Monday, December 11, 2017

The Mysterious Morrison

I'm a huge fan of the Morrison Formation; the famous North American formation from which Brontosaurus and Allosaurus and Stegosaurus come.  Although it's very well known and kind of established what was once seen as a "standard" Jurassic fauna, there are actually some real questions i it is representative, or unusual.

In large part, this is because of the strange prevalence of diplodocid dinosaurs.  The Morrison is teeming with them.  It's got Diplodocus itself of course (two species, probably separated in time) plus Apatosaurus, Barosaurus, Supersaurus, Galeamopus, Kaatedocus, the newly re-split out Brontosaurus plus (probably) Amphicoelias

The most common sauropod was the macronarian Camarasaurus, however, and Brachiosaurus appeared rarely, probably living more in the highlands where it's remains were less likely to be fosssilized.  Nowhere else is this diversity represented, and even in contemporary faunas that are often compared to the Morrision, the diplodocids are much more rare and less diverse than they are in the Morrison.  In the Lourinha formation, for example, there's only Dinherirosaurus (which may be simply a different species of Supersaurus instead) and in the Tendaguru, there's only Tornieria.  Portugal's fossil needs more surveying (the same is true for Africa's) but in Tendaguru it seems abundantly clear that the analog to Brachiosaurus (Giraffatitan) is relatively much more common.

So the diplodocids are actually present in only a fairly narrow window of time, and really only had amazing diversity in one (albeit a large one) location; the Morrison itself.  Other than that, they don't really appear anywhere else, except in small numbers in some formations that are contemporary in time with them.  What does that mean?  Were they a briefly flourishing local family, or do we just need to find more of them in the rest of the world as more formations get to be better known?

It seems clear that the macronarians, especially the titanosaurs, are the most diverse and successful clade of sauropods; they lasted the longest, were by far the most widespread, and had by far the largest number of clades and specimens both.

But somehow, I can't get over the classic "Brontosaurus" and "Diplodocus" that I grew up with.  Along with "Brachiosaurus" (which was actually based on the African skeleton rather than the related but different North American one, so it isn't actually Brachiosaurus after all) they were the three sauropods that everyone knew; the most well-known, the longest and the biggest, respectively.  None of those is true anymore, but it's weird to think that the classic dinosaurs I knew... aren't.

No comments: