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A complete titanosaur femur from West Texas with comments regarding hindlimb posture
Cretaceous Research 49 (2014) 39e44
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A complete titanosaur femur from West Texas with comments regarding hindlimb posture Steven L. Wick a, *, Thomas M. Lehman b a b
Division of Science and Resource Management, Big Bend National Park, TX 79834, United States Department of Geosciences, Texas Tech University, Lubbock, TX 79409, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2013 Accepted in revised form 2 February 2014 Available online
An isolated titanosaur femur recovered from the Javelina Formation (Maastrichtian) of Big Bend National Park, Texas is the most complete example yet reported from North America. The specimen is likely referable to Alamosaurus sanjuanensis, the only titanosaur thus far known from Upper Cretaceous strata in North America, but cannot be attributed with certainty to that taxon. Compared to femora from other titanosaurs, the specimen has a relatively reduced abductor crest, a less elevated femoral head, and a distal joint surface that is orthogonal to the long axis of the shaft. These differences suggest that the Big Bend femur pertains to a species where hindlimb stance was closer to vertical, and with a comparatively narrower gait than other titanosaurids. Published by Elsevier Ltd.
Keywords: Late Cretaceous Texas Sauropod Alamosaurus Hindlimb
1. Introduction Titanosaur sauropod bones are the most common large vertebrate fossils found in Maastrichtian strata of Big Bend National Park, in southwestern Texas (hereafter “Big Bend”). It is likely that these remains pertain to Alamosaurus sanjuanensis or a closely related species (Gilmore, 1922; 1946). A. sanjuanensis is thus far the only titanosaur recognized from Upper Cretaceous strata in North America (e.g., Lucas and Hunt 1989; Lucas and Sullivan, 2000; Lehman and Coulson, 2002; Woodward and Lehman, 2009; D’Emic et al. 2010; Fowler and Sullivan 2011; Fronimos and Lehman, in press). Although some authors have questioned the taxonomic status of A. sanjuanensis, Wilson and D’Emic (2011) and D’Emic et al. (2011) presented a compelling argument in favor of its validity. Even so, referral of fragmentary titanosaur specimens to Alamosaurus has been problematic because its key apomorphic features are found only in a few skeletal elements (e.g., scapula). As a result, specimens lacking elements that overlap with the A. sanjuanensis hypodigm are doubtfully attributed. This is the case for the complete titanosaur femur recently collected from the Javelina Formation described below. Although several large femora attributed to Alamosaurus sanjuanensis have been reported from the Kirtland Formation in * Corresponding author. E-mail addresses: [email protected] (S.L. Wick), [email protected] (T. M. Lehman). http://dx.doi.org/10.1016/j.cretres.2014.02.003 0195-6671/Published by Elsevier Ltd.
northern New Mexico (Lucas et al., 1987; Lucas and Hunt, 1989; Lucas and Sullivan, 2000; Fowler and Sullivan, 2011), and from the McRae Formation in southern New Mexico (Lozinsky et al., 1984), all are either fragmentary or uncollected, and provide little morphological information. Several large titanosaur femora have also been collected from Big Bend (AMNH FARB 21531; TMM 41541-1; TTU 542; UTEP P-25), but their morphology has been only briefly described (Lehman and Coulson, 2002). Other fragmentary titanosaur femora are known from Big Bend, but most of these have been dissected for use in histological studies (Woodward and Lehman, 2009), or remain uncollected (e.g., a badly-weathered, 180 cm long in-situ specimen, BIBE locality # P-138). A complete, undistorted isolated titanosaur femur (TMM 460521) recently collected from the Javelina Formation in Big Bend offers the first opportunity for a comprehensive description of a titanosaur femur from North America. TMM 46052-1 is herein conservatively referred to an indeterminate titanosaur, although we consider it likely that the specimen may ultimately prove to be attributable to Alamosaurus sanjuanensis. Institutional abbreviations: AMNH, American Museum of Natural History, New York; BIBE, Big Bend National Park, Texas; TMM, Vertebrate Paleontology Laboratory at the Jackson School of Geosciences, University of Texas at Austin; TTU, Museum of Texas Tech University, Lubbock; UTEP, Centennial Museum at the University of Texas at El Paso.
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2. Geologic setting TMM 46052-1 was collected from light gray mudstone in the fluvially-derived Javelina Formation at a site known informally as the ‘Willow’ locality in western Big Bend National Park (Fig. 1). Exact locality information is available at the Vertebrate Paleontology Laboratory at the Jackson School of Geosciences, University of Texas at Austin. The exposed part of the Javelina Formation at ‘Willow’ is 65 m thick. The lower part of the formation here is truncated by faulting, and the upper part is largely covered by alluvium. For comparison, complete sections of the Javelina Formation nearby are 120 m (Dawson Creek) and 133 m (Peña Mountain; Lehman et al. in prep.). An exposure up-dip from the ‘Willow’ locality is near the contact with the overlying Black Peaks Formation, and indicates that the section here exposes the upper portion of the Javelina. If this correlation is correct, the stratigraphic position of TMM 46052-1 is within the middle of the formation. The Javelina Formation is correlative with Maastrichtian strata (¼Edmontonian through Lancian Land Mammal Ages; Lehman et al., 2006) elsewhere in North America. 3. Description TMM 46052-1 is a large left femur, 151 cm in length (Fig. 2). Only a small anterior portion of the medial condyle is missing due to modern erosion; otherwise the specimen retains its natural shape, and exhibits no evidence for pre-burial weathering or transport. In contrast to other titanosaurs, the femoral head of TMM 46052-1 is not markedly elevated, and positioned only slightly
*"Willow" meters
Hwy. 385
cl si ss cg
N
alluvium Hwy. 118
60
*
Javelina Formation
50
Big Bend National Park
15 km
103 W 29 N
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Titanosauridae indet. Tyrannosauridae indet. TMM 46028-1
30
TMM 46052-1 20
10
0
fault
Fig. 1. “Willow” locality in Big Bend National Park and stratigraphic exposure of the Javelina Formation discussed in text with stratigraphic position of TMM 46052-1 indicated.
dorsomedial to the greater trochanter, as in Phuwiangosaurus (Suteethorn et al., 2009). The femoral head is roughly round in dorsal view and convex in medial view. The greater trochanter is separated from the femoral head by a very shallow dorsolateral depression. The rugose surface of the greater trochanter extends down both anterior and posterior portions of the femoral shaft. As in Opisthocoelicaudia, the greater trochanter is slightly enlarged posteriorly (Borsuk-Bialynicka, 1977). As in other titanosaurs, the proximal third of the femur projects medially. The proximolateral corner of TMM 46052-1 is sharply defined, resulting in an angle of approximately 115 between the lateral margin of the shaft and the dorsal surface of the greater trochanter, and is similar in this regard to Aeolosaurus sp. (Garcia and Salgado, 2013). Also as in Aeolosaurus sp., the abductor crest (¼“lateral bulge” of authors) is reduced, and occupies approximately one-fourth of the proximolateral surface of the bone. On TMM 46052-1, the lateral margin of the shaft is deflected at the apex of the abductor crest, resulting in an angle of approximately 155 . The abductor crest projects laterally 6.5 cm from a straight line connecting the lateral terminus of the greater trochanter to a point along the lateral margin of the shaft where it reaches its minimum mediolateral width of 24.5 cm. The ratio between the two measurements is 0.265, a value less than in most other titanosaurids where it is typically above 0.30 (character 19 of Salgado et al., 1997). On its lateral and posterior surfaces, the abductor crest exhibits longitudinal attachment scars for the abductor musculature. Posteriorly, the abductor crest is delineated from the remainder of the shaft by a long, prominent trochanteric shelf similar to that described for Neuquensaurus and Lirainosaurus (Otero, 2010; Díaz et al., 2013). The medial and lateral margins of the diaphysis in TMM 46052-1 are sub-parallel, but to a lesser degree than in Aeolosaurus sp. The shaft is anteroposteriorly compressed. The mid-shaft posteromedial surface presents a very low fourth trochanter; however, the cortex in this area and the trochanter itself are poorly preserved. The proximal anterior surface of the femoral shaft is less concave than its posterior counterpart. Faint muscle attachment scars are visible on medial and lateral sides of the anterior surface for the extensor musculature; the largest scar (for M. femorotibialis internus) occupies almost the entire proximolateral portion of the shaft. Separating these scars is a low, dorsoventrally elongate crest (the linea intermuscularis cranialus described in Neuquensaurus; Otero, 2010); this crest extends distally from the level of the abductor crest to the intercondylar area. As in other titanosaurs, TMM 46052-1 attains its minimum diameter at about one third of its height. It is, however, proportionally wider than in Rinconsaurus or Rapetosaurus (Calvo and González Riga, 2003; Curry-Rogers, 2009). At its midpoint, the shaft is elliptical in cross-section with the posterior surface nearly flat. The ratio of transverse to anteroposterior diameters of the shaft is 2.04, and comparable to that found in Opisthocoelicaudia, Neuquensaurus, and Saltasaurus (>1.85; character 198 of Wilson, 2002). Viewed laterally the medial portion of the shaft is slightly bowed anteriorly. The diaphysis flares markedly approaching the distal end; diverging into robust medial and lateral condyles. On the anterior surface of the bone, both condyles are low and extend dorsally onto the shaft a short distance. Between them, the crural extensor fossa is shallow anteroposteriorly; however, this area is slightly weathered and the extent of each condyle, and fossa depth, are partly obscured. On the posterior side of the bone, the condyles are considerably more robust and the joint surfaces extend further proximally than on the anterior surface. Here, the intercondyloid fossa is very deep. The medial condyle is considerably larger and broader than the lateral condyle and its anteroposterior extent
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Fig. 2. TMM 46052-1 in A) anterior; B) posterior; C) proximal; D) medial; E) distal views; 1, 2 denote distal views at midshaft breaks; pos indicates posterior side of shaft. Abbreviations: abductor crest (ac); crural extensor fossa (cef); femoral head (fh); fourth trochanter (ftr); greater trochanter (gt); intercondyloid fossa (icf); lateral condyle (lc); lateral epicondyle (lec); linea intermuscularis cranialus (lic); medial condyle (mc); trochanteric shelf (ts). Missing portions of anterior medial condyle represented by dashed lines. Horizontal plane denoted by double lines with long axis of the bone indicated by arrows. Scale bar ¼ 50 cm.
