Forelimb Posture in Neoceratopsian Dinosaurs: Implications for · Gait and Locomotion

Paleobiology, 26(:3), 2000, pp. 450-4.65 FORELIMB POSTURES IN NEOCERATOPSIAN DINOSAURS 451

Forelimb posture in neoceratopsian dinosaurs: implications for · gait and locomotion

· Gregory S: Paul and Per Christiansen"

Abstract. ~Ceratopsid dinosaurs traditionally have been restored with sprawling forelimbs and were considered unable to run at high speeds. An alternative view restores the ceratopsids as rhinoceros-like ",ith parasagittal forelimb kinematics and the ability to run faster than extant ele- .phants ..Several anatomical difficulties concerning the mounting of ceratopsid skeletons with nearly parasagittal forelimbs stem not from the forelimb itself, but from errors in rib and vertebral articulation. Matching a skeletal restoration to a probable ceratopsid trackway shows that the hands ·were placed directly beneath the glenoids, and that manu~l impressions were directed laterally, not medially as in sprawling reptiies. Pedal impressions in trackways are medial to the manual im- · pressions, owing to the slightly averted elbow and to the asymmetrical distal femoral condyles, . · which directed the crus slightly medially. The limbs of ceratopsians of all sizes display substantial joint flexure, strongly indicating that the elephantine forelimb posture that has sometimes been suggested as the alternative to a sprawling posture is erroneous. The articular surfaces of uncru- shed ceratopsian scapulocoracoids and forelimb joints confirm that the forelimb operated in a near- ·parasagittal plane with the elbows only slightly averted. The maximal running speed of even the largest ceratopsids is inferred to have Significantly exceeded that of elephants and was probably . broadly similar to that of rhinos.

Gregory S. Paul. 3109 North Calvert Station, Side Apartment, Baltimore, Maryland 21218. Esmail: [email protected] Per Christiansen. Department of Vertebrates, Zoological Museum, Universitetsparken 15, 2100 Copen- hagen 0, Denmark. E-mail: [email protected] Accepted: 20 March 2000· rIiI ~- Introduction The forelimb posture and locomotion of cer- ~ G -- Large ceratopsids (Dinosauria: Ornithis- atopsians, particularly the ceratopsids, has chia) have often been considered the dinosau- been controversial. As noted below, most pre- rian equivalent to the Rhinocerotidae because vious attempts to solve this problem have of- of convergent resemblances in overali mor- ten either neglected several anatomical as- phology (e.g., Alexander 1985, 1989, 1991i pects or failed to fully incorporate the important evidence provided by trackways. Fore- FIGURE 1. Skeletal restorations ih side- and multi view of ceratopsians compared with large perissodactyls. A, Lep- Bakker 1986, 1987). Dodson and Farlow ioceraiops NMC 8889 (93 kg). B, Cenirosaurus AMNH 5351 (1460 kg). C, Chasmosaurus NMC 2280 (1520 kg). D, An- (1997), however, cautioned that these resem- limb posture in ceratopsians has been chiceraiops NMC 8538 (1180 kg). E, Triceratops or Torosaurus, based primarily on USNM 4842 (6420 kg). F, Rhinoceros blances in some cases appear to have been restored as wide-gauge and sprawling by Gil- USNM 12450 (1000 kg). G, Brontaps YPM 12048 (3300 kg). Scale bar, 1 m. . overemphasized, as dinosaurs are phyloge- more (1905), Sternberg (1927), Tait and Brown " netically distinct from mammals. Nonethe- (1928), Osborn (1933), Russell (1935), Erickson difficult to restore than that of the hindlimb, ly. In fact, the common use of the word "erect" less, solutions to the problems of terrestrial (1966), Farlow and Dodson (1975), Russell as the latter presents the advantage of a pelvis can cause confusion (e.g., Dodson 1996i Dod- support of mass and locomotion with.a large (1977), Czerkas and Czerkas (1990), and most firmly attached to the vertebral column. son and Farlow 1997), as this could be taken body mass may well be constrained, implying recently Johnson and Ostrom (1995) and Dod- The debate over forelimb posture in cera- to suggest a nearly columnar stance. By this similar solutions to similar problems, as ap- son (1996). A narrower forelimb gauge and a topsians often appears unnecessarily polar- definition most large mammals are not erect pears to be the case in the sauropod-probes- more parasagittal limb posture were favored ized, forcing a choice between a fully erect, al- either (Fig. 1). The issue of forelimb posture in cidean analogy (e.g., Christiansen 1997). A by Marsh (1891a,b), Hatcher et al. (1907), Bak- most elephantine posture, with all three long- ceratopsians is primarily a question of wheth- representative sample of several ceratopsian ker (1971, 1986, 1987), Ostrom and WeHnhofer bones kept essentially vertical, or a sprawling er the humerus operated in a largely parasag- genera is presented in Figure 1 and compared (1986: Fig. 8), Paul (1987, 1991, 1997), Adams posture, with the humerus held horizontally ittal fashion or employed significant medio- with large perissodactyls (Paul 1997: Appen- (1991), Tereshchenko (1994), Lockley and (e.g., Czerkas and Czerkas 1990: p. 212; John- lateral rotation during locomotion. dix figs. 2,6). Hunt (1995), Ford (1997), Garstka and Burn- son and Ostrom 1995: Fig. 12.1; Dodson 1996: The question of restoring limb posture has ham (1997), and. in part Dodson and Farlow Fig. 9.7). This is an oversimplification and is two basic and related aspects. One is fore and • Corresponding author (1997), the latter view in accord with available actually misleading, because the available ev- aft limb posture in lateral vie~hich ranges trackway .evidence. Forelimb posture is more idence suggests that neither posture was like- from highly flexed, the plesiomClrphic tetra-

