Is Seismosaurus a valid name or should it be regarded as a genus of Diplodocus?

Much debate has taken place over the years as to which of the dinosaurs was the biggest, the longest or the heaviest.  The origins of this controversy amongst scientists can be traced back to the “Dinosaur Wars” between the likes of Cope and Marsh in the 19th Century, as expeditions competed with each other to provide the biggest and best specimens for their wealthy sponsors.  Despite improvements in technology, the much more accurate and detailed study of fossil locations, plus of course improvements in research techniques and the greater number of specimens around today to study, it is still unclear as to which genus or even family of dinosaurs can lay claim to being the biggest.

There are certainly some spectacular contenders out there, Titanosaurs are well represented with genera such as Andesaurus, Antarctosaurus and Argentinosaurus being heralded as true “heavy weights” in the Dinosauria clade.  However, the Titanosaurs do not have everything their own way, although these Cretaceous leviathans are certainly very impressive, many are matched in terms of size by the earlier Brachiosaurids, Camarasaurids and Diplodocids of the Jurassic.

These huge animals do have a number of common characteristics that frustrate teams of field workers, tasked with the job of excavating such finds.  For one thing, the remains of these large animals are relatively rare within the fossil record in comparison to other herbivorous groups such as the Ornithopods for example.  Another problem is the lack of fossil bones which represent one individual specimen.  Many of these large dinosaurs have been named and described from just a few isolated bones, or at best a new genus has been announced based on fossils found in association with each other – rarely do articulated fossils turn up.

Although, as with most scientific matters there are always exceptions to this rule, for example the recent discovery of another large Titanosaur from Argentina:

Huge Titanosaur Specimen uncovered in Cretaceous “Lost World”

The jumble of Sauropod bones to be found in the Morrison Formation has still to be unravelled, no doubt more surprising discoveries will be made, but discussion has turned recently to the validity of another contender for certainly the longest dinosaur yet known – Seismosaurus.  Seismosaurus, the name means “earth-shaking lizard”; as it was imaginatively speculated that such a huge beast would cause the ground to shake as it walked by, could be reclassified as a Diplodocus.

The single specimen found to date of this animal was discovered in 1979, two hikers walking in New Mexico, literally stumbled upon some strange fossil bones eroding out of sandstone sediments.  David Gillette, an American palaeontologist whose work has focused mainly on the Jurassic Morrison Formation in New Mexico, organised a team to begin the long process of extracting the fossils from the matrix.  This process took many years, as the sandstone entombing the fossils was as hard of concrete.  This excavation helped with the design and modification of ground penetrating radar, as this technique was used extensively during the excavations to help locate fossils still buried in the sediment.

Seismosaurus was named and described by Gillette in 1991 (S. hallorum), the remains of this animal consisting of vertebrae, ribs and part of the pelvis.  Based on this evidence it is clear that this particular animal was a contender for the longest dinosaur yet to be described with an estimated length of around 40 metres for this late Jurassic giant (Kimmeridgian faunal stage).  Like all other Diplodocids, the majority of this length was made up of the long neck and the very long tail.  In the New Mexico specimen, the body is not particularly big for a Diplodocoid, it had longer back legs than front legs, a characteristic of this family, but a study of the pelvic area and the subsequent assumed position and length of the legs indicated that they may have been quite short and stubby in comparison to other closely related dinosaurs.  The impressive whip-like Diplodocid tail seems to have had a “kink” in it, perhaps indicating that unlike other Diplodocids, which are believed to have held their tails out straight behind them, perhaps the end portion of the tail of Seismosaurus was trailed on the ground.

However, further work on this fossil has led to a number of reviews some of which have questioned whether the name Seismosaurus would be valid.  In 2004, a case was made for Seismosaurus to be regarded as a Diplodocus, certainly from the fossil evidence these two animals do seem to be very closely related.  Whether or not there are enough differences found to regard Seismosaurus as a separate genus is still being debated.

In 2006, a scientific paper was published, following the most detailed analysis of the fossil bones made at the time, the validity of the Seismosaurus name was challenged, as indeed was the actual size of the beast.  Put into simple terms, it is largely a matter of how you construct the vertebrae, the order in which they are placed together and which part of the backbone is associated with them.  The authors of this 2006 paper renamed Seismosaurus as Diplodocus hallorum, but also speculated that it could actually be a large specimen of another Diplodocus species D. longus.

A Model of a Typical Diplodocid

Picture Credit: Everything Dinosaur

The model shown above is the new scale model of Diplodocus produced by Carnegie.  It represents the latest interpretation of these huge dinosaurs, with a relatively stiff neck, not capable of obtaining the so-called “swan neck” position.  This model is one of the largest scale models currently available with a length of nearly 60 cms.  We love this new interpretation, the Everything Dinosaur packing team who were given the job of finding suitable packaging to enable this item to be posted out to customers are not so keen!

To view this model: Dinosaur Toys for Boys and Girls – Dinosaur Models

As Diplodocus was named before Seismosaurus, the first Diplodocus being named and described in 1878, 113 years before Seismosaurus was named and described, the nomenclature Diplodocus would take precedence.  Seismosaurus would be a junior synonym of Diplodocus.  A synonym is another name for an object.  In taxonomic circles, the earliest of several names given to an organism is considered the senior synonym while later names are junior synonyms.  Perhaps the most famous example of this concerns another Diplodocid – Apatosaurus.  The name Apatosaurus predates Brontosaurus but both are synonyms of the same animal (genus).  Thus Apatosaurus is the senior synonym and Brontosaurus the junior synonym.  The name Brontosaurus, means thunder lizard, a great description for such a huge dinosaur was officially dropped by palaeontologists in 1974.

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