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Re: Study Backs Quick End of Dinosaurs

By way of Larry Dunn:

Sheehan said that through the whole 180-foot depth of
the Hell Creek formation, the species mix and numbers
of dinosaurs were the same, with Tyrannosaurus as the
most common carnivore and the Triceratops the most
common plant eater. [snip]

``The abundance of dinosaur fossils in the upper three
meters (9 feet, 9 inches) of sediment immediately
underlying the impact layer is well within the range
of many intervals lower in the Hell Creek formation,''
the study says.

I thought the argument of the "gradualists" was not necessarily that the overall _number_ of dinosaurs declined in the last few million years of the Mesozoic, but the overall number of dinosaur _species_ declined compared to older strata. In other words, there was a drop in dinosaur diversity in the late Maastrichtian. The Hell Creek Formation (and equivalently-aged strata in North America) is dominated by a relatively small number of dinosaur genera (_Tyrannosaurus_, _Triceratops_, _Torosaurus_, _Anatotitan_, _Thescelosaurus_, _Stygimoloch_ and the like). Underlying strata may show the same or similar number of dino fossils, but they belong to a higher number of dinosaur species.

William Clemens addresses this point:

Clemens said that the weakness of the Sheehan study is
that it fails to go back far enough in history. He
said that deposits five million and six million years
old contain a much richer variety and number of
dinosaur fossils, suggesting the animals were
declining when the Hell Creek formation was deposited.

On this point (and perhaps I'm overstating this here) is that the number of dino taxa described from a given stratum does depend upon how researchers "package" the fossils into individual taxa.

For example, the material assigned to _Triceratops horridus_ may represent just one species of late Maastrichtian chasmosaurine, or as many as three species (_T. horridus_, _T. prorsus_, _Diceratops hatcheri_) depending on who you ask.

Same goes for _Thescelosaurus neglectus_. Already one specimen has been split of as a new taxon (_Bugenasaura infernalis_), and it's been mentioned on this list that among the remaining _T. neglectus_ specimens may lurk more than one ornithopod species.

However, I'm willing to believe that most (if not all) of the North American tyrannosaurid material from this time can be pooled into _T. rex_. _Nanotyrannus_ and _Dinotyrannus_ can be safely sunk into _T. rex_, although I'm less certain about the fragmentary _Aublysodon_ material found scattered across the continent. (Unserrated premaxillary D-shaped teeth considered to be diagnostic for the genus _Aublysodon_, though I wonder if the serrations were worn down by wear or if the teeth were swallowed and smoothed down by stomach acid.)

``You need to consider the whole fauna,'' Clemens
said. ``Why did amphibians go through this period
unaffected? There was a diversity of birds and they go
through this period unaffected.''

I think this is probably true for neornithines (assuming that all those fragmentary, bits-and-piecey birds from the Maastrichtian of North America are indeed neornithines). I don't think it can be shown to be true for enantiornithines, nor for the various oddball birds (alvarezsaurids included) known from the Late Cretaceous. The discovery of hesperornithiform material (_Canadaga arctica_) from the Maastrichtian of Canada suggests that this group may have been around at the very end of the Cretaceous.


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