pod condition (e.g., Romer 1956;Young 1981), laeontological Institute of Uppsala University; to fully erect. The other is transverse limb pos- TMM, Texas Memorial Museum; UNM, Uni- ture, which ranges from sprawling (the ple- versity.of New Mexico;USNM, National Mu- siomorphic tetrapod condition), through seum of Natural History; UTEI', Centennial semierect. largely parasagittal, to nearly co- Museum, University of Texas;WFQ, Warfield lumnar. It is important to bear in mind that Fossils; YPM, YalePeabody Museum. even fully erect limbs usually do not operate in an entirely parasagittal manner, as humeral Limb Posture and Trackways and femoral aversion and inversion is often Trackways can provide good evidence for present (Muybridge 1957; Gambaryan 1974; limb posture in the transverse plane for ex- Paul 1991). Most ceratopsids were large ani- tinct animals, providing that the track maker mals, although no larger than extant rhinos, can be identified with a reasonable degree of but Triceratops, Torosaurus (Fig. 1), and Penta- certainty. It is therefore important to recog- ceraiops were elephant-sized (Paul 1997). nize the role trackways can play in restoring Restoring limb posture in dinosaurs pre- limb posture in dinosaurs. Any restoration of sents problems, however, largely because their normal walking posture and limb action must distal articulating facets often show a consid- be in full accord with available trackways in erably pO.orerdegree of ossification than those terms of manual and pedal placement, orien- of mammalian longbones. This often pre- tation, and stride length. cludes articulating the bones and observing Ceratopsian tracks have been reported pre- their tightly controlled limits of protraction- viously (e.g., Lockley 1986,1991;Currie 1993; retraction and abduction-adduction. Attempt- Ford 1997), but probably one of the best cer- ing to restore dinosaur limb posture is some- atopsid trackways, Ceratopsipes goldenensis, has what analogous to restoring limb posture in been recognized only recently (Lockley and ••• immature birds and mammals, in which-car- Hunt 1995). The track maker was almost cer- .. C tilaginous joints are still. present. Whether tainly a gigantic ceratopsid, perhaps even Tri- - FIGURE2. A, Multiview limb-skeletal restoration of Triceratops or Torosaurus based primarily on USNM 4842 (see protoceratopsids sometimes employed bipe- ceratops, as it was not only by far the most com- Fig. JE), shown walking out a probable ceratopsid trackway (from Lockley and Hunt 1995). Anteriormost manus dality during fast locomotion (Bakker 1968; mon ceratopsian at the time (Lockley and print about to be impressed is outlined (modified after Paul 1991: Fig. 2). Skeleton and trackway shown to scale; Coombs 1978) or remained quadrupedal Hunt 1995;Dodson and Farlow 1997)but it is scale bar, 1 m. Because breadth of manus (about 400 mm) and pes (about 500 m) and estimated glenoaeetabular length (1.5-1.7 m) are similar in both skeleton and trackway, the skeleton appears to approximately match or only (Thulborn 1982; Tereshchenko 1994)has been also one of the few ceratopsids so far discov- slightly exceed the size of the track maker. B, C. Trackways of walking Diceros (B) and eonl1ochaetes (e) (modified controversial, but it is unlikely that ceratop- ered that was large enough to have been able after Bird 1987). All track ways drawn to same manus stride length. Manus prints indicated by short medial lines. sids used bipedality. to make the trackway. In this paper we aim to show that the most The trackway is about 1.25 meters across likely posture of the ceratopsian forelimb was (Lockleyand Hunt 1995).However, this figure one suggested by Gilmore (1905),Sternberg probably also imply that the hands would largely parasagittal, albeit far from columnar, represents the width across the lateral edges (1927), Osborn (1933), Russell (1935), Hal- tend to "smear" the toe imprints, but this is and that several of the difficulties encountered of the manual impressions, whereas their me- stead and Halstead (1981),Czerkas and Czer- not present in the trackway series. No one has in mounting skeletons with a parasagittal dial edges are less than one meter apart, ap- kas (1990),Johnson and Ostrom (1995), and yet attempted to mount a ceratopsid skeleton forelimb posture, e.g., the medial tuberosity proximately the width between the glenoids Dodson (1996),among others. in a walking pose that paces out one of these proximally on the humerus discussed by Dod- of a Triceratops with similar-sized hands. The Dodson (1996) suggested ·that the fact that trackways. In this paper, only results that suc- son and Farlow (1997),in fact appear to be the trackway shows that the hands were almost the manual impressions were slightly lateral cessfully place the feet of ceratopsids into result of errors in articulating the axial skele- directly .beneath the glenoids, and thus to the pedal impressions was inconsistent their trackway positions will be accepted as ton. Thus, morphology of the axial skeleton strongly argues against a sprawling forelimb with a parasagittal forelimb posture, because likely. the pelvic region is wider than the shoulder will also be discussed. We will also use the posture (Fig. 2A) (Ford 1997). The trackway Axial and Girdle Morphology trackway evidence as a reference for assessing indicates that forelimb gauge was, however, region in ceratopsids. However, as noted be- limb posture and gait. slightly wider than shown in previous resto- low and shown in Figure 1, the pelvic region Presacral Column.