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RE: Maastrichtian extinction and synonymous sauropods

Tim Donovan

> > Note the loss of some diversity about 2 million years before the
> > end, which suggests another factor causing a preliminary extinction.

> Diversity of large dinos definitely declined with the loss of
> centrosaurines etc.

Apologies if this response seems rushed...

Extinction is a normal part of evolution. Throughout the Mesozoic there was
a steady turnover of dino species - no contest there. Certainly, there
appears to have been such a turnover of species *within* the Maastrichtian
of North America and Europe.

What happened c mid Maastrichtian in North America hardly seems like a normal part of evolution. Among large herbivores, diversity waned considerably on the generic and family level. Lambeosaurs were essentially gone by the Lancian, and probably disappeared entirely, as they are known from a single specimen found stratigraphically low in the Hell Creek. Even hadrosaurine diversity was diminished with the loss of Saurolophus. These losses were in addition to the near 100% turnover among surviving lineages e.g. Euoplocephalus-Ankylosaurus.

In North America, centrosaurines disappear;
but chasmosaurines are still alive and well.  If Cathy Forster is correct,
there were no less than four chasmosaurine species existing at the very end
of the Cretaceous: _Triceratops horridus_, _T. prorsus_, _Diceratops
hatcheri_ and _Torosaurus latus_.

True, but IIRC she indicated that only one-horridus-was common. Is it certain that Diceratops persisted t the end of the Lancian btw? Edmontonia may not have.

Thus, I would be hesitant about referring
to such faunal turnovers

Not just turnover but significant extinction.

The Maastrichtian gets extra attention - after all, it was the final stage of the Mesozoic. But even if you can demonstrate there was a decline in diversity (!) during the Maastrichtian, you then have to demonstrate that the K/T extinction represents a culmination of this decline.

This is possible but very hard to test. If the mid Maastrichtian decline in NA was due to the advent of Tyrannosaurus, the isolation of the affected region owing to seaways may explain why other regions experienced no losses at the time. But regression may ultimately have led to even more profound losses in regions where the prey lacked any prior exposure to tyrannosaurs or coevolutionary preparation to withstand them. It is possible that the archpredator made at least some contribution to he final extinction, perhaps a very major one, which was foreshadowed by the mid Maastrichtan event.

Maybe there
were peaks and troughs in dino diversity (!) during other stages of the
Mesozoic.  Again, when it comes to the Maastrichtian, you have to delineate
correlation from causation; I think you're confusing the two.

Maybe the impactors are. Do you have a better explanation for the mid Maastrichtian extinctions and turnover?

> One difference between the Morrison and Nemegt is the presence of
> therizinosaurs-presumably high feeders-in the latter, which probably was
> at the expense of sauropod diversity.

Only if you can demonstrate (a morphometric study of teeth would be useful)
that sauropods and therizinosaurs targeted the same types of vegetation.

Pretty likely given the long necks of therizinosaurs, and their upright stance.


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