REAPPRAISAL OF SEISMOSAURUS, A LATE JURASSIC SAUROPOD DINOSAUR FROM NEW MEXICO

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.