-We agree with Osborn Institutional abbreviations are as follows: rations (compare Figs. IE, 2 with Paul 1987: of ceratopsids is not markedly wider than the (1933)that the presacral vertebrae of ceratop- AMNH, American Museum of Natural His- Fig. 23). Rough calculations indicate that the shoulder region. Additionally, the morpholo- sians articulated in a gentle S-curve, with the tory; CM, Carnegie Museum of Natural His- width across the lateral edges of the manual gy of the distal femoral articulating facet in- dorsally convex part of this curve along the tory; FMNH, Field Museum of Natural His- impressions of a. Triceratops-sized animal dicates that the crus was turned slightly me- dorsals (Fig. lA-E) (Hatcher et aL 1907:Plate tory; MPM, Milwaukee Public Museum; would have been in excess of two meters if the dially during normal walking. We agree with XL;Brown and Schlaikjer 1940:Fig. 24; Stern- NMC, National Museum of Canada; PMD, Pa- animal walked with a posture similar to the Ford (1997)that the sprawling posture would berg 1951:Plate LIVB;Garstka and Burnham 455 454 GREGORY S. PAUL AND PER CHRISTIANSEN FORELIMB POSTURES IN NEOCERATOPSIAN DINOSAURS

1997).This curvature is also presentin the best (Edwards 1976). Certain large mammals also naturally articulated specimens, e.g., Centro- use lateral trunk movements during slow lo- saurus AMNH 5351 (Brown 1917; Lull 1933) comotion, e.g., plantigrade ursids, but rela- and is correctly reproduced in most restora- tively stiff-trunked ungulates have no such tions and mounted skeletons. In life this cur- shift in locomotory mechanics. There is little vature was fixed not only by the angulation of reason to suppose that this was the case in cer- the central articulating facets, but also by a se- atopsids either, particularly as they also ap- ries of well-developed ossified tendons, as can pear to have had relatively stiff trunk regions. be seen in Triceratops USNM 4928 (Hatcher et Rib Cage.-The posterior ribs of ceratopsids al. 1907) Centrosaurus AMNH 5351 (Brown were subvertical in lateral view. In anterior 1917),and Triceratops WFQ 9309 (Garstka and view they were curved, thus forming a broad, Burnham 1997), among others. A dorsally roundish abdominal cavity, similar to the concave dorsal vertebral column is therefore large subcircular abdominal cavity present in the result of postmortem distortion due to many extant ungulates (see, for instance, pos- desiccation of epaxial muscles and ligaments, terior views of Bos and Equus in Muybridge rather than the natural condition during life 1957). The belly appears to have been rela- (contra Garstka and Burnham 1997: Fig. 5). tively broadest in Chasmosaurus (Fig. 1C), Two important, and often overlooked piec- which in part could be the reason for modifi- es of evidence in the presacral column of cer- cation of the distal femoral condyles, de- atopsians that contradict a sprawling forelimb scribed below. Rib cages from well-preserved posture are the above-mentioned ossified ten- ceratopsid specimens indicate that the mid- dons and the tight fit of the prepubic process and posterior ribs became increasingly curved with the posterior ribs, described below. in lateral view, closely approaching each other These would have significantly stiffened the distally, e.g., Centrosaurus AMNH 5351 or An- trunk. Sprawling reptiles extensively use lat- FIGURE3. Multiple views of right scapular glenoid of Triceratops Or To,·osaurus USNM 8013 in jateral (A), posterior chiceraiops NMC 8535 (Brown 1917;Lull 1933). (B), posteroventral (C), and ventral (D) views. The scapula is oriented to show the gle(loid in natural ar ticu lation, eral undulations of the vertebral column dur- Osborn (1933) suggested that one of the as per Figure 1. ing locomotion (e.g., Romer 1956;Young 1981; posterior ribs articulated with the anterolat- Carrier 1987; Sennikov 1989; Reilly and De- eral edge of the prepubic process. The articu- mals (Ellenberger et al. 1956; Paul 1987; Car- lancey 1997). These undulations are an exten- lated contact is preserved in Anchiceraiops archosaurs, the anterior ribs app<:ar to have penter et al. 1994).The shoulder region, how- sion and slight modification of the lateral NMC 8535 (Lull 1933), and the articular ru- been swept strongly posteroventrally (Paul ever, remained almost as transversely wide as undulations employed by fish (Romer 1956; gosity is present on a partly disarticulated 1987, 1997; Carpenter et al. 1994; Ford 1997; the hip region (Fig. lA-E), a condition similar Edwards 1976; Young 1981). posterior rib of Cenirosaurus AMNH 5351, in correctly mounted in Centrosaurus YPM 2015, to that of extant large utlgulates and elephants Large mammals walk and run with signif- which the rib heads are partially disarticulat- Plate IIA in Lull 1933). icantly less crouched limbs than small mam- ed from the diapophyses. The great majority This rib cage morphology had two effects. (Ellenberger et al. 1956). Pectoral Gird/e.-The second consequence of mals, in order to maintain bone stresses with- of skeletal restorations and mounts, however, First, as the capitulum and tuberculum are set the posterior sweep of the anterior ribs is the in reasonable limits (e.g.,Biewener 1990),even lack this unusual posterior rib cage articula- at a quite acute angle with respect to the long more posterior placement of the pectoral gir- though their longbones scale almost isomet- tion, probably erroneously. The bracing pro- axis of the rib, a posterior inclination also dle (Paul 1987,1997;Ford 1997)than is usually rically (Christiansen 1999).Accordingly, small vided by this tight-fitting complex appears to twists the ribs medially, thus narrowing the displayed in most skeletal Iflounts (Gilmore mammals run in a slightly different fashion have further stiffened and strengthened the anterior chest region (Fig. 1B,C,E)(Paul 1987, 1905;Sternberg 1927;Lu!11933: Plate IIA; Os- than larger mammals (Alexander 1991). Like trunk. 1997; Carpenter et al. 1994; Ford 1997). This born 1933; Brown and Schlail