slightly greater. The lateral condyle also includes a distinct, posteriorly-directed epicondyle. The epicondyle is well delineated from the lateral condyle, but both form part of a contiguous joint surface. Unlike in other titanosaurids, the transverse axis of the combined joint surfaces of the distal condyles is not beveled dorsomedially, but is instead orthogonal relative to the long axis of the femoral shaft. 4. Discussion TMM 46052-1 exhibits features typical of other titanosaur femora; the proximal third of the shaft is directed medially, the femoral head is elevated, the abductor crest is marked. CurryRogers (2009) recognized that femoral morphology is very similar among most titanosaurs, but variations occur in the morphology of the abductor crest, shape of femoral head, and development of the distal condyles; however, TMM 46052-1 deviates from the norm in all three aspects of femoral morphology noted by Curry-Rogers (2009). First, TMM 46052-1 exhibits a less elevated femoral head compared to most other titanosaurs. Second, it exhibits a comparatively low abductor crest. Third, and most significantly, it retains a transverse distal joint surface that is orthogonal to the long axis of the bone. In other titanosaurids, the medial condyle is positioned somewhat elevated to the lateral condyle requiring that the femur
be held outwardly in order to properly articulate with the tibia during ambulation e a posture developed in response to the redistribution of body mass during “wide-gauge” locomotion (Wison and Carrano, 1999; Wilson, 2002; Curry-Rogers, 2005; Salgado et al., 2005; Curry-Rogers, 2009; Vila et al., 2013). CurryRogers (2005, character 338) coded Alamosaurus as possessing a dorsomedially-beveled distal condylar surface; however, this morphology is clearly not exhibited by TMM 46052-1. The unbeveled distal morphology in TMM 46052-1 is similar to that in less derived somphospondylians (e.g., Ligabuesaurus, Euhelopus) where hindlimb stance was closer to vertical and the gait comparatively narrower. 4.1. Comparison of Big Bend titanosaur femora The five titanosaur femora thus far known from Big Bend likely represent an ontogenetic series from sub-adult through fully mature animals (Fig. 3, Table 1). All are remarkably similar in form, especially the three specimens where the orthogonal distal joint surface is preserved. In two of the specimens (TTU 542, TMM415411), the distal end is completely restored in plaster. TTU 542 also appears to have a more highly elevated femoral head, but this is due to erosional loss of the greater trochanter. Another specimen (UTEP P-25) appears to have a relatively narrow shaft and distal end, but
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Fig. 3. A) Comparison of Big Bend titanosaur femora: from left to right: AMNH FARB 21531 right femur (image reversed for comparison), TTU 542 right femur (image reversed for comparison), TMM 46052-1 left femur, UTEP P-25 left femur, TMM 41541-1 right femur (anterior view), others are shown in posterior view; parts restored in plaster are crosshatched. B) Medial views of several femora discussed in text with comparison to TMM 46052-1 (shaded), with TTU 542 likely exhibiting an incorrect (posteriorly bowed) reconstruction. C) Outline drawings compare each specimen in posterior view with TMM 46052-1 (shaded) reduced to a common length; tick marks indicate position of abductor ridge. D) Bivariate plot shows variation in length relative to minimum mediolateral width of the shaft in the five specimens, with selected other titanosaurs for comparison (Na ¼ Neuquensaurus australis, Os ¼ Opisthocoelicaudia skarzynskii, Rk ¼ Rapetosaurus krausi, Sl ¼ Saltasaurus loricatus; shaded arrow represents increasing degree of distal joint surface bevel; outlines and measurements from von Huene, 1929; Borsuk-Bialynicka, 1977; Bonaparte and Powell, 1980; Curry-Rogers, 2009).
Table 1 Measurements (in mm) of five titanosaur femora from the Javelina Formation in Big Bend National Park, Texas; length ¼ maximum length from apex of femoral head to distal end of tibial condyle, proximal width ¼ transverse width from medial end of femoral head to lateral edge of greater trochanter, shaft width ¼ minimum transverse width of shaft, circumference ¼ minimum circumference of shaft, distal width ¼ transverse width from medial edge of tibial condyle to lateral edge of fibular condyle, tibial condyle ¼ maximum anteroposterior width of condyle, fibular condyle ¼ maximum anteroposterior width, x ¼ dimension not measurable due to lack of preservation,^¼ estimated measurement due to incomplete preservation.
Length Proximal width Shaft width Shaft circumference Distal width Tibial condyle Fibular condyle
AMNH FARB 21531
TTU 542
TMM 46052-1
UTEP P-25
TMM 41541-1
970 320 173 451 294 213 210
1460^ 420^ 230 610 x x x
1510 450 245 640 460 270 260
1675 525 275 690 x 321 x
1730^ 523 270 737 x x x
this is likewise due to erosional loss of the lateral surface of the shaft and fibular condyle. These apparent differences are all attributable to incomplete preservation. Otherwise, in those specimens where comparable features are preserved, there are notable similarities: 1) the femoral head is not highly elevated above the greater trochanter; 2) where the greater trochanter, abductor crest, and minimum transverse shaft width are preserved, the abductor crest ratios are lower than in other titanosaurs (0.265 for TMM46052-1; 0.287 for AMNH FARB 21531, and 0.296 for TMM 41541-1; character 19 of Salgado et al., 1997); and 3) where the distal end is preserved (AMNH FARB 21531, UTEP P-25, and TMM 46052-1), the transverse distal joint surface is orthogonal to the long axis of the shaft (contra Curry-Rogers, 2005). These similarities pertain in the smallest as well as largest of the five specimens. In the two largest Big Bend specimens, the depth of the intercondyloid fossa appears to be greater, separation of medial and lateral condyles more pronounced, and the condyles are more expanded anteroposteriorly than in the smaller specimens (Fig. 3A). Similarly, in the largest specimen (TMM 41541-1) the femoral head is more expanded anteroposteriorly (Fig. 3B). These differences may be attributable to ontogenetic stage; however,
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preservation of the distal ends in these specimens is poor. Among the five specimens, the summit of the abductor crest varies in its position along the lateral margin, but this does not appear to be related to size; both the smallest and largest specimens (AMNH FARB 21531, TMM 41541-1) exhibit a more distal placement. These two specimens also have a proportionally more robust proximal end (Fig. 3C). Such differences might be attributable to sexual variation; however, these two specimens are both also from near the top of the Javelina Formation. In contrast, TMM 46052-1 is from the middle of the formation (c. 30 m lower), and although the precise stratigraphic positions of the other two specimens are unknown (TTU 542, UTEP P-25), both are likely from near the base of the formation. The general similarity of the five specimens, their occurrence in the same stratigraphic unit, and restricted geographic region (within c. 50 km of each other), suggests that they pertain to a single taxon; however, it remains possible that subtle morphological differences noted above may reflect stratigraphically successive species (e.g., Fronimos and Lehman, in press). 5. Conclusions The orthogonal distal joint surface, low abductor crest, and less elevated femoral head probably comprise a linked set of character states, all related to hindlimb stance, and plesiomorphic for titanosaurs. This combination of character states in small as well as large femora suggests that throughout life, the Big Bend titanosaur retained a hindlimb morphology and stance less derived compared to other members of the clade. The outwardly angled femora and offset tibial condyle in derived titanosaurs is therefore not correlated with body size; the Big Bend titanosaur attained much larger size than many other titanosaurs, yet retained a plesiomorphic femur morphology. If TMM 46052-1 is attributable to Alamosaurus, then its femur morphology may offer additional means to diagnose the taxon. Recovery of similar femora in association with skeletal elements that exhibit features apomorphic for Alamosaurus (e.g., scapula, first caudal vertebra; D’Emic et al., 2011) may ultimately permit identification of TMM 46052-1 as well as the other Big Bend specimens. Acknowledgments The authors gratefully acknowledge the support of NPS Geologist Don Corrick of the Division of Science and Resource Management for his support of this project. Members of the Big Bend National Park law enforcement community helped with the excavation of the specimen. NPS Rangers Billie Brauch, Jorge Sanchez, and Mike Ryan were especially innovative regarding collection of the specimen. A special word (and apple) of gratitude was extended to the NPS horse “Ringo” for his determination in helping us move a heavy load over rugged terrain. We also thank Bill Mueller at the Museum of Texas Tech, Scott Cutler at the Centennial Museum of the University of Texas at El Paso, and Chris Sagebiel at the Vertebrate Paleontology Laboratory at the Jackson School of Geosciences, University of Texas at Austin, and Ruth O’Leary at the American Museum of Natural History for their help with access to and information about the specimens described herein. Our late colleague Wann Langston, Jr. at Texas Memorial Museum provided access to his notes, photographs, and measurements for some of the specimens, including TTU 542, one of the first specimens he discovered in Big Bend in 1938, and that he returned to collect after WW II in 1947. Jack McIntosh recognized the titanosaur affinity of AMNH FARB 21531, which was collected by Barnum Brown in 1940 from northern Tornillo Flat in what would later become Big Bend National Park. UTEP P-25 was collected in 1939 near Talley Mountain in what would later become Big Bend National Park by a crew
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supported by the Works Progress Administration (Strain, 1940). Appreciation is offered also to J. Wick and E. Lehman for their continued indulgence of the authors’ paleontological endeavors.
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Journal of Vertebrate Paleontology 29, 1046e1086. D’Emic, M.D., Wilson, J.A., Thompson, R., 2010. The end of the sauropod dinosaur hiatus in North America. Paleogeography, Paleoclimatology, Paleoecology 297, 486e490. D’Emic, M.D., Wilson, J.A., Williamson, T.E., 2011. A sauropod dinosaur pes from the latest Cretaceous of North America and the validity of Alamosaurus sanjuanensis (Sauropoda, Titanosauria). Journal of Vertebrate Paleontology 31, 1072e1079. Díaz, V.D., Suberbiola, X.P., Sanz, J.L., 2013. Appendicular skeleton and dermal armour of the Late Cretaceous titanosaur Lirainosaurus astibiae (Dinosaurua: Sauropoda) from Spain. Palaeontologica Electronica 16 (2), 19A. Fowler, D.W., Sullivan, R.M., 2011. The first giant titanosaurian sauropod from the Upper Cretaceous of North America. Acta Palaeontologica Polonica 56, 685e690. Fronimos, J.A., Lehman, T.M., 2014. New specimens of a titanosaur sauropod from the Maastrichtian of Big Bend National Park. 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First isotopic (U-PB) age for the Late Cretaceous Alamosaurus vertebrate fauna of West Texas and its significance as a link between two faunal provinces. Journal of Vertebrate Paleontology 26, 922e928. Lozinsky, R.P., Hunt, A.P., Wolberg, D.L., Lucas, S.G., 1984. Late Cretaceous (Lancian) dinosaurs from the McRae Formation, Sierra County, New Mexico. New Mexico Geology 6, 72e77. Lucas, S.G., Hunt, A.P., 1989. Alamosaurus and the sauropod hiatus in the Cretaceous of the North American Western Interior. In: Farlow, J.O. (Ed.), Paleobiology of the Dinosaurs. Geological Society of America Special Paper, 238, pp. 75e85. Lucas, S.G., Mateer, N.J., Hunt, A.P., O’Neill, F.M., 1987. Dinosaurs, the age of the Fruitland and Kirtland formations, and the Cretaceous-Tertiary boundary in the San Juan Basin, New Mexico. In: Fassett, J.E., Rigby Jr., J.K. (Eds.), The CretaceousTertiary Boundary in the San Juan and Raton Basins, New Mexico and Colorado. Geological Society of America Special Paper, 209, pp. 35e50. Lucas, S.G., Sullivan, R.M., 2000. The sauropod dinosaur Alamosaurus from the Upper Cretaceous of the San Juan Basin, New Mexico. New Mexico Museum of Natural History and Science Bulletin 17, 147e156. Otero, A., 2010. The appendicular skeleton of Neuquensaurus, a Late Cretaceous saltasurine sauropod from Patagonia, Argentina. Acta Palaeontologica Polonica 55, 399e426. Salgado, L., Coria, R.A., Calvo, J.O., 1997. Evolution of titanosaurid sauropods. I: Phylogenetic analysis based upon the postcranial evidence. Ameghiniana 34, 3e32. Salgado, L., Apesteguía, S., Heredia, S.E., 2005. A new specimen of Neuquensaurus australis, a Late Cretaceous saltasaurine titanosaur from north Patagonia. Journal of Vertebrate Paleontology 25, 623e634. Strain, W.S., 1940. Report on Works Progress Administration paleontological project sponsored by the Texas College of Mines and Metallurgy. W.P.A. progress report for W.P.10106/0.P. 465-66-3-374. Unpublished, p. 21. Suteethorn, S., Le Loeuff, J., Buffetaut, E., Suteethorn, V., Talubmook, C., Chonglakmani, C., 2009. A new skeleton of Phuwiangosaurus sirindhornae (Dinosaurua, Sauropoda) from NE Thailand. In: Geological Society of London Special Publications, 315, pp. 189e215.
