[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: earliest tetrapods
>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?
Well, yes, here's a comment: you misunderstand the "crown group" concept.
As used in this context (and, indeed, by most systemicists), a crown group
is the node composed of the most recent common ancestor of all LIVING
species in the clade. Since the only living dinosaurs are birds, the common
ancestor of this crown group would be the most recent common ancestor of all
living birds. This clade already has a couple of names: Neornithes or Aves!
Unless George Olshevsky, Larry Martin, or others are correct, and modern
birds evolved from something other than the Megalosaurus + Iguanodon clade,
then birds are the descendants of dinosaurs. As such, you cannot both
exclude birds from the Dinosauria and use only monophyletic groups.
Thomas R. Holtz, Jr.
Dept. of Geology
University of Maryland
College Park, MD 20742