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RE: GSA Abstracts Posted




Looks like _Seismosaurus_ has been cut down to size, both length-wise and taxonomically. Lucas & co. revise the body length from 52 to 33m, and formally sink _Seismosaurus_ as a species of _Diplodocus_ (_D. hallorum_).


REAPPRAISAL OF SEISMOSAURUS, A LATE JURASSIC SAUROPOD DINOSAUR FROM NEW
MEXICO. LUCAS, Spencer G.1, HERNE, Matthew C.2, HECKERT, Andrew B.3, HUNT, Adrian
P.1, and SULLIVAN, Robert M.4,

The holotype and only known specimen (NMMNH P-3690) of Seismosaurus hallorum Gillette is from the Upper Jurassic Brushy Basin Member of the Morrison Formation near San Ysidro, New Mexico. It consists of several dorsal vertebrae, several cervical and dorsal ribs, an incomplete pelvis, a sacrum, and ~ 20 caudal vertebrae including two chevrons. Axial length estimates of Seismosaurus of as much as 52 m have been used to identify it as the longest dinosaur. However, they are largely based on mistaken placement of the middle caudal vertebrae and do not stand up to close critical scrutiny. Instead, an axial length estimate of 33 m is well supported. A single (right) femur (NMMNH P-25079) found as float at the type locality was excluded from the type series but appears to belong to the same individual as the holotype. As preserved, this femur is ~1680 mm long, and compares well with a reconstructed skeleton based on the axial length estimate used here. The supposed 240 gastroliths of Seismosaurus have been revealed to be highly polished quartzite pebbles that lack an unambiguous skeletal association; they are stream-deposited cobbles of a channel-lag deposit. The taxonomic validity of Seismosaurus also is questionable. Many of the supposedly diagnostic features of the genus are those of the caudal vertebrae that are spurious artifacts of misplacing the middle caudals as vertebrae 20-27 although, in fact, they are vertebrae 12-19. This correct placement reveals a caudal morphology strikingly similar to that of Diplodocus. Other supposedly diagnostic characters of Seismosaurus can be summarized as a relatively robust pelvis that has an unusual distal expansion of the ischium. This expansion, however, appears to be pathologic and is not mirrored in the second ischium (prepared subsequently), and the differences in proportions between the pelves of Seismosaurus and Diplodocus are minor---most metric ratios differ by less than 10%. Therefore, we consider Seismosaurus to be a junior subjective synonym of Diplodocus, though a case can be made for recognizing the New Mexican fossil as a valid species of Diplodocus, D. hallorum.




ABELISAUROID THEROPOD DINOSAURS FROM THE LATE CRETACEOUS OF EGYPT: EVIDENCE
FOR GONDWANAN COSMOPOLITANISM
SMITH, Joshua B.,

Recent paleogeographic and paleobiogeographic scenarios postulate the isolation of continental Africa throughout the Late Cretaceous. The absence of abelisauroid theropods from Upper Cretaceous African strata has been employed as evidence of hypothesized African isolation, with acknowledgement that the paucity of African abelisauroids may be related to sampling rather than to their actual absence from the continent. Here we report on a shed theropod tooth from the Upper Cretaceous (Campanian-?Maastrichtian, ~70Ma) Duwi (Phosphate) Formation of the Eastern Desert of Egypt that was referred to Megalosaurus crenatissimus (=Majungatholus atopus) in 1921. Stepwise discriminant function analyses (DA) using squared Mahalanobis distances (D2) and including eight size and shape variables were run to test for morphological congruence between the Egyptian tooth and the dental morphologies of 16 well-established theropod taxa. The DA correctly classified 96.1% of the teeth in the sample and assigned the tooth to the Malagasy abelisaurid Majungatholus (29.95 D2, p <.0001). Given that current paleogeographic reconstructions posit that Madagascar had attained its current position relative to Africa before the beginning of the Late Cretaceous, it is unlikely that the Egyptian tooth actually pertains to Majungatholus. Nevertheless, its classification as an abelisaurid constitutes the first definitive evidence of this clade from the post-Cenomanian Cretaceous of the African continent. This result, combined with recent discoveries of abelisauroids from ~95Myr and older rocks in Niger and Morocco and ~95Myr-old remains from the Bahariya Oasis of Egypt that share dental characters with the abelisauroid Masiakasaurus, indicate that Abelisauroidea was actually a diverse group in continental Africa during the Late Cretaceous, existing in multiple places for at least ~25Myr. These results suggest that abelisauroids may have been more cosmopolitan than demonstrated by previous evidence and that faunal communication between South America and Africa might have remained possible for longer than has been generally hypothesized.


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