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Vila, B., Oms, O., Galobart, À., Bates, K.T., Egerton, V.M., Manning, P.L., 2013. Dynamic similarity in titanosaur sauropods: Ichnological evidence from the Fumanya dinosaur tracksite (southern Pyrenees). PloS ONE 8 (2), e57408. http:// dx.doi.org/10.1371/journal.pone0057408. Wilson, J.A., 2002. Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnaean Society 136, 217e276. Wison, J.A., Carrano, M.T., 1999. Titanosaurs and the origin of “wide-gauge” trackways: a biomechanical and systematic perspective on sauropod dinosaurs. Paleobiology 25, 252e267.
Wilson, J.A., D’Emic, M.D., 2011. The validity and paleobiogeographic history of the titanosaur sauropod Alamosaurus sanjuanensis from the latest Cretaceous of North America. In: Program and Abstract, Technical Session XI, 71st Annual Meeting. Society of Vertebrate Paleontology, Las Vegas. Woodward, H.N., Lehman, T.M., 2009. Bone histology and microanatomy of Alamosaurus sanjuanensis (Sauropoda: Titanosauria) from the Maastrichtian of Big Bend National Park, Texas. Journal of Vertebrate Paleontology 29, 807e821.
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Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes
A complete titanosaur femur from West Texas with comments regarding hindlimb posture Steven L. Wick a, *, Thomas M. Lehman b a b
Division of Science and Resource Management, Big Bend National Park, TX 79834, United States Department of Geosciences, Texas Tech University, Lubbock, TX 79409, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2013 Accepted in revised form 2 February 2014 Available online
An isolated titanosaur femur recovered from the Javelina Formation (Maastrichtian) of Big Bend National Park, Texas is the most complete example yet reported from North America. The specimen is likely referable to Alamosaurus sanjuanensis, the only titanosaur thus far known from Upper Cretaceous strata in North America, but cannot be attributed with certainty to that taxon. Compared to femora from other titanosaurs, the specimen has a relatively reduced abductor crest, a less elevated femoral head, and a distal joint surface that is orthogonal to the long axis of the shaft. These differences suggest that the Big Bend femur pertains to a species where hindlimb stance was closer to vertical, and with a comparatively narrower gait than other titanosaurids. Published by Elsevier Ltd.
Keywords: Late Cretaceous Texas Sauropod Alamosaurus Hindlimb
1. Introduction Titanosaur sauropod bones are the most common large vertebrate fossils found in Maastrichtian strata of Big Bend National Park, in southwestern Texas (hereafter “Big Bend”). It is likely that these remains pertain to Alamosaurus sanjuanensis or a closely related species (Gilmore, 1922; 1946). A. sanjuanensis is thus far the only titanosaur recognized from Upper Cretaceous strata in North America (e.g., Lucas and Hunt 1989; Lucas and Sullivan, 2000; Lehman and Coulson, 2002; Woodward and Lehman, 2009; D’Emic et al. 2010; Fowler and Sullivan 2011; Fronimos and Lehman, in press). Although some authors have questioned the taxonomic status of A. sanjuanensis, Wilson and D’Emic (2011) and D’Emic et al. (2011) presented a compelling argument in favor of its validity. Even so, referral of fragmentary titanosaur specimens to Alamosaurus has been problematic because its key apomorphic features are found only in a few skeletal elements (e.g., scapula). As a result, specimens lacking elements that overlap with the A. sanjuanensis hypodigm are doubtfully attributed. This is the case for the complete titanosaur femur recently collected from the Javelina Formation described below. Although several large femora attributed to Alamosaurus sanjuanensis have been reported from the Kirtland Formation in * Corresponding author. E-mail addresses: [email protected] (S.L. Wick), [email protected] (T. M. Lehman). http://dx.doi.org/10.1016/j.cretres.2014.02.003 0195-6671/Published by Elsevier Ltd.