AMNH 5372 (Brown and SchIaikjer 1937), atopsids (Fig. 4B-D). The slight lateral flaring have cotyloid glenoids that mainly face pos- of the ventral part of the glenoid probably al- teroventrally and that have only a moderate lowed the humerus to slope an estimated 20- ventrolateral flaring, especially on the cora- 250 ventrolaterally (Figs. 1A-E, 3A, 4E, 6D). coid part. Good examples are Triceratops The orientation of the scapula ranges from USNM 4800 (Hatcher et a1.1907),Chasmosau- horizontal to almost vertical in articulated cer- rus TMM42303-1(Lehman 1989),Peniaceraiops atopsian specimens (Lull 1933:Plate IXA).The UNM FKK-081 (Lehman 1993). Crushing former is depicted in some restorations (Stern- sometimes exaggerates this lateral flaring, as berg 1927;Erickson 1966;Johnson and Ostrom in Torosaurus MPM VP6841 (Johnson and Os- 1995), but horizontal scapulae appear to be trom 1995). largely limited to specialized tetrapods with The dorsal part of the glenoid appears very elongated coracoids, such as certain ad- mainly to have faced slightly medially in most vanced non-avian theropods, birds, and cases. With a correctly oriented, sharply in- pterosaurs (Paul 1987, 1988). In most tetra- clined scapula and coracoids that almost meet pods the scapula is posteriorly inclined, and at the midline, the dorsal part of the glenoid this is the most likely orientation in ceratop- would face even slightly more medially, as sian dinosaurs as well (Fig. 1A--G) (Russell also concluded by Ford (1997).This appears to 1970; Ostrom and Wellnhofer 1986; Teresh- have been the case in well-preserved ceratop- chenko 1994; Ford 1997; Garstka and Burn- sids as well as protoceratopsids. This poster- ham 1997). oventral orientation of the glenoid probably Scapular Mobility.-One aspect of ceratop- would have prevented the humerus from sig- sian limb function that has generated substan- nificant lateral mobility. tial controversy is scapular mobility. Well-de- The cotyloid glenoid fossa also strongly ar- veloped scapular mobility is favored by Bak- gues against a sprawling forelimb, as the ker (1986, 1987), Paul (1987), Tereshchenko thrust from carrying the body mass would FTGURE4. Comparison of relative forelimb flexion, with left limb elements in planar lateral view, drawn to same (1994),and in part Gilmore (1919),while Ben- humerus length. A, Columnar-limbed Apatosaurus (A). B-D, Animals with joint flexure in the limbs: Leptoceratops have been directed mediodorsally upon con- nett and Dalzell (1973),Coombs (1978),Thul- (B), Chasltlosaurus (C), and Triceratops (D). E, Estimated maximum protraction and retraction of humerus in Tricer- traction of the presumably massive M. pecto- born, (1982), Adams (1991), and Dodson atops. ralis. However, the glenoid does not have a (1996) suggested that it was either very lim- wide flaring to brace the humeral caput me- ited or did not occur. Scapular mobility is pre- to the ribs (Lull 1933:Plate IIA; Osborn 1933; girdle was set far posteriorly in ceratopsians, dially, and thus there appears to have been lit- sent in a variety of amniote groups; it occurs Erickson 1966)are probably erroneous, as also the posterior sweep of the anterior chest ribs tle osteological support for a lateral thrust, as in several lizards (Jenkins and Goslow 1983) concluded by Adams (1991) and Ford (1997). meant that the glenoid was still situated an- noted for the acetabulum-femur articulation and is well developed in chameleons (Peterson In laterally compressed, articulated speci- terior to the ribs (Fig. lA-E). This is similar to in Plaieosaurus by Christian et al. (1996).Even 1984; Bakker 1986, 1987), crocodilians, and mens the sternals are usually displaced antero- the condition in many extant birds and rep- columnar-limbed sauropods often have some many mammals (Fig. 5B-D) (Gambaryan ventrally, perhaps because of bloating of the tiles, but it differs from the condition in mam- lateral flaring of the glenoid, especially in the 1974; Jenkins and Weijs 1979). Although the carcass (e.g., Lull 1933:Plates IXA, X).Antero- mals, in which the glenoid is lateral to the ribs coracoid part, although it is usually less than functional anatomy differs markedly among ventral displacement of the sternals would (Fig. lEG). in ceratopsians. The rugosity of the sauropod the various groups, it is clear, however, that also tend to pull the episternum and scapu- The glenoids of sprawling reptiles face pre- glenoid often makes this character hard to ver- scapular mobility is common among arnni- locoracoid anteriorly, thus giving the false im- dominately laterally to allow the humerus to ify, however, and some specimens appear to otes. pression of a substantial part of the shoulder perform significant dorsoventral and antero- lack this feature. In ceratopsids the glenoid A substantial degree of mobility is retained girdle being anterior to