A large Cretaceous theropod from Patagonia, Argentina, and the evolution of carcharodontosaurids
- 514 Downloads
The Cretaceous Carcharodontosauridae is the latest clade of carnosaurs, including the largest predatory dinosaurs yet recorded. Albeit spectacular for their size, the skeletal anatomy of these theropods remains poorly-known, and their diversity was until recently restricted to two Cenomanian species: the highly derived Giganotosaurus carolinii, from southern South America, and the incompletely known Carcharodontosaurus saharicus, from northern Africa. Here we describe an older and basal member of the group, Tyrannotitan chubutensis gen. et sp. nov., from Aptian strata of Patagonia, Argentina. The new taxon gives new insights into the systematics and evolution of carcharodontosaurids and offers a better understanding of the evolution of Southern theropod faunas. We suggest that carcharodontosaurids radiated in Gondwana sharing with spinosaurids the role of top-predators until their extinction in Cenomanian–Turonian times. During this interval, the diplodocoid sauropods and giant titanosaurians went extinct (probably as part of a global-scale crisis), and the smaller abelisaurid theropods took dominance, reigning until the end of the Cretaceous. Electronic Supplementary Material is available.
The fossil record of Aptian dinosaurs from Gondwana is favourably increased with the discovery of a new carcharodontosaurid theropod. The specimens, recovered in central Patagonia, belong to the oldest carcharodontosaurid yet recorded.
Description of specimens
Two partially disarticulated skeletons found 1 km apart from each other (Rich et al. 2000).
Tetanurae Gauthier, 1986
Allosauroidea Currie and Zhao, 1993
Carcharodontosauridae Stromer 1934
Tyrannotitan chubutensis gen. et sp. nov.
The generic name is derived from the Latin words tyrannus (tyrant) and titan (giant), the specific name from the Chubut province, Argentina.
MPEF-PV 1156 (Museo Paleontológico “Egidio Feruglio,” Trelew): Partial dentaries, isolated teeth, dorsals 3–8 and 11–14, proximal caudal vertebra, isolated ribs and haemal arches, incomplete left scapulocoracoid and right humerus and ulna; pubes, ischia, and fragments of left ilium; almost complete left femora, fibula and metatarsal II.
MPEF-PV 1157: jugals, right dentary, isolated teeth, atlas, cervical 9?, dorsals 7?, 10 and 13, partially preserved fused centra of sacrals 1–5, isolated distal caudals, ribs, right femur, incomplete left metatarsal II, pedal phalanges 2.I, 2.II, and 3.III. Paratype specimen is approximately 7% larger than that of the holotype.
Locality and horizon
As preserved, the largest specimen (MPEF-PV 1157) has a dentary 68 cm long and 14 cm deep at its rostral end. It has a deep, squared off symphyseal region, with a ventral process or “chin,” as in Giganotosaurus (Calvo and Coria 2000) (Fig. 2). It is ornamented by oblique grooves along the ventral half of its lateral surface, passing through a band of smooth bone surface along the dental margin, forming a pattern of ornamentation resembling that of Giganotosaurus and abelisaurids (e.g., Carnotaurus). Up to 16 alveoli are present on the dentaries. As in other carcharodontosaurids, the teeth bear marginal arcuate enamel wrinkles on the labial side of the caudal carina (Sereno et al. 1996). However, tooth denticles from the cranial carina are bilobate in side view, a character that seems unique among theropods.
Postaxial cervical vertebrae are strongly opisthocoelous. Presacral vertebrae bear well developed pneumatic foramina and fossae, in particular a pair of pleurocoels on cervical and dorsal vertebrae. Caudal centra lack pleurocoels or nutrient foramina, in contrast to the large pleurocoel reported for Carcharodontosaurus (Stromer 1931), and the pair of small nutrient foramina present on proximal and mid-caudals of Acrocanthosaurus (Currie and Carpenter 2000; Harris 1998) and Giganotosaurus. The neural spines of the dorsal vertebrae are craniocaudally long, dorsoventrally deep and transversely thick, and with strong ligament scars protruding both cranially and caudally.
