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Re: Brusatte et al. 2008



On Sat, Sep 13, 2008 at 10:32 AM, David Peters <davidpeters@att.net> wrote:
>
> Having seen the paper and supporting materials, I see that Brusatte
> et al. 2008 have a very interesting cladogram.

Looks fairly typical to me.

> 1. The [theropod + theropod-like forms] form a clade separate from
> [the rest of the dinosaurs + pterosaurs  + 'Crurotarsi.']

They have _Eoraptor_ and _Herrerasaurus_ as basal theropods, if that's
what you mean. There is no "the rest of the dinosaurs + pterosaurs  +
'Crurotarsi'" clade -- that would be quite bizarre. Theropods are the
sister group to _Sauropodomorpha_ (represented by _Plateosaurus_,
_Antetonitrus_, and _Pantydraco_), just like they always are. Are you
looking at Figure S1?

Serial outgroups to _Coelophysis_, from closest to furthest:
_Liliensternus_; _Zupaysaurus_; _Herrerasaurus_; _Eoraptor_;
_Sauropodomorpha_; _Ornithischia_ (represented by _Eocursor_,
_Pisanosaurus_, and _Heterodontosaurus_); (_Lewisuchus_ +
_Silesaurus_); _Sacisaurus_; _Eucoelophysis_; _Pseudolagosuchus_;
_Marasuchus_; (_Dromomeron_ + _Lagerpeton_); _Pterosauromorpha_ (incl.
_Scleromochlus_); _Crurotarsi_; _Proterochampsidae_; _Euparkeria_; and
_Erythrosuchus_.

> The big question for you to ponder is this: do all these nestings
> make sense in terms of gradualism in evolution? If 'sister taxa'
> don't blend morphologically from one to another, isn't that a red flag?

No.

Sister taxa are just that -- sister taxa. They are allowed to each
have their own apomorphies, as long as there are other apomorphies
which they share.

As for gradualism, sure, if the fossil record were more complete. It's
not very complete for Triassic vertebrates, though. And even in strata
where it is more complete (e.g., Pleistocene vertebrates), there are
still entire biomes that are barely represented at all (rainforests,
mountains, etc.). Consider: when we do not have even a single
stem-group chimpanzee fossil*, how can we expect to have complete
representation of animals 200 million years older?

* Unless _Ardipithecus_ is one, although that's not the majority
consensus. Even that would leave a four-million-year gap between it
and the only other chimpanzee fossils, some teeth belonging to the
crown group, dating from around half a million years ago.

-- 
T. Michael Keesey
Technical Consultant and Developer, Internet Technologies
Glendale, California