the chest ribs (e.g., posterior movements, and it is usually some- was directed as much as 500 more posteriorly among large mammals, although it is reduced Centrosaurus AMNH 5351; Brown 1917: Plate what saddle-shaped (e.g., Romer 1956;Young along the parasagittal plane than in columnar- in columnar-limbed elephants (Fig. 5D) XI). Apart from being unrealistic, such a po- 1981; Bakker 1986,1987). The glenoids of cer- limbed sauropods and stegosaurs (Fig.4A-D). (Gambaryan 1974). To attain scapular mobili- sition would leave the girdle with insufficient atopsians did not look like this (Fig. 3). The Glenoid morphology indicates that the hu- ty the coracoids can be lost, as in most mam- support from the M. serratus ventralis. glenoids of parasagittal mammals are coty- meral posture was ungulate-like (Figs. 5B,C mals, or alternatively, the coracoids can be al- Rather, the scapula was astride and con- loid in shape and the articulating facet faces and 6) with flexion primarily in the antero- lowed to slide fore and aft in a groove on the nected to the chest ribs as in extant tetrapods posteroventrally. Well-preserved, largely un- posterior plane, rather than transverse as in lateral edge of the sternum, as in Varanus (Jen- except turtles (Romer 1956; Pritchard 1979; crushed ceratopsian scapulocoracoids, e.g., reptiles. In ceratopsids the glenoids were kins and Goslow 1983). Paired clavicles are Young 1981;Ford 1997).It is important to bear Pentaceratops PMUR268 (Wiman 1930),Centro- probably directed approximately 10° more present in Protoceratops and Leptoceratops but in mind, however, that although the pectoral saurus YPM 2015 (Lull 1933), or Styracosaurus ventrally than was the case in small protocer- have not been found in ceratopsids yet (Dod- f·

458 GREGORY S. PAUL AND PER CHRISTIANSEN FORELIMB POSTURES IN NEOCERATOPSIAN DINOSAURS 459

FIGURE6. Forelimb articulation and posture studies. Shoulder glenoids in naturally articulated lateral and posterior views of Leptoceratops (A) AMNH 5205 (in part after Brown 1914) and Triceratops or Torosaurus (C) USNM 8013 and other specimens (modified after Paul 1987). Left forelimb in neutral standing pose of Leptoceratops (B) AMNH 5205, in naturally articulated lateral view, and Triceratops or Torosauru$ (C) USNM 4842, USNM 8013, AMNH 970, and other specimens, in lateral, anterior, and posterior views. In D, stippling on proximal ulna delin- eates articular facets from olecranon process; manus print orientation Indicated. Illustrations not to scale.

mobility. As described above, the anterior with sprawling forelimbs (e.g., Sternberg chest region in ceratopsians was deep and 1927; Osborn 1933; Brown and Schlaikjer FICURE5. Motion studies showing limb flexion and action. A, Restored galloping Triceratops or Torosaurus based only gently curved, and correspondingly, cer- 1940; Erickson 1966; Johnson and Ostrom primarily on USNM 4842, includes anterior view of forelimb and possible point of scapular rotation indicated by atopsian scapulocoracoids a~e not strongly 1995; or the Triceratops mount at FMNH) the a white dot. B, Fast galloping Equus. C, Slow galloping juvenile Rhinoceros. D, Ambling Elaphus. Band Dafter Muy- bridge 1957, and C after film. Not to scale. • curved (see also Ford (1997).We suggest that ribs are too vertical and broad, as noted above, ceratopsian scapulocoracoids were free to ro- and the coracoids set too far apart. As a result, tate (Fig. SA),although the great scapular mo- the glenoids are rotated laterally and the hu- son and Currie 1990).Ceratopsians lacked in- There appear to be no anatomical difficulties bility displayed by many larger mammals merus is prevented from operating in an ap- terclavicles and fused clavicles, and thus no in rotating a posteriorly inclined scapulocor- (Gambaryan 1974)appears less likely.Because proximately parasagittal manner (see also anatomical structure impaired scapular mo- acoid (Fig. SA). Coombs (1978) and Dodson the coracoid-sternal articulation is lacking, the Ford 1997), although it probably did diverge bility. (1996)suggested that ceratopsian scapulocor- exact amouri.t of scapular mobility possible slightly laterally from the parallel plane with No realistic figure illustrating exactly what acoids were too large to be mobile, but it re- cannot be estimated, nor the exact point of ro- respect to the long axis of the body (see would have prevented ceratopsian scapulo- mains elusive why size and mobility should tation. above). Additionally, the humeral caput often coracoid mobility has yet been published. Il- be correlated. In contrast, a long scapular does not articulate properly with the glenoid lustrations depicting a mobile coracoid pressed blade could potentially provide great mechan- Appendicular Morphology in mounts with sprawling or semierect fore- against the ventral part of the neck or a scap- ical leverage and thus enhance propulsive Forelimbs.-Many aspects of forelimb mor- limbs (Osborn 1933: Fig. 2; Erickson 1966: ula projecting far anteriorly to the ribs, as per power from the forelimb. phology in conjunction with the trackways Plate IB; Johnson and Ostrom 1995:Figs. 12.2, Bennett and Dalzell (1973),involve an initial Dodson (1996) further argued that the also strongly argue against a sprawling fore- 12.7, and 12.8). incorrect, horizontal scapulocoracoid posture. strong scapulocoracoid curvature prevented limb posture. In many ceratopsian mounts Dodson (1996) and Dodson and Farlow 460 GREGORY S. PAUL AND PER CHRISTIANSEN FORELIMB POSTURES IN NEOCERATOPSIAN DINOSAURS 461

jer 1940; Erickson 1966; Bakker 1971, 1986, laterally, as this would rotate the manus me- 1987; Russell 1977; Paul 1987; Tereshchenko dially, especially during protraction (see, e.g., 1994; Johnson and Ostrom 1995; Ford 1997). Gilmore 1905: Plate 1; Lull 1933: Plates IlIA, This orientation is easily recognized because XIVB,XVII;and illustrations in Dodson 1996: the humeral caput extends much onto the cau- P: 275, and in Czerkas and Czerkas 1990: p. dal side of the humerus and the distal con- 212]). dyles face anteriorly, thus undoubtedly caus- As a result of the most likely orientation of ing substantial elbow flexure. It is the plane of the glenOid and the orientation of the footprint flexion that is in dispute. The distal condyles, impressions in trackways, the elbow is bowed however, do not face ventrally as in sprawling slightly laterally (contra the perfectly parasag- reptiles (see also Ford 1997). Both the mor- ittal forelimbs restored by Tereshchenko phology of the glenoid and the lateromedially [1994J), the radius and ulna slope ventrome- rectangular humeral caput strongly suggest dially about 15-20°, and the distal end of the

F'GURE 7. Left humeri in posterior view, drawn to same length. A, Caman,saurus YPM 1901. B, Apatosaurus CM that this flexion was primarily in the antero- radius is beveled to form a proper articulation 3018. C, Toros",,,us MPM VP6841. D, Ctmsmosourus UTEP P.37.7.006. E, Leptoceratops AMNH 5205. F, Rhinoceros. G, posterior, not the transverse, plane. In fact, the with the carpus (Fig. 6D). In most large extant Eou us. Proximomed ial lesser tubercle is black; short lines indicate deltopectoral crests. morphology of the humeral caput makes it mammals the elbows are also bowed slightly doubtful that ceratopsians would have been laterally and the manus imprints almost touch able to protract the humerus past vertical. In the midline, especially in the long-limbed el- (1997)argued that the prominent lesser tuber- (1935),Johnson and Ostrom (1995), and Dod- this respect they resemble large ungulates and ephants. Manus gauge is usually the same as, cle of ceratopsids (Fig. 7C-E), which is usually son (1996) suggested that the associated mus- rhinos and differ from elephants (Fig. 5A-D). or slightly less than, pedal gauge, hence clear- greatly reduced in mammals (Fig. 7F,G) culature was similar to that of sprawling rep- In various reconstructions with sprawling ly narrower than was the case in ceratopsids would interfere with the ribs if the humerus tiles, but perissodactyls also have well-devel- forelimbs (Gilmore 1905; Sternberg 1927; Os- (Fig. 2B) (Bird 1987; Lockley and Hunt 1995). moved in a near-parasagittal plane. However, oped deltopectoral crests (Fig. 7F,G)(Garrbar- born 1933; Erickson 1966; Johnson and Os- Extant wildebeest, however, also place the this process is often fairly well developed in yan 1974). Dodson (personal communication trom 1995)the radius and ulna are straight. It manus impressions lateral to the pedal im- columnar-limbed sauropods, which often 1999) suggested that deltopectoral crest mor- is more likely, however, that the radius and pressions (Fig. 2C). have a narrow gauge (Fig. 7A,B) (Paul 1987). phology in ceratopsians differs markedly ulna crossed over (Fig. 6D), albeit probably The fact that most dinosaurs are hindlimb With the scapulocoracoid properly articulated from that of large mammals, indicating dif- not markedly (see Carpenter 1982;Paul 1987; dominant and most mammals forelimb dom- there is also a free space medial to the proxi- ferent axes of humeral rotation. Rhinos, how- Carpenter et al. 1994; Ford 1997). A straight inant (e.g.,Alexander 1985,1989;Christiansen mal part of the humerus (Adams 1991; Ford ever, like ceratopsians, have large deltopector- radius also makes parasagittal mounting of 1997;Christiansen and Paul in press) may also 1997). Additionally, there is little conflict be- al crests, virtually at right angles to the long the forelimbs difficult, as the distal articulat- affect this comparison, although it is difficult tween a prominent lesser tubercle and a para- axis of the bone, and they are mainly rugose ing facet then appears disarticulated from the to assess to what extent. Exact forelimb pos- sagittal gait in dinosaurs, as the glenoid was on the proximolateral sides. Overall they proximal carpal; however, this problem large- ture and placement of the manus may have anterior to the ribs, as noted above. show great resemblance to 'the deltopectoral ly disappears once the radius crosses the ulna varied among ceratopsians, although the sim- As the humerus retracted and the caput ar- crests of ceratopsids. for support of the lateral humeral condyle. ilarity in joint orientation suggests that vari- ticulated increasingly with the more laterally The slight differences from extant rhinos in Thus articulated, the proximal radial articu- ation was minor. oriented coracoidal part of the glenoid. the projection of the crest relative to the long axes lating surface also fits properly into the radial Hindlimbs.