Re: Tanystropheus & archosaur evolution

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 08:30 AM 11/24/98 +1100, M.Alan wrote:

>yes - there seem to be two interpretations - one has Herrerasaurus & co
>as basal theropods, another as basal dinosaurs.  I prefer the 2nd
>interpretation, if only because:
>
>(a) there logically WERE basal dinos somewhere, and
>
>(b) one would expect that at least SOME (even if fragmentary) their
>remains are therefore likely to have turned up in the various rich
>early-late Triassic (Carnian) tetrapod assemblages (Ischigualasta, Santa
>Maria, Popo Agie, Lower Docum, Maleri, etc,) and so therefore
>
>(c) if Herrerasaurus & co are NOT basal dinos, then where the frig ARE
>those beasts?
>
>Of course, one way around this cunundrum would be to say that the
>Theropoda themselves are the basal stem from which the other lines of
>dinos (sauropodamorph & ornithischia) evolved.

And so we return to the old days... :-(  Welcome back Thecodontia, Pisces,
and Invertebrata :-)

It seems part of the problem here is that we are talking in different
lingos.  Both the Sereno position and the extreme Holtz & Padian position
are necessarily within the cladisitic framework (i.e., Dinosauria = all
descendants of the most recent common ancestor of Triceratops and
Neornithes; Saurischia = Neornithes and all taxa closer to it than to
Triceratops; Theropoda = Neornithes and all taxa closer to it than to
Sauropodomorpha).

In the most parsimonious distribution of the data as Sereno et al. interpret
them, _Eoraptor_ and _Herrerasaurus_ fall closer to birds than to
sauropodomorphs, and are therefor theropod dinosaurs.

In the most extreme of the Holtz & Padian trees as presented in 1995 (again,
one not supported in the current database), _Eoraptor_ and Herrerasauridae
fell outside of the clade Ornithischia + Saurischia, and were thus not
dinosaurs by definition.  They were really damn close, the next best things,
but just outside.

In either scenario, these taxa represent the closest thing known to a basal
dinosaur.


Oh, and to address Luis Rey's comment:
>Agreed one hundred percent with this message... next step is to officially
>recognize "Class Dinosauria"... what are the academics waiting for?
>
>
Luis, have you missed something here?  As in the general abandonment of the
rank concept in taxonomy?  Most academic paleontologists are HAPPY with Aves
within Dinosauria (that's where the data keep putting them!); it's the whole
"Class" thing that's not worth the effort (since Dinosauria is clearly
within the "Class" Reptilia at the same time it contains the "Class" Aves).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661