The patella of KAN N2/875 has a flat superior margin and a prominent apex. The lateral margin is relatively flat, and the mesial margin is strongly rounded. These characteristics fall within the range of brontothere patellar morphologies. The superior surfaces of brontotheres patellae are sometimes slightly concave (e.g., Brontops robustus ). The mesial and lateral margins can be slightly concave as well (e.g., Rhinotitan mongoliensis ), giving the patella a somewhat ‘‘waisted’’ appearance, although this is not the case in A. hippopotamopus . The lower elements of the left hind limb (fig. 16) consist of an articulated tibia, fibula, and pes. The tibia is much shorter and stouter than the femur; its relative length is similar to that of other large, horned brontotheres. The metatarsals are very short and flat in proportion to the length of the more proximal limb elements and the calcaneal tuber. The calcaneal tuber is comparatively very long and is actually longer than the third metatarsal. The calcaneum itself is not necessarily derived in this respect; the calcaneal tubers of other brontothere tend to be elongate; however, the very short metatarsals of Aktautitan is an extreme condition among brontotheres whose limbs are known. The articulated feet of Rhinotitan mongoliensis , cf. Parabrontops gobiensis , Brontops robustus and more primitive hornless brontotheres are taller and more slender.