northern New Mexico (Lucas et al., 1987; Lucas and Hunt, 1989; Lucas and Sullivan, 2000; Fowler and Sullivan, 2011), and from the McRae Formation in southern New Mexico (Lozinsky et al., 1984), all are either fragmentary or uncollected, and provide little morphological information. Several large titanosaur femora have also been collected from Big Bend (AMNH FARB 21531; TMM 41541-1; TTU 542; UTEP P-25), but their morphology has been only briefly described (Lehman and Coulson, 2002). Other fragmentary titanosaur femora are known from Big Bend, but most of these have been dissected for use in histological studies (Woodward and Lehman, 2009), or remain uncollected (e.g., a badly-weathered, 180 cm long in-situ specimen, BIBE locality # P-138). A complete, undistorted isolated titanosaur femur (TMM 460521) recently collected from the Javelina Formation in Big Bend offers the first opportunity for a comprehensive description of a titanosaur femur from North America. TMM 46052-1 is herein conservatively referred to an indeterminate titanosaur, although we consider it likely that the specimen may ultimately prove to be attributable to Alamosaurus sanjuanensis. Institutional abbreviations: AMNH, American Museum of Natural History, New York; BIBE, Big Bend National Park, Texas; TMM, Vertebrate Paleontology Laboratory at the Jackson School of Geosciences, University of Texas at Austin; TTU, Museum of Texas Tech University, Lubbock; UTEP, Centennial Museum at the University of Texas at El Paso.
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2. Geologic setting TMM 46052-1 was collected from light gray mudstone in the fluvially-derived Javelina Formation at a site known informally as the ‘Willow’ locality in western Big Bend National Park (Fig. 1). Exact locality information is available at the Vertebrate Paleontology Laboratory at the Jackson School of Geosciences, University of Texas at Austin. The exposed part of the Javelina Formation at ‘Willow’ is 65 m thick. The lower part of the formation here is truncated by faulting, and the upper part is largely covered by alluvium. For comparison, complete sections of the Javelina Formation nearby are 120 m (Dawson Creek) and 133 m (Peña Mountain; Lehman et al. in prep.). An exposure up-dip from the ‘Willow’ locality is near the contact with the overlying Black Peaks Formation, and indicates that the section here exposes the upper portion of the Javelina. If this correlation is correct, the stratigraphic position of TMM 46052-1 is within the middle of the formation. The Javelina Formation is correlative with Maastrichtian strata (¼Edmontonian through Lancian Land Mammal Ages; Lehman et al., 2006) elsewhere in North America. 3. Description TMM 46052-1 is a large left femur, 151 cm in length (Fig. 2). Only a small anterior portion of the medial condyle is missing due to modern erosion; otherwise the specimen retains its natural shape, and exhibits no evidence for pre-burial weathering or transport. In contrast to other titanosaurs, the femoral head of TMM 46052-1 is not markedly elevated, and positioned only slightly
*"Willow" meters
Hwy. 385
cl si ss cg
N
alluvium Hwy. 118
60
*
Javelina Formation
50
Big Bend National Park
15 km
103 W 29 N
40
Titanosauridae indet. Tyrannosauridae indet. TMM 46028-1
30
TMM 46052-1 20
10
0
fault
Fig. 1. “Willow” locality in Big Bend National Park and stratigraphic exposure of the Javelina Formation discussed in text with stratigraphic position of TMM 46052-1 indicated.
dorsomedial to the greater trochanter, as in Phuwiangosaurus (Suteethorn et al., 2009). The femoral head is roughly round in dorsal view and convex in medial view. The greater trochanter is separated from the femoral head by a very shallow dorsolateral depression. The rugose surface of the greater trochanter extends down both anterior and posterior portions of the femoral shaft. As in Opisthocoelicaudia, the greater trochanter is slightly enlarged posteriorly (Borsuk-Bialynicka, 1977). As in other titanosaurs, the proximal third of the femur projects medially. The proximolateral corner of TMM 46052-1 is sharply defined, resulting in an angle of approximately 115 between the lateral margin of the shaft and the dorsal surface of the greater trochanter, and is similar in this regard to Aeolosaurus sp. (Garcia and Salgado, 2013). Also as in Aeolosaurus sp., the abductor crest (¼“lateral bulge” of authors) is reduced, and occupies approximately one-fourth of the proximolateral surface of the bone. On TMM 46052-1, the lateral margin of the shaft is deflected at the apex of the abductor crest, resulting in an angle of approximately 155 . The abductor crest projects laterally 6.5 cm from a straight line connecting the lateral terminus of the greater trochanter to a point along the lateral margin of the shaft where it reaches its minimum mediolateral width of 24.5 cm. The ratio between the two measurements is 0.265, a value less than in most other titanosaurids where it is typically above 0.30 (character 19 of Salgado et al., 1997). On its lateral and posterior surfaces, the abductor crest exhibits longitudinal attachment scars for the abductor musculature. Posteriorly, the abductor crest is delineated from the remainder of the shaft by a long, prominent trochanteric shelf similar to that described for Neuquensaurus and Lirainosaurus (Otero, 2010; Díaz et al., 2013). The medial and lateral margins of the diaphysis in TMM 46052-1 are sub-parallel, but to a lesser degree than in Aeolosaurus sp. The shaft is anteroposteriorly compressed. The mid-shaft posteromedial surface presents a very low fourth trochanter; however, the cortex in this area and the trochanter itself are poorly preserved. The proximal anterior surface of the femoral shaft is less concave than its posterior counterpart. Faint muscle attachment scars are visible on medial and lateral sides of the anterior surface for the extensor musculature; the largest scar (for M. femorotibialis internus) occupies almost the entire proximolateral portion of the shaft. Separating these scars is a low, dorsoventrally elongate