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Re: earliest tetrapods



At 7:17 AM 8/15/95, David Brez Carlisle wrote:
>I have just been reading a paper in the Zoological Journal of the
>Linnean Society, vol.114 (3), pp. 307-348, July, 1995, on the
>post-cranial skeleton of the Devonian tetrapod _Tulerpeton curtum_
>Lebedev.  (I'll get back to dinos eventually).  The authors are
>Oleg A. Lebedev and Michael I. Coates.
>
>They conclude that batrachiomorphs (Amphibia) and Reptiliomorphs
>(everything else of land vertebrates) are sister groups and that
>the latter did not evolve from the former.  At least, I think
>that is what they are saying.  Tulerpeton has six digits on each
>foot (manus and pes).  Thet conclude that the primitive condition
>in tetrapods is polydactyly, not pentadactyly, and that the five-
>toed condition has eveloved separately in batrachiomorphs and
>reptiliomorphs.  That's "evolved".  Penadactyly is a derived
>condition in rept.  In bact. the derived condtion (separately
>derived) is a four-fingered manus, and a five-toed pes.  So
>much for the pentadactyl limb as diagnostic of tetrapods!
>
>They p ropose the  following classification (in part):
>INFRACLASS Osteolepiformes
>        Plesions - several families and isolated spp.
>        DIVISION Tetrapoda
>                Plesion _Greerertepon_
>                SUBDIVISION Batrachomorpha
>                sUBDIVISION Reptiliomorpha
>                        Plesion _Tulerpeton_
>                        d
>                        several more plesions
>                        INFRADIVISION Amniota
>
>They  state: "A node-based definition of tetrapods is probabaly
>of greater biological use, butr this requires a decision between
>the alternative choices of either 'total-group' (Patterson, 1993)
>or 'crown-group' ... definitons.  A total-group definition of
>tetrapods would i nclude all fosil spp . that are  more closely
>related to the crown-group that to its extant sister group
>(dipnoans)...This definition has the the merit of disstinguishing
>clearly the most fundamental cladistic events, but in this case
>a CROWN-GROUP definition is prefered, restricting the tetrapoda
>to those taxa which include... the basal synapomorphies
>uniting all living lineages."
>If this pro posal is acceptable, then it seems to me to open
>the possibility of a crown-group definition of dinosaurs,
>excluding the birds.  Has this ever been suggested?
>I have never liked i ncluding the birds as dinos, even though
>strict cladistics suggests that they belong th ere.  This
>leads to such awkward phrases as "non-avian dinos".
>Any co mments?
>It's an important p aper in my view.
>David (an unrepentant Linnean)
>
>--
>From: David Brez Carlisle
>bk090@Freenet Carleton.CA

        Is it just me, or isn't the "total group" as here defined, the same
as a crown group?  Also, you can't have dinosaurs minus aves as a crown
group, as there are no derived characters shared between ornithischia +
saurnishia that are lacking in aves.  However, this doesn't mean that you
MUST consider birds as dinosaurs.   While birds should be nested (sorry) w/
in Dinosauria in the same sense that all tetrapods could be considered
sarcopteryigian fish, recognition of paraphyletic groups is not as great an
evil as you might think.  This is especially true in paleontology, where
paraphyletic groups are often exclusively studied.  For example, its
possible to study mesonychids w/out studying archaeocetes and its possible
to discuss archaeocetes w/ out having to include odontocetes and
mysticetes.  Being an "unrepentant Linnean" is to hold a pre-evolutionary
view of natural history,  so you might want to think that over.  Or at
least attempt to define quantitatively what a "hepaxyorder", etc. is.  Or
go into the woods and try to find one.  Linnean taxonomy is a system of
arbitrarily created pigeonholes for insertion of groups of taxa, and
doesn't necessarily reflect phylogeny/ natural history, in any way.


Jason Head

Shuler Museum of Paleontology

SMU

Dallas, TX.

75275

(214) 768-2750