The coracoid and scapula are fused. The scapular blade is narrow, and the acromial process rises abruptly from the scapula at an angle approaching 90 °. The slender shoulder girdle of Tyrannotitan is sharply different from the unusually robust and highly derived one of Giganotosaurus (Calvo 1999), in which the coracoid is reduced. Preserved portions of humerus and ulna (MPEF-PV 1156; Fig. 2) indicate that forelimbs were short and robust in this carcharodontosaurid (as it also occurs in Acrocanthosaurus; Currie and Carpenter 2000). Hindlimb bones are also massive, and exhibit two remarkable carcharodontosaurid synapomorphies: the femoral head is proximomedially projected, and the fibula is proportionally short with respect to femoral length (less than 70%). The femur of MPEF-PV 1157 is almost complete; its estimated length of 140 cm is slightly shorter than that in Giganotosaurus (143 cm; Coria and Salgado 1995). The transverse width of the femoral shaft of Tyrannotitan is 16.5 cm.
Tyrannotitan helps to clarify the confusing aspects of the skeletal anatomy of its close relative Carcharodontosaurus. This Saharan taxon was recently diagnosed (Sereno et al. 1996) and reconstructed (Currie 1996) on the supposed overlapping characters of specimens of Carcharodontosaurus saharicus (Stromer 1931) and the problematic theropod “Spinosaurus B” (Stromer 1934). Pivotal in the purported overlap is a stout cervical vertebra, characterized by its low and very broad centrum, strong ventral keel, and reduced neural spine (Sereno et al. 1996). However, this vertebra (not found in association with specimens of C. saharicus; P. Sereno, personal communication) shows clear distinctions with cervicals of Tyrannotitan, Giganotosaurus, and the holotype specimen of C. saharicus. On the contrary, the cervical in question closely resembles that of Sigilmassasaurus (Russell 1996), a theropod of uncertain phylogenetic relationships. Besides, the flattened and acuminate pedal unguals of “Spinosaurus B” (Stromer 1934) purportedly referred as to Carcharodontosaurus by Sereno et al. (1996, 1998), are sharply different from the robust and curved ones of Tyrannotitan (Fig. 1, ph). Such differences do not correspond with digit position. Also, femur, tibia, dorsal and caudal vertebrae originally referred as to “Spinosaurus B” show clear distinctions from those of Tyrannotitan and Giganotosaurus. In sum, diagnosis and reconstruction of Carcharodontosaurus recently offered (Sereno et al. 1996; Currie 1996) are based on the chimaeric association of specimens corresponding to different theropod clades. In this context, we do not regard Sigilmassasaurus brevicollis as a subjective junior synonym of C. saharicus, as recently proposed (Sereno et al. 1998).
From Aptian through Cenomanian times Gondwana was inhabited by large theropods including carcharodontosaurids, spinosaurids, and the bizarre tetanuran Bahariasaurus (Stromer 1934; Sereno et al. 1998). This “mid”-Cretaceous fauna was also composed of huge titanosaurs (e.g., Argentinosaurus, Argyrosaurus, Paralititan), basal diplodocoids (e.g., dicraeosaurids and rebbachisaurids), and in northern Africa, crocodiles reached up to 12 m in length (e.g., Stomatosuchus and Sarcosuchus) (Stromer 1936; Smith et al. 2001; Salgado 2001; Sereno et al. 2001). In the post-Turonian, carcharodontosaurids and spinosaurids become rare or absent in South America, being replaced by smaller abelisauroids. Coincidently, the following reptiles are no longer present in the Southern landmasses after the Turonian: huge pholidosaurid crocodiles, large basal iguanodontians, and diplodocoids (following Chiappe et al. 2001; Currie Rogers and Forster 2004, and Apesteguía 2004, we interpret Antarctosaurus wichmanianus as a titanosaurid, thus countering Sereno et al. 1999, who envisaged this Patagonian taxon as a rebbachisaurid). After the strong decline of carcharodontosaurids and the virtual extinction of spinosaurids at the end of the Cenomanian, theropod assemblages from South America, Madagascar and India consisted mainly of comparatively smaller abelisauroids, and secondarily of a wide array of tetanurans (e.g., Megaraptor, coelurosaurians). Although abelisauroid diversification was underway at least from the Early Cretaceous (e.g., Carrano et al. 2002; Rauhut 2003), they become abundant and large during Late Cretaceous times.