crest (the linea intermuscularis cranialus described in Neuquensaurus; Otero, 2010); this crest extends distally from the level of the abductor crest to the intercondylar area. As in other titanosaurs, TMM 46052-1 attains its minimum diameter at about one third of its height. It is, however, proportionally wider than in Rinconsaurus or Rapetosaurus (Calvo and González Riga, 2003; Curry-Rogers, 2009). At its midpoint, the shaft is elliptical in cross-section with the posterior surface nearly flat. The ratio of transverse to anteroposterior diameters of the shaft is 2.04, and comparable to that found in Opisthocoelicaudia, Neuquensaurus, and Saltasaurus (>1.85; character 198 of Wilson, 2002). Viewed laterally the medial portion of the shaft is slightly bowed anteriorly. The diaphysis flares markedly approaching the distal end; diverging into robust medial and lateral condyles. On the anterior surface of the bone, both condyles are low and extend dorsally onto the shaft a short distance. Between them, the crural extensor fossa is shallow anteroposteriorly; however, this area is slightly weathered and the extent of each condyle, and fossa depth, are partly obscured. On the posterior side of the bone, the condyles are considerably more robust and the joint surfaces extend further proximally than on the anterior surface. Here, the intercondyloid fossa is very deep. The medial condyle is considerably larger and broader than the lateral condyle and its anteroposterior extent
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Fig. 2. TMM 46052-1 in A) anterior; B) posterior; C) proximal; D) medial; E) distal views; 1, 2 denote distal views at midshaft breaks; pos indicates posterior side of shaft. Abbreviations: abductor crest (ac); crural extensor fossa (cef); femoral head (fh); fourth trochanter (ftr); greater trochanter (gt); intercondyloid fossa (icf); lateral condyle (lc); lateral epicondyle (lec); linea intermuscularis cranialus (lic); medial condyle (mc); trochanteric shelf (ts). Missing portions of anterior medial condyle represented by dashed lines. Horizontal plane denoted by double lines with long axis of the bone indicated by arrows. Scale bar ¼ 50 cm.
slightly greater. The lateral condyle also includes a distinct, posteriorly-directed epicondyle. The epicondyle is well delineated from the lateral condyle, but both form part of a contiguous joint surface. Unlike in other titanosaurids, the transverse axis of the combined joint surfaces of the distal condyles is not beveled dorsomedially, but is instead orthogonal relative to the long axis of the femoral shaft. 4. Discussion TMM 46052-1 exhibits features typical of other titanosaur femora; the proximal third of the shaft is directed medially, the femoral head is elevated, the abductor crest is marked. CurryRogers (2009) recognized that femoral morphology is very similar among most titanosaurs, but variations occur in the morphology of the abductor crest, shape of femoral head, and development of the distal condyles; however, TMM 46052-1 deviates from the norm in all three aspects of femoral morphology noted by Curry-Rogers (2009). First, TMM 46052-1 exhibits a less elevated femoral head compared to most other titanosaurs. Second, it exhibits a comparatively low abductor crest. Third, and most significantly, it retains a transverse distal joint surface that is orthogonal to the long axis of the bone. In other titanosaurids, the medial condyle is positioned somewhat elevated to the lateral condyle requiring that the femur
be held outwardly in order to properly articulate with the tibia during ambulation e a posture developed in response to the redistribution of body mass during “wide-gauge” locomotion (Wison and Carrano, 1999; Wilson, 2002; Curry-Rogers, 2005; Salgado et al., 2005; Curry-Rogers, 2009; Vila et al., 2013). CurryRogers (2005, character 338) coded Alamosaurus as possessing a dorsomedially-beveled distal condylar surface; however, this morphology is clearly not exhibited by TMM 46052-1. The unbeveled distal morphology in TMM 46052-1 is similar to that in less derived somphospondylians (e.g., Ligabuesaurus, Euhelopus) where hindlimb stance was closer to vertical and the gait comparatively narrower. 4.1. Comparison of Big Bend titanosaur femora The five titanosaur femora thus far known from Big Bend likely represent an ontogenetic series from sub-adult through fully mature animals (Fig. 3, Table 1). All are remarkably similar in form, especially the three specimens where the orthogonal distal joint surface is preserved. In two of the specimens (TTU 542, TMM415411), the distal end is completely restored in plaster. TTU 542 also appears to have a more highly elevated femoral head, but this is due to erosional loss of the greater trochanter. Another specimen (UTEP P-25) appears to have a relatively narrow shaft and distal end, but
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Fig. 3. A) Comparison of Big Bend titanosaur femora: from left to right: AMNH FARB 21531 right femur (image reversed for comparison), TTU 542 right femur (image reversed for comparison), TMM 46052-1 left femur, UTEP P-25 left femur, TMM 41541-1 right femur (anterior view), others are shown in posterior view; parts restored in plaster are crosshatched. B) Medial views of several femora discussed in text with comparison to TMM 46052-1 (shaded), with TTU 542 likely exhibiting an incorrect (posteriorly bowed) reconstruction. C) Outline drawings compare each specimen in posterior view with TMM 46052-1 (shaded) reduced to a common length; tick marks indicate position of abductor ridge. D) Bivariate plot shows variation in length relative to minimum mediolateral width of the shaft in the five specimens, with selected other titanosaurs for comparison (Na ¼ Neuquensaurus australis, Os ¼ Opisthocoelicaudia skarzynskii, Rk ¼ Rapetosaurus krausi, Sl ¼ Saltasaurus loricatus; shaded arrow represents increasing degree of distal joint surface bevel; outlines and measurements from von Huene, 1929; Borsuk-Bialynicka, 1977; Bonaparte and Powell, 1980; Curry-Rogers, 2009).