-It is almost universally agreed humerus may have swung slightly more pos- of the humeri and the humeral heads could fossa anteroproximally on the ulna (see Ford that the hindlirnbs of ceratopsians worked in terolaterally, helping to clear the capacious suggest that ceratopsid humeri were turned 1997). a near-parasagittal manner (Gilmore 1905, gut. The more lateral orientation of the ante- slightly more outward than are the humeri of Ceratopsian trackways are incompatible 1919; Sternberg 1927; Osborn 1933; Russell rior glenoid surface suggests that it may have large mammals (e.g..Hatcher et al. 1907:Plate. with wide-gauge forelimbs on an additional 1935; Brown and SchIaikjer 1940; Erickson been possible to adopt a secondary, wider XLIXB).Deltopectoral crest morphology does point besides the width of the manual prints 1966;Bakker 1971,1986,1987; Paul 1987, 1991; forelimb gauge, when the humerus was held not indicate major differences in the axes of compared with the width between the gle- Johnson and Ostrom 1995; Lockley and Hunt almost horizontally. This could have been use- rotation of the humerus between ceratopsians noids, as described above. The manus im- 1995; Dodson 1996; Dodsorr and Farlow 1997; ful under special circumstances, such as graz- and large mammals. Prominent deltopectoral prints face anterolaterally (Figs. 2A, 6D), a fea- Ford 1997). This in accord with the medially ing, drinking, or static interspecific agonistic crests among quadrupedal dinosaurs are also ture common in quadrupedal dinosaur track- directed femoral caput, the open acetabulum, behavior. found in the Ankylosauria and Stegosauri a, ways (e.g., Carpenter 1982; Paul 1987; Currie the morphology of the knee, and the hemicy- Another feature of forelimb morphology and in the columnar-limbed Sauropoda. 1993; Thulborn 1993; dos Santos et al. 1994; Iindrical meso tarsal ankle. Trackways, which that some have argued supports a Wide-gauge There has been virtually unanimous agree- Lockley and Barnes 1994;Lockley et al. 1994). show the hindfeet touching the midline (Figs. forelimb posture is the extremely well devel- ment that ceratopsian forelimbs were flexed Carpenter (1982),Paul (1987),and Ford (1997) IB,C,E and 2A; Dodson and Farlow 1997:Fig. oped deltopectoral crest in ceratopsids, par- (Gilmore 1905; Sternberg 1927; Tait and all noted, that if the manus is rotated laterally 4), support this view. ticularly in the large chasmosaurines. Russell Brown 1928;Osborn 1933;Brown and Schlaik- the elbow could not also be directed strongly As described above, the ceratopsian dorsal 462 GREGORY S. PAUL AND PER CHRISTIANSEN FORELIMB POSTURES IN NEOCERATOPSIAN DINOSAURS 463 vertebral column was rather rigid, and a nar- tilted markedly toward the ground in a body mammals, not large reptiles (Christiansen gated compared with those of elephants, the row pedal gauge could not have been due to posture that appears highly unusual for a and Paul in press). To restore locomotion in number of digits was reduced, and the herni- extensive spinal flexion as suggested by Car- large terrestrial quadruped. Such a recon- extinct forms Paul has instead used several of cylindrical astragalus allowed the ankle to ro- penter et a!. (1994). Because the pelvic region struction would also imply that ceratopsian the anatomical characters outlined more spe- tate. POSSiblyindricotheres retained the ability was broad, the hindlimbs would have had to locomotion was by default slow and cumber- cificallyin this paper. It would appear that Os- to attain at least a trot. slope a few degrees ventromedially to achieve some, but it cannot be labeled elephantine, be~ trom and Johnson (1995)have based their con- Recently extinct rhinos, such as Elasmoiher- this narrow pedal gauge. To clear the capa- cause of numerous differences in appendicu- clusions on a questionable anatomical recon- ium or Megacerops, strongly resembled extant cious stomach during walking, the knees lar functional anatomy between ceratopsids struction, inconsistent with trackway evi- rhinos. Elasmotherium and Megacerops also had would have had to be bowed somewhat lat- and proboscideans. As Bakker (1986) noted, dence. longbones indicating permanent joint flexure, erally. In most ceratopsids the femur was kept the discrepancy in inferred limb kinematics Although we believe that it is likely, it is not and they reached 2-5 metric tons in body nearly vertical and the tibia and fibula sloped between forelimbs and hindlimbs of such an certain that ceratopsids were able to attain a mass (Paul 1997). Uintatheres, arsinoitheres, an estimated 15° ventromedially. This was animal would be very pronounced, and that in full gallop (Christiansen and Paul in press). and brontotheres were archaic mammalian gi- possible because of the asymmetrical distal itself appears dubious. The rather inflexible dorsal series may have ants, approximately the same size as extant femoral condyles, with the lateral condyle ex- Analyses of limb proportions and bone imposed limits on the asymmetry of the gaits rhinos, with roughly similar appendicular tending slightly distally beyond the medial strengths of ceratopsians (Alexander 1985, possible, although this appears not to be the anatomy. Osborn (1929)restored titanotheres, condyle (e.g., Marsh 1891a:Plate VIII; Hatcher 1989, 1991; Christiansen and Paul in press) case among relatively stiff-trunked extant un- which reached an estimated 3-5 metric tons, et a!. 1907: Plate XIV;Brown 1917:Plate XVII; tend to reveal similarities with large mam- gulates. However, scapular mobility among with flexed limbs, and giant theropods and Langston 1975:Fig. 10;Lehman 1.989:Fig. 20). mals for both forelimbs and hindlimbs. There extant ungulates is usually well developed, hadrosaurs appear to have had flexed knees Dodson and Currie (1990) suggested that has been little dispute that ceratopsid hind- and although we suggest that it was present also (Paul 1987,1988).It is true, as Dodson and this femoral morphology implied a non-para- limbs were kinematically similar to the limbs