Notably, the “mid”-Cretaceous faunal transformations described above for South America may parallel the faunal replacement that occurred in North America, where Aptian carnosaurs (e.g., Acrocanthosaurus), basal titanosauriforms, and large basal iguanodontians, were replaced in the Cenomanian by hadrosaurs, ceratopsians and tyrannosaurids (Bakker 1977; Kirkland 1997; Britt and Stadman 1997; Harris 1998). This suggests that a faunal replacement took place probably at a global scale at the same general time interval. Studies of the still poorly known “mid”-Cretaceous terrestrial communities will contribute to better understanding of the major ecological changes that preceded the terminal Mesozoic mass extinction event.
We thank L. Guerrero, P. Puerta, R. Vacca and their team for the discovery, excavation and preparation of the specimens; L. Salgado, P. Currie, and P. Posadas for suggestions on early drafts; and R. Coria and R. Carolini for access to specimen of Giganotosaurus carolinii. Finnancial support received from Agencia Nacional de Promoción Científica y Técnica, CONICET, National Geographic Society, and The Jurassic Foundation (to FEN) is gratefully acknowledged. Fieldwork was sponsored by Museo Paleontológico “Egidio Feruglio.”
- Apesteguía S (2004) Bonitasaura salgadoi gen. et sp. Nov.: a beaked sauropod from the Late Cretaceous of Patagonia. Naturwissenschaften 91:493–497Google Scholar
- Bakker RT (1977) Tetrapod mass extinctions—a model of the regulation of speciation rates and inmigration by cycles of topographic diversity. In: Hallam A (ed) Patterns of evolution as illustrated by the fossil record. Elsevier Scientific Publishing, Amsterdam, pp439–468Google Scholar
- Britt BB, Stadman KL (1997) Dalton Wells Quarry. In: Currie P, Padian K (eds) Encyclopaedia of dinosaurs, Academic Press, New York, pp 165–166Google Scholar
- Calvo JO (1999) Dinosaurs and other vertebrates of the Lake Ezequiel Ramos Mexía Area, Neuquén-Patagonia, Argentina. In: Tomida Y, Rich TH, Vickers-Rich P (eds) Proceedings of the Second Gondwanan Dinosaur Symposium, National Science Museum Monographs 15, Tokyo, pp 13–45Google Scholar
- Calvo JO, Coria RA (2000) New specimen of Giganotosaurus carolinii (Coria and Salgado, 1995), supports it as the largest theropod ever found. GAIA 15:117–122Google Scholar
- Carrano MT, Sampson SD and Forster CA (2002) The osteology of Masiakasaurus knopfleri, a small abelisauroid (Dinosauria: Theropoda) from the Late Cretaceous of Madagascar. J Vert Paleontol 22:510–534Google Scholar
- Chiappe LM, Salgado L, Coria RA (2001) Embryonic skulls of titanosaur sauropod dinosaurs. Science 293:2444–2446Google Scholar
- Codignotto J, Nullo F, Panza J, Proserpio C (1978) Estratigrafía del Grupo Chubut entre Paso de Indios y Las Plumas, Provincia del Chubut, Argentina. Actas VII Congreso Geológico Argentino, pp 471–480Google Scholar
- Coria RA, Currie PJ (2003) The braincase of Giganotosaurus carolinii, (Dinosauria: Theropoda) from the Upper Cretaceous of Argentina. J Vertebr Paleontol 4:802–811Google Scholar
- Coria RA, Salgado L (1995) A new giant carnivorous dinosaur from the Cretaceous of Patagonia. Nature 377:224–226Google Scholar
- Currie PJ (1996) Out of Africa: meat-eating dinosaurs that challenge Tyrannosaurus rex. Science 272:971–972Google Scholar
- Currie PJ, Carpenter K (2000) A new specimen of Acrocanthosaurus atokensis (Theropoda, Dinosauria) from the Lower Cretaceous Antlers Formation (Lower Cretaceous, Aptian) of Oklahoma, USA. Geodiversitas 22:207–246Google Scholar
