Measuring diversity in the fossil record can be a tricky task. Short of inventing time travel, there will be always be some uncertainty about how many species of dinosaur existed at any one place and time, and as we learn more about the fossil record it may turn out that what we once thought to be distinct species or genera really belonged to
Earlier this week I wrote about a new study describing skin impressions and other soft-tissue traces of the stegosaur Hesperosaurus mjosi. What I did not mention was that some paleontologists have proposed that this dinosaur was actually a smaller species of the more famous Stegosaurus genus, which would make its name Stegosaurus mjosi. Carpenter, who was one of the scientists who named Hesperosaurus in 2001, disputes this, but notes that whether the contentious stegosaur falls into one group or the other relies on more than anatomy alone.
Back in the Bone Wars era, when Stegosaurus was first described, paleontological rivals E.D. Cope and O.C. Marsh were in uncharted territory as far as taxonomy was concerned. The bits and pieces of the fossil animals they found had not been seen before, so it is not surprising that they created a vast accumulation of names to label them all (to say nothing of the competition between them that likely influenced their scientific practices). Given what we know now, though, any paleontologist who applied a new name to every bone scrap they found would be derided by the paleontological community. The naming of a new species—or the synonymy of two old ones—must be explained in minute detail, but even then different scientists have different perspectives on how different two fossils have to be in order to be designated as two different species.
That different species of dinosaur actually existed is immediately obvious. Tyrannosaurus rex and Stegosaurus stenops were so different from each other that it is at once apparent that they were two distinct species of dinosaur. Where a scientist's personal views come into play are cases where there are two groups of animals which are only slightly different from each other. Do these two groups represent different growth stages of the same animal, different populations of the same species, different species of the same genus, or well-distinguished genera which can readily be told apart? Since, as Carpenter notes, dinosaur taxonomy is based on the comparison of bones alone, disputes can easily arise over how much variation a species had and what falls outside that range.
As for Hesperosaurus, the debate over its validity has been greatly influenced by the material O.C. Marsh used to create the name Stegosaurus armatus in 1877. The fossils were very scrappy, and compared to skeletons discovered since the 19th century, are not very useful in distinguishing these bones from other better-established Stegosaurus species such as S. stenops and S. ungulatus. This means that almost any restoration of the first species Marsh described, Stegosaurus armatus, is going to be a composite of other specimens and therefore obscure the defining characteristics of Stegosaurus as seen in the other species. As a result, it would be possible to lump almost any dinosaur with characteristics similar to the sparse materials Marsh found into the genus Stegosaurus, and it was on that basis that Hesperosaurus was proposed to be a unique species of Stegosaurus.
As Carpenter (and, in the same volume, Peter Galton) argues, however, Stegosaurus armatus is not the best dinosaur to use for determining differences between Stegosaurus species. If the more complete Stegosaurus stenops is taken as representative of the genus, it clearly differs in enough characteristics from Hesperosaurus for both to be considered separate genera. In fact, the differences between them have only become more apparent since more complete specimens of Hesperosaurus have become known.
Overall, I think Carpenter makes a solid case for Hesperosaurus; when compared to the better-known species of Stegosaurus, it was clearly a very different animal. Nevertheless, the fact that two groups of animals were easily distinguishable from each other does not tell us whether we should group them as different species or genera. That is something that is proposed, debated and revised according to the ideas of scientists, and there is no doubt that paleontologists will continue to play the dinosaur name game as research continues.
Carpenter, K. (2010). Species concept in North American stegosaurs Swiss Journal of Geosciences, 103 (2), 155-162 DOI: 10.1007/s00015-010-0020-6