Arctosaurus the poposaurid

Continuing our treck through ex-theropods is the mysterious vertebra named Arctosaurus.

Arctosaurus Adams, 1875
A. osborni Adams, 1875
Late Triassic
Heiberg Formation, Nunavut, Canada
Holotype- (NMI 62 1971) incomplete third cervical vertebra (33 mm)

Diagnosis- (suggested) differs from Ctenosauriscus in low cervical neural spine; differs from Lotosaurus in offset centrum faces.

Arctosaurus osborni holotype NMI 62 1971, third cervical vertebra in A. anterior, B. left lateral, C. posterior, D. ventral and E. dorsal views.  Scale = 25 mm. (after Galton and Cluver, 1976)

Comments- This was discovered in 1853 and though originally identified as a Teleosaurus vertebra, was described by Adams (1875) as a new reptilian genus more similar to lizards than crocodiles. Non-dinosaurian identifications include Huene's (1902) suggestion it is a testudine. This was based on comparison to his new genus Chelyzoon, which he believed to be a turtle but has since been synonymized with the archosauromorph Tanystrophaeus. Thus a relationship with turtles can be discounted. Baird (in Russell, 1984) suggested it was a trilophosaurid. This needs to be seriously considered, as trilophosaurs have epipophyses and spinopostzygapophyseal laminae. However, they differ in having platycoelous to procoelous centra (in Spinosuchus and Trilophosaurus) which lack a posterior centrodiapophyseal lamina and have larger epipophyses. Furthermore, Arctosaurus has closely situated postzygapophyses which lack the broad horizontal transpostzygapophyseal laminae characteristic of Trilophosaurus and Spinosuchus. The rhynchosaur Mesosuchus also has epipophyses, but similarly differs in lacking posterior centrodiapophyseal laminae as well as having prominent posterodorsally placed diapophyses.

Arctosaurus a dinosaur? Lydekker (1890) placed it in Theropoda fam. indet., stating it was probably related to Anchisaurus (which he viewed as a theropod). Indeed, authors such as Lydekker (1889) and Marsh (1895) placed it in Anchisauridae. A relationship to basal sauropodomorphs remained the consensus until Galton and Cluver (1976) placed it in Theropoda instead due to the short and high proportions and supposed pleurocoel. Additional authors who have placed Arctosaurus in Theropoda include Steel (1970), Welles (1984; as a nomen dubium) and Weishampel (1990). The elongate centrum with offset articular surfaces is like dinosauriforms, and the presence of epipophyses is like dinosaurs. Within Dinosauria, the lack of a pleurocoel excludes it from Avepoda, the relatively short centrum excludes it from Sauropodomorpha (even compared to Panphagia and Pantydraco), and the shortness of the epipophyses exclude it from Theropoda. It is unlike basal ornithischians like Eocursor and Heterodontosaurus in the anteroventral placement of the diapophysis and thus probably parapophysis, and in the well developed posterior centrodiapophyseal lamina which marks a lateral fossa (pleurocoel of Galton and Cluver).

Arctosaurus an archosauriform? The specimen was most recently reviewed by Nesbitt et al. (2007), who placed it as Archosauriformes indet.. Ignoring dinosaurs, epipophyses are only known in some ornithosuchians (e.g. Revueltosaurus, Lotosaurus), while the elongate centrum with offset sides resembles those of Xilousuchus, some poposaurids and crocodylomorphs. Thus Poposauroidea is the most likely candidate. It is more similar to poposaurids than basal poposauroids (Yarasuchus, Qianosuchus) in the well developed posterior centrodiapophyseal lamina. Within poposaurids, Arctosaurus is more similar to Lotosaurus, Ctenosauriscus and Poposaurus than Arizonasaurus, Sillosuchus, Effigia and Shuvosaurus in having a shorter centrum. As in Arizonasaurus but unlike Sillosuchus, Effigia and Shuvosaurus, the centrum is transversely compressed. It is further similar to Arizonasaurus and Lotosaurus, but unlike Poposaurus, Sillosuchus, Effigia and Shuvosaurus in lacking a pleurocoel. Like Lotosaurus and apparently Ctenosauriscus, but unlike Arizonasaurus, Poposaurus, Effigia and Shuvosaurus, Arctosaurus has postaxial epipophyses. Thus it seems most similar to Lotosaurus and Ctenosauriscus. As it differs from both taxa, it is not indeterminate, contra Welles and Nesbitt et al..

References- Adams, 1875. On a fossil saurian vertebra, Arctosaurus osborni, from the Arctic regions. Proceedings of the Royal Ireland Academy. 2, 177-179.

Lydekker, 1889. Notes on new and other dinosaurian remains. Geological Magazine, decade 3. 6(8), 352-356.

Lydekker, 1890. Catalogue of the Fossil Reptilia and Amphibia in the British Museum (Natural History), Cromwell Road, S.W., Part IV. Containing the orders Anomodontia, Ecaudata, Caudata and Labyrinthodontia; and supplement. British Museum of Natural History, London. 295 pp.

Marsh, 1895. On the affinities and classification of the dinosaurian reptiles. American Journal of Science. 50(300), 483-498.

Huene, 1902. Übersicht über die Reptilien der Trias [Review of the Reptilia of the Triassic]. Geologische und Paläontologische Abhandlungen (Neue Serie). Gustav Fischer Verlag, Jena. 6, 1-84.

Steel, 1970. Part 14. Saurischia. Handbuch der Paläoherpetologie/Encyclopedia of Paleoherpetology. Gustav Fischer Verlag, Stuttgart. 1-87.

Galton and Cluver, 1976. Anchisaurus capensis (Broom) and a revision of the Anchisauridae. Annals of the South American Museum. 69, 121-159.

Russell, 1984. A check list of the families and genera of North American dinosaurs. Syllogeus Series No. 53. National Museums of Canada.

Welles, 1984. Dilophosaurus wetherilli (Dinosauria, Theropoda), osteology and comparisons. Palaeontographica. Beiträge zur Naturgeschichte der Vorzeit. Abteilung A: Paläozoologie, Stratigraphie. 185, 85-180.

Weishampel, 1990. Dinosaurian distribution. in Weishampel, Dodson and Osmolska (eds.). The Dinosauria. University of California Press. 63-139.

Nesbitt, Irmis and Parker, 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology. 5(2), 209-243.