It’s nice to see that the genus Brontosaurus is being rehabilitated after over a century of being rudely lumped together with Apatosaurus. For many of us amateurs, of course, it never needed rehabilitation, having been far more iconic than its alter-ego throughout the last century.
When I was suddenly astonished by seeing “prehistoric monsters” in Walt Disney’s Fantasia at the age of five, my father told me they were Brontosaurus. When my brother and I rampaged in the house, Dad suggested it should be renamed Brontosaurus Villa. Even James White’s medical sci-fi story The Trouble with Emily (so that’s why I went into medicine) had a title punning the sick Brontosaurus of the title (think about it).
As the Smithsonian article shows, the recent work was to review the fossil evidence of the Family Diplodociae, tracing 477 anatomical landmarks across 81 specimens. These specimens, including the newly restored Brontosaurus genus, represent around sixteen species of which the Apatosaurinae I’m considering constitute a sub-family of five or six.
All are found in the Morrison formation of North America which, to quote the always infallible Wikipedia (which has already been updated with the new results) “records an environment and time dominated by gigantic sauropod dinosaurs.” The time (the late Jurassic) covers around ten million years, and the number of separate individuals found shows the pattern that I’ve mentioned before in relation to the fossil record: a relatively small number of forms with several specimens (on average) of each, found over a broad area (in this case of Western USA), rather than transitional sequences of individual fossils in a restricted area.
As the article points out, the taxonomy of such fossils is always subject to dispute between “lumpers” and “splitters”, but in broad terms there has been agreement up to now on the differences: a Diplodocus is quite distinct from an Apatosaurus. An Atlantosaurus is also easily distinguished from an Apatosaurus, and the question is only one of proper relationship, the former still only being considered a possible member of the same sub-family.
To me the significant thing, as has been the case in other groups I have looked into more or less at random, is that all these distinct species existed in the same area at the same time. They appear suddenly, are apparently static over their time in the fossil record, and then go extinct rather than leaving obvious descendants.This, of course, is the pattern to which Stephen Jay Gould drew attention many years ago, though it was already an open secret in palaeontology.
The new classification seems to me to show more of the same pattern on the smallest available scale, so I draw attention to it once more. The original distinction between Brontosaurus and Apatosaurus seems to have arisen from Cope and Marsh’s 19th century rivalry (The “Dinoasur Wars”), and limited and fragmentary fossil material – including the attribution of a completely foreign skull to Brontosaurus. The distinction was dissolved early in the last century at the expense of the Brontosaurus taxon, and the new reinstatement, whether or not it remains fully accepted, is based on careful distinctions between multiple features that have evaded notice in the interim. In other words, they are closely related and very similar genera, from which only five species are distinguished across multiple individuals.
If Darwinian gradualism, or even punctuated equilibria, were the pattern of nature here, these two genera would seem to be ideal test cases. One or other genus (or their constituent species) might be expected to occur earlier, or in different locations if allopatric speciation occured. But in fact, no such pattern was discerned across the material for a century, and none seems to have become apparent in the light of the new work: both Apatosaurus and Brontosaurus, in their various species, lived 150 million years ago, in the same sites in the Western USA – they apparently shared the same territory. As far as the resolution of the relatively rich fossil record goes, there was a sudden appearance of each (from uncertain or lost precursors), then stasis for a few million years, and then extinction. Brontosaurus and Apatosaurus are apparently distinct branches from a stem, not part of a gradual evolutionary continuum. Once again, the actual evolution is happening somewhere else than the fossil record.
That’s been the pattern since the nineteenth century. Time to wonder again what it means, perhaps. But in the meantime, some more magnificent dinosaurs from a lost past: