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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):
Plesions - several families and isolated spp.
several more plesions
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