Wednesday, November 28, 2012

"Allosaurus/Chilantaisaurus" sibiricus is probably a megaraptoran

So the Database is still up for some reason, but I've come through with my promise to examine what Riabinin named Allosaurus sibiricus and which has been going by Molnar et al.'s reassignment as Chilantaisaurus sibiricus.  The description's in Russian (translations are welcome), but the holotype is figured clearly in all views, allowing us to make comparisons to other taxa (thanks to Molnar for sending it to me).  I can send the pdf to any who ask.

"Allosaurus" sibiricus Riabinin, 1915
= Antrodemus sibiricus (Riabinin, 1915) Huene, 1932
= Chilantaisaurus? sibiricus (Riabinin, 1915) Molnar, Kurzanov and Dong, 1990
Berraisian-Hauterivian, Early Cretaceous
Tignin Formation or Turgin Formation or Zugmar Formation, Chitinskaya Oblast, Russia
Holotype
- (PIN coll.) distal metatarsal II (~300-400 mm)
Late Barremian-Mid Aptian, Early Cretaceous
Mogoito Member of Murtoi Formation, Buryatia, Russia
Referred
- ? bone (Ivanov, 1940)
Diagnosis- (suggested) (combination of) metatarsal II lateral condyle with ventral surface width ~46% of condylar depth; metatarsal II medial condyle dorsal surface ~73% as wide as surface of lateral condyle and angled 35 degrees from lateral edge of lateral condyle.

Holotype of "Allosaurus" sibiricus, distal left metatarsal II in (top left to right) medial, dorsal, lateral, ventral, (bottom left to right) distal and proximal views (after Riabinin, 1915).  The distal end is 68 mm wide.

Comments- Note the description was actually published in 1915, though the volume was intended for 1914. The holotype was discovered in 1912 and deposited in what was then the Geological Museum of the Russian Academy of Sciences in Petrograd (Tolmachoff, 1924) (now St. Petersburg), which has since moved to Moscow. Riabinin (1915; partially translated in Chure, 2000) named it Allosaurus (?) sibiricus based on what he identified as a distal metatarsal IV. Huene (1932) said only that it did not permit exact characterization and probably belonged to an allosaurid (he renamed it Antrodemus? sibiricus as he thought that was a senior synonym of Allosaurus). Molnar et al. (1981) felt sibiricus resembled Ceratosaurus so closely that they "hesitate to accept it as an allosaurid". Molnar et al. (1990) on the other hand stated it was "almost identical with that of C. tashuikouensis in form and proportions of the distal condyle", so questionably referred it to Chilantaisaurus. Nessov (1995) agreed the species were similar and that sibiricus may belong in Chilantaisaurus. He noted stratigraphic data gave three possible formations the specimen was discovered in. Chure (2000) incorrectly said Riabinin did not illustrate the material, as he apparently had only a small portion of the original document. This error was repeated by Benson and Xu (2008). Chure excluded it from Allosauridae because he believed the distal outline was rectangular, but it is actually trapezoidal in both Allosaurus and sibiricus. Both Benson and Xu and Carrano et al. (2012) incorrectly credited Holtz et al. (2004) as being responsible for assigning it to Chilantaisaurus, though Carrano et al. correctly identified it as a second metatarsal. They believed it "too fragmentary to be assigned to a known taxon or identified as a distinct form" and noted similarity to Allosaurus, Neovenator, Torvosaurus and Afrovenator (though no metatarsal II has been reported for the latter).

Ivanov (1940) reported a bone referred to Allosaurus sibiricus from the Mortoi Formation, though without more data this referral is uncertain.

The holotype is ~70% the size of Chilantaisaurus tashuikouensis in distal width and depth, which would make it ~286 mm if similarly stout. If from an elongate metatarsus like Australovenator's though, it would be ~398 mm long. It differs from Allosaurus in having a more medially oriented dorsal curve to the lateral condyle, having a larger and more medially flaring medial condyle, having a lateral condyle which is recessed ventrally, and lacking the lateral flare on the ventral edge of Allosaurus' lateral condyle. Chilantaisaurus' medial condyle flares slightly more than Allosaurus' and it has a ventrally recessed lateral condyle, but it has the small medial condyle and a lateral flare like Allosaurus and has the dorsal curve oriented even further laterally. Of course, Riabinin and Molnar et al. were comparing sibiricus with the fourth metatarsals of Ceratosaurus, Allosaurus and Chilantaisaurus, not the second metatarsals (all three fourth metatarsals differ from sibiricus in having a ventrally pointed medial condyle and lacking a ventrally inset medial condyle, while Ceratosaurus' and  Allosaurus' element is much narrower, and Chilantaisaurus' has a larger dorsolateral bulge; Molnar et al. were correct that Chilantaisaurus' is more similar, but it is not almost identical). Taxa with similarly medially flaring medial condyles are Ceratosaurus, Torvosaurus, Sinraptor, Acrocanthosaurus, Fukuiraptor, Australovenator, Megaraptor and Harpymimus, though only Australovenator's and Acrocanthosaurus' are close in size. Taxa with a ventrally inset lateral margin are Rajasaurus, Megalosaurus, Chilantaisaurus, Fukuiraptor, Australovenator, Megaraptor, Appalachiosaurus and Alxasaurus.

Comparison of metatarsals in distal view (some reversed; scaled to equal depth). Top row, left to right- right metatarsal IV (medial to right) of Chilantaisaurus tashuikouensis (after Hu, 1964),  Allosaurus fragilis (after Madsen, 1976) and Ceratosaurus (after Gilmore, 1920).  Middle row, left to right- left metatarsal II (medial to left) of Chilantaisaurus tashiukouensis (after Benson and Xu, 2008), Allosaurus fragilis (after Madsen, 1976), Megaraptor (after Lamanna, 2004), "Allosaurus" sibiricus (after Riabinin, 1915), Australovenator (after Hocknull et al., 2009), Neovenator (after Brusatte et al., 2008), Torvosaurus (after Britt, 1991) and Alxasaurus (after Russell and Dong, 1994).  Bottom row, left to right- left metatarsal II (medial to left) of Ceratosaurus (after Gilmore, 1920), Rajasaurus (after Wilson et al., 2003), Megalosaurus (after Benson et al., 2010), Sinraptor (after Churrie and Zhao, 1994), Acrocanthosaurus (after Harris, 1998), Fukuiraptor (after Azuma and Currie, 2000), Appalachiosaurus (after Carr et al., 2005) and Harpymimus (after Kobayashi, 2004).

 Overall, it is most similar to Australovenator, differing in being 9% broader compared to depth, a broader lateral condyle ventrally, and having a more medially oriented dorsal surface of the lateral condyle. Next most similar is Megaraptor, which it differs from in having a broader lateral condyle ventrally with ventral surface angled more laterally, and a less rounded dorsal surface of the lateral condyle. The amount of ventral inset of the lateral condyle and dorsal exposure of the medial condyle is in between these two taxa. This suggests sibiricus may be a megaraptoran, which is congruent with its age, size and location. As it is intermediate in two variables, more similar to Megaraptor in depth, more similar to Australovenator in the orientation of the lateral condyle's ventral surface, and differs from both in the lateral condyle's ventral width, placing it in any named genus is not possible. As it does differ from all known theropod metatarsals, it is not a nomen dubium, contra Rauhut (2003), Holtz et al. and Carrano et al..

References- Riabinin, 1915. Zamtka o dinozavry ise Zabaykalya [A note on a dinosaur from the trans-Baikal region]. Trudy Geologichyeskago Muszeyah Imeni Petra Velikago Imperatorskoy Academiy Nauk. 8(5), 133-140.

Tolmachoff, 1924. On dinosaurs in northern Asia. American Journal of Science. 5(7), 489-490.

Huene, 1932. Die fossile Reptil-Ordnung Saurischia, ihre Entwicklung und Geschichte. Monographien zur Geologie und Palaeontologie. 4(1), viii + 361 pp.

Ivanov, 1940. [On the age of the coal-bearing deposits of Transbaikalia]. Sovietskaya Geologiya. 11, 45-54.

Molnar, Flannery and Rich, 1981. An allosaurid theropod dinosaur from the early Cretaceous of Victoria, Australia. Alcheringa. 5, 141-146.

Molnar, Kurzanov and Dong, 1990. Carnosauria. In Weishampel, Dodson and Osmolska (eds.). The Dinosauria. Berkeley: University of California Press. 169-209.

Nessov, 1995. Dinozavri severnoi Yevrazii: Novye dannye o sostave kompleksov, ekologii i paleobiogeografii [Dinosaurs of northern Eurasia: new data about assemblages, ecology, and paleobiogeography]. Institute for Scientific Research on the Earth's Crust, St. Petersburg State University, St. Petersburg. 156 pp.

Chure, 2000. A new species of Allosaurus from the Morrison Formation of Dinosaur National Monument (Utah-Colorado) and a revision of the theropod family Allosauridae. Ph.D. thesis. Columbia University. 964 pp.

Rauhut, 2003. The interrelationships and evolution of basal theropod dinosaurs. Special Papers in Palaeontology. 69, 213 pp.

Holtz, Molnar and Currie, 2004. Basal Tetanurae. In Weishampel, Dodson and Osmolska (eds.). The Dinosauria Second Edition. University of California Press. 71-110.

Benson and Xu, 2008. The anatomy and systematic position of the theropod dinosaur Chilantaisaurus tashuikouensis Hu, 1964 from the Early Cretaceous of Alanshan, People’s Republic of China. Geological Magazine. 145(6), 778-789.

Carrano, Benson and Sampson, 2012. The phylogeny of Tetanurae (Dinosauria: Theropoda). Journal of Systematic Palaeontology. 10(2), 211-300.

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