Photo-montage of the reconstructed Seismosaurus
in the Age of Super Giants hall at the
New Mexico Museum of Natural History & Science.
In 1979, two hikers came across several enormous vertebrae weathering out of Jurassic-aged mudstones near the top of a mesa northwest of Albuquerque, New Mexico. They reported their find, which was on federal land, to the local office of the Bureau of Land Management. BLM paleontologists identified the bones as coming from a large sauropod dinosaur, but lacked the resources to begin a proper excavation of the site. The bones were buried to protect them from further erosion (or unauthorized excavation) until 1985, when the New Mexico Museum of Natural History was about to open in Albuquerque.
A friend of one of the fossil’s disoverers reported the find to Dr. David Gillette, then Curator of Paleontology at the fledgling NMMNH. After a trip to the site, Gillette began making the necessary arrangments to excavate the bones that had been discovered and, eventually, a much greater amount of fossil material that was buried within the mesa. Over the next decade, several tons of rock and bone were chipped from the site and transported to the NMMNH. There, staff and volunteer preparators began the arduous process of separating the fossils from the surrounding rock (matrix)—a process complicated by the fact that both fossil and matrix were similar in color, hardness, and in some areas, texture.
After some of the first bones (vertebrae from the middle of the tail) were prepared, Gillette began comparing them to their counterparts in other sauropods. Although they were closest in form to the well-known Diplodocus, they were much larger and differed in their overall proportions. Gillette concluded that they were unique enough to warrant their own genus, and in 1991 he published an article naming the fossil Seismosaurus halli, or “Hall’s Earth-shaking Lizard.” (Because the name was intended to honor both Reverend James Hall and his wife, Ruth, for their contributions to New Mexico paleontology, it was later emended to the plural possessive form Seismosaurus hallorum.) In his description, Gillette estimated that, when alive, Seismosaurus might have been up to 52 meters (170 feet) long.
By the closing years of the 20th century, enough of the fossils had been prepared to begin the creation of an accurate skeletal reconstruction of Seismosaurus. As new teams of researchers began to compare Seismosaurus to Diplodocus, they began to suspect that some of the bones that Gillette had placed towards the middle of the tail had, in fact, been located much closer to the hips. This realization significantly shortened the animal’s overall length, from a near-record 170-foot maximum down to a “mere” 110 feet long.
More importantly, this revised interpretation eliminated several of the anatomical characters that Dr. Gillette had used to distinguish Seismosaurus from Diplodocus. Most of the characters that remained had to do with relative proportions of the vertebrae, and several researchers suggested that these were well within the range of variation one might expect in a dinosaur genus like Diplodocus.
Lucas et al. 2006 Figure 2: A diagram I worked up showing the differing interpretations
of bone location in the Seismosaurus holotype.
By 2004, when a reconstructed skeleton was put on display in the New Mexico Museum of Natural History and Science (the institution’s name has lengthened with age), only one feature remained that was clearly unique to Seismosaurus—an unusual prong at the tip of its ischium (the backward-pointing bone in its hip). But when this bone was prepared more thoroughly, it became apparent that the ‘prong’ was actually a piece from one of the vertebrae that had broken off post-mortem and drifted beneath the animal, coming to rest at the tip of the ischium. Eventually the two bones were cemented together by the nearly indistinguishable matrix.
A new paper in the NMMNHS Bulletin discusses the features unique to Seismosaurus, shows the before-and-after ischium, and, with this last notable character gone, sinks the earth-shaking Seismosaurus into the long-established genus Diplodocus. It is kept as a separate, over-sized species—Diplodocus hallorum—based on its relative proportions and the paddle-like shape of some of the chevron bones in its tail. However, the authors note that the other four named species of Diplodocus are in need of re-examination, and there is no guarantee that the dinosaur formerly known as Seismosaurus would retain even a specific level of distinction in the light of current taxonomic thought.
Further Reading:
Gillette, D. D. 1991. Seismosaurus halli, gen. et sp. nov., a new sauropod dinosaur from the Morrison Formation (Upper Jurassic/Lower Cretaceous) of New Mexico, USA. Journal of Vertebrate Paleontology, v. 11, pp. 417–433.
Gillette, D. D. 1994. Seismosaurus: The Earth Shaker. New York, Columbia University Press. HTML.
Herne, M. C. and Lucas, S. G. 2006. Seismosaurus hallorum: Osteological reconstruction from the holotype. New Mexico Museum of Natural History and Science Bulletin 36. pp. 139–148.
Lucas, S. G., Spielmann, J. A., Rinehart, L. F., Heckert, A. B., Herne, M. C., Hunt, A. P., Foster, J. R., and Sullivan, R. M. 2006. Taxonomic status of Seismosaurus hallorum, a Late Jurassic sauropod dinosaur from New Mexico. New Mexico Museum of Natural History and Science Bulletin 36. pp. 149–161.
—Matt Celeskey.
File under: Dinosaurs, Jurassic.
Comments on record: (1)