in ceratopsians also, it would probably have Farlow (1997)stated, that ceratopsians are not sagittal femoral action, but this is probably in- of extant large mammals. One could reason- been less intense. Russell (1977) suggested mammals, but the functional anatomy of their correct. Rather, it would most likely have ably suppose that the allometry of ceratopsian that ceratopsids lacked special structures for limbs clearly resembled that of large mam- served to direct the crus medially, thus allow- forelimbs, if their kinematics differed so support of the large head during a gallop; mals much more than that of large reptiles. In ing the knees to be fairly wide apart while markedly from their hindlirnbs, would differ however, withers were present in at least Chas- this respect the ceratopsians are no different maintaining a narrow pedal gauge. Femoral more from extant large mammals than the al- mosaurus (Fig. 1), and the cervicodorsal series from the other dinosaur groups. action would probably not have been affected. lometric trends of their hindlimbs. This, how- of ceratopsids appears more massively built Lehman (1989) noted that the strong lateral ever, is not the case (Christiansen and Paul in than in comparably sized elephants or peris- Conclusion curve of the femoral diaphysis in Chasmosau- press). Analyses of bone strengths (Alexander sodactyls (Fig. 1). The validity of Russell's The traditional mounting of ceratopsid skel- rus indicated that the hindlimbs were bowed 1985, 1989, 1991; Christiansen and Paul in suggestion thus appears dubious. etons with either very wide gauge forelimbs, out more than in other ceratopsids, probably press) suggest that ceratopsids may have been It might seem that giant terrestrial animals as in sprawling reptiles, or near-columnar, el- owing to the great broadness of their abdo- capable of running in a fashion comparable to would need pillarlike limbs to support their ephantine forelimbs is strongly at odds with men (Fig. 1C)."It is possible, although by no that of extant rhinos, and considerably faster own mass, as in elephants, stegosaurs, and both trackway evidence and anatomy. Rather, means certain, that pedal gauge in Chasmosau- than elephants. . sauropods (Paul 1987; Christiansen 1997). both trackways and anatomy appear to sup- rus was wider than in other ceratopsians. A Johnson and Ostrom (1995: P: 216) are in- Large ceratopsids, however, were not the only port a forelimb posture similar to that of large, similar morphology is present in Pentaceratops correct in their assertion that Paul's (1991)con- possible examples of extinct .terrestrial giants extant mammals, albeit probably with slightly (Wiman 1930: Plate VI). clusions on ceratopsid limb posture and lo- whose appendicular anatomy suggests a lo- more averted elbows. Several problems with comotory capability are based not on "com- comotory potential exceeding that of extant mounting the forelimbs in flexed, near-para- Discussion pelling anatomical evidence, but rather on as- elephants. Granger and Gregory (1936)noted sagittal fashion stem from errors in mounting It is evident that a wide-gauge, sprawling sertions that such energetic locomotion was that joint flexion was present in the largest the axial skeleton. Some of the anatomical forelimb posture with a horizontal humerus consistent with the suggestion that all dino- known land mammals, the indricothere rhi- characters usually cited as evidence of a runs counter not only to available trackway saurs were endothermic with a high metabolic noceroses. The limbs of indricotheres are mor- sprawling forelimb posture, such as a promi- evidence, but also to forelimb anatomy. This rate." Paul has not linked metabolism with a phologically significantly different from those nent lesser tubercle or the morphology of the reconstruction must be rejected. If a large cer- running gait because extant endothermic and of elephants and are more similar to those of deltopectoral crest, are erroneous. We con- atopsid with sprawling forelimbs and a hori- ectothermic tetrapods include both species ca- extant running ungulates, even at body mas- clude that the best extant analogues for fore- zontal humerus were made to walk out track- pable and those incapable of galloping (see ses of up to 15-20 metric tons (Fortelius and limb posture in ceratopsid dinosaurs are rhi- ways of Ceraiopsipes (Lockley and Hunt 1995) Webb and Gans 1982for galloping in reptiles). Kappelman 1993;Paul 1997).The humerus is nos. The two groups share a number of con- or Tetrapodosaurus, the epipodium would have Among extant terrestrial tetrapods, however, short and the elbow appears to have been vergent anatomical resemblances probably to be directed sharply medially (Ford 1997: traditional anatomical adaptations for sus- flexed and held posterior to the glenoid, rather forced upon them by the influence of gravity Fig. 8C), the hands would be twisted medially, tained fast locomotion are found in the skele- than the long, subvertical humeri of probes- and by the need to support a large body mass which is contrary to trackway evidence, and ton of endotherrns only. In this respect cera- cideans. The knees and ankles of indricotheres while retaining the ability to perform true the whole anterior part of the body would be topsids again show some resemblance to large were also flexed. The metapodials were elon- running with a suspended phase. 464 GREGORY S. PAUL AND PER CHRISTIANSEN FORELIMB POSTURES IN NEOCERATOPSIAN DINOSAURS 4651

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