- Currie Rogers K, Forster C (2004) The skull of Rapetosaurus krausei (Sauropoda: Titanosauria) from the Late Cretaceous of Madagascar. Journ Vert Pal 24:121–144Google Scholar
- Harris JD (1998) A reanalysis of Acrocanthosaurus atokensis, its phylogenetic status and paleobiogeographic implications, based on a new specimen from Texas. Bull New Mexico Mus Nat Hist Sci 13:1–75Google Scholar
- Holtz TR (2000) A new phylogeny of the carnivorous dinosaurs. GAIA 15:5–61Google Scholar
- Kirkland J (1997) Cedar Mountain Formation. In: Currie P, Padian K (eds) Encyclopaedia of Dinosaurs, Academic Press, New York, pp 98–99Google Scholar
- Musacchio E, Chebli W (1975) Ostrácodos no marinos y carófitas del Cretácico inferior de las provincias de Chubut y Neuquén, Argentina. Ameghiniana 12:70–96Google Scholar
- Rauhut OWM (2003) The interrelationships and evolution of basal theropod dinosaurs. Special Papers in Palaeontology 69:1–213Google Scholar
- Rich TH, Vickers-Rich P, Novas F, Cúneo R, Puerta P, Vacca R (2000) Theropods from the “Middle” Cretaceous Chubut Group of the San Jorge sedimentary basin, Central Patagonia. A preliminary note. GAIA 15:111–115Google Scholar
- Russell DA (1996) Isolated dinosaur bones from the Middle Cretaceous of the Tafilalt, Morocco. Bulletin du Muséum National d’Histoire Naturelle de Paris, 18:349–363Google Scholar
- Salgado L (2003) Los Saurópodos de Patagonia: sistemática evolución y paleobiologia. II Jornalas internacionales sobre paleontologia de Dinosaurios y su Entorno, Actas, Sala de Los Inpantes, España, pp 139–168Google Scholar
- Sereno PC, Beck AL, Dutheil DB, Gado B, Larsson HC, Lyon GH, Marcot JD, Rauhut OWM, Sadleir RW, Sidor CA, Varrichio DJ, Wilson GP, Wilson JA (1998) A long-snouted predatory dinosaur from Africa and the evolution of the spinosaurids. Science 282:1298–1300Google Scholar
- Sereno PC, Beck AL, Dutheil DB, Larsson HCE, Lyon GH, Moussa B, Sadleir RW, Sidor CA, Varricchio DJ, Wilson GP, Wilson JA (1999) Cretaceous sauropods from the Sahara and the uneven rate of skeletal evolution among dinosaurs. Science 286:1342–1347Google Scholar
- Sereno PC, Dutheil DB, Iarochene M, Larsson HC, Lyon GH, Magwene PM, Sidor CA, Varrichio DJ, Wilson JA (1996) Predatory dinosaurs from the Sahara and Late Cretaceous faunal differentiation. Science 272:986–991Google Scholar
- Sereno PC, Larsson HCE, Sidor CA, Gado B (2001) The giant crocodyliform Sarcosuchus from the Cretaceous of Africa. Science 294:1516–1519Google Scholar
- Smith JB, Lamanna MC, Lacovara KJ, Dodson P, Smith JR, Poole JC, Giegengack R, Attia Y (2001) A giant sauropod dinosaur from an Upper Cretaceous mangrove deposit in Egypt. Science 292:1704–1706Google Scholar
- Stromer E (1931) Wirbeltierreste der Baharíje-Stufe (unterstes Cenoman). 10. Ein Skelett-Rest von Carcharodontosaurus nov. gen. Abh. bayer. Akad. Wissensch., math-naturwiss. Abt., N.F. 9:1–23Google Scholar
- Stromer E (1934) Wirbeltierreste der Baharíje-Stufe (unterstes Cenoman). 13. Dinosauria. Abh. bayer. Akad. Wissensch., math-naturwiss. Abt., N.F. 22:1–79Google Scholar
- Stromer E (1936) 7.Wirbeltierreste der Baharíje-Stufe (unterstes Cenoman). Baharije-Kesse und Stufe mit deren Fauna und Flora Eine erganzende Zusammenfassung. Abh. bayer. Akad. Wissensch., math-naturwiss. Abt., N.F. 33:1–102Google Scholar