Table 1 Measurements (in mm) of five titanosaur femora from the Javelina Formation in Big Bend National Park, Texas; length ¼ maximum length from apex of femoral head to distal end of tibial condyle, proximal width ¼ transverse width from medial end of femoral head to lateral edge of greater trochanter, shaft width ¼ minimum transverse width of shaft, circumference ¼ minimum circumference of shaft, distal width ¼ transverse width from medial edge of tibial condyle to lateral edge of fibular condyle, tibial condyle ¼ maximum anteroposterior width of condyle, fibular condyle ¼ maximum anteroposterior width, x ¼ dimension not measurable due to lack of preservation,^¼ estimated measurement due to incomplete preservation.
Length Proximal width Shaft width Shaft circumference Distal width Tibial condyle Fibular condyle
AMNH FARB 21531
TTU 542
TMM 46052-1
UTEP P-25
TMM 41541-1
970 320 173 451 294 213 210
1460^ 420^ 230 610 x x x
1510 450 245 640 460 270 260
1675 525 275 690 x 321 x
1730^ 523 270 737 x x x
this is likewise due to erosional loss of the lateral surface of the shaft and fibular condyle. These apparent differences are all attributable to incomplete preservation. Otherwise, in those specimens where comparable features are preserved, there are notable similarities: 1) the femoral head is not highly elevated above the greater trochanter; 2) where the greater trochanter, abductor crest, and minimum transverse shaft width are preserved, the abductor crest ratios are lower than in other titanosaurs (0.265 for TMM46052-1; 0.287 for AMNH FARB 21531, and 0.296 for TMM 41541-1; character 19 of Salgado et al., 1997); and 3) where the distal end is preserved (AMNH FARB 21531, UTEP P-25, and TMM 46052-1), the transverse distal joint surface is orthogonal to the long axis of the shaft (contra Curry-Rogers, 2005). These similarities pertain in the smallest as well as largest of the five specimens. In the two largest Big Bend specimens, the depth of the intercondyloid fossa appears to be greater, separation of medial and lateral condyles more pronounced, and the condyles are more expanded anteroposteriorly than in the smaller specimens (Fig. 3A). Similarly, in the largest specimen (TMM 41541-1) the femoral head is more expanded anteroposteriorly (Fig. 3B). These differences may be attributable to ontogenetic stage; however,
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preservation of the distal ends in these specimens is poor. Among the five specimens, the summit of the abductor crest varies in its position along the lateral margin, but this does not appear to be related to size; both the smallest and largest specimens (AMNH FARB 21531, TMM 41541-1) exhibit a more distal placement. These two specimens also have a proportionally more robust proximal end (Fig. 3C). Such differences might be attributable to sexual variation; however, these two specimens are both also from near the top of the Javelina Formation. In contrast, TMM 46052-1 is from the middle of the formation (c. 30 m lower), and although the precise stratigraphic positions of the other two specimens are unknown (TTU 542, UTEP P-25), both are likely from near the base of the formation. The general similarity of the five specimens, their occurrence in the same stratigraphic unit, and restricted geographic region (within c. 50 km of each other), suggests that they pertain to a single taxon; however, it remains possible that subtle morphological differences noted above may reflect stratigraphically successive species (e.g., Fronimos and Lehman, in press). 5. Conclusions The orthogonal distal joint surface, low abductor crest, and less elevated femoral head probably comprise a linked set of character states, all related to hindlimb stance, and plesiomorphic for titanosaurs. This combination of character states in small as well as large femora suggests that throughout life, the Big Bend titanosaur retained a hindlimb morphology and stance less derived compared to other members of the clade. The outwardly angled femora and offset tibial condyle in derived titanosaurs is therefore not correlated with body size; the Big Bend titanosaur attained much larger size than many other titanosaurs, yet retained a plesiomorphic femur morphology. If TMM 46052-1 is attributable to Alamosaurus, then its femur morphology may offer additional means to diagnose the taxon. Recovery of similar femora in association with skeletal elements that exhibit features apomorphic for Alamosaurus (e.g., scapula, first caudal vertebra; D’Emic et al., 2011) may ultimately permit identification of TMM 46052-1 as well as the other Big Bend specimens. Acknowledgments The authors gratefully acknowledge the support of NPS Geologist Don Corrick of the Division of Science and Resource Management for his support of this project. Members of the Big Bend National Park law enforcement community helped with the excavation of the specimen. NPS Rangers Billie Brauch, Jorge Sanchez, and Mike Ryan were especially innovative regarding collection of the specimen. A special word (and apple) of gratitude was extended to the NPS horse “Ringo” for his determination in helping us move a heavy load over rugged terrain. We also thank Bill Mueller at the Museum of Texas Tech, Scott Cutler at the Centennial Museum of the University of Texas at El Paso, and Chris Sagebiel at the Vertebrate Paleontology Laboratory at the Jackson School of Geosciences, University of Texas at Austin, and Ruth O’Leary at the American Museum of Natural History for their help with access to and information about the specimens described herein. Our late colleague Wann Langston, Jr. at Texas Memorial Museum provided access to his notes, photographs, and measurements for some of the specimens, including TTU 542, one of the first specimens he discovered in Big Bend in 1938, and that he returned to collect after WW II in 1947. Jack McIntosh recognized the titanosaur affinity of AMNH FARB 21531, which was collected by Barnum Brown in 1940 from northern Tornillo Flat in what would later become Big Bend National Park. UTEP P-25 was collected in 1939 near Talley Mountain in what would later become Big Bend National Park by a crew
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supported by the Works Progress Administration (Strain, 1940). Appreciation is offered also to J. Wick and E. Lehman for their continued indulgence of the authors’ paleontological endeavors.
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