Where, oh where do those turtles come from?

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

Not dinosaurs, but a couple of interesting papers on vertebrate paleontology.

(I don't think anyone's posted on them yet, so I thought I'd bring them to
peoples' attention).

The latest issue (vol. 120, issue 3) of the Zoological Journal of the
Linnean Society is nothing but TWO long (and long-awaited) papers on the
origin of turtles among the other reptiles.  They are both (from first
glance, at least) very well written, well organized, and well researched.
And they come up with totally different conclusions.

The papers are:

Lee, M.S.Y. 1997.  Pareiasaur phylogeny and the origin of turtles.  ZJLS
120:197-280.

and

DeBraga, M. & O. Rieppel.  1997. Reptile phylogeny and the
interrelationships of turtles.  ZJLS 120: 281-354.

Lee concludes that turtles are the derived descendants of the pareiasaurs,
those lumpy bulky Permian herbivores.  Pareiasauria (including turtles),
procolophonids, and some other forms form a monophyletic
Anaspdia/Parareptilia.  More detail here on the pareiasaurs than almost any
other published source.

DeBraga & Rieppel, on the other hand, support the idea that turtles are
derived diapsids, are lepidosauromorphs, and are the sister taxon to
Sauropterygia.  Pareiasaurs (not including turtles), procolophonids, and
others form a monophyletic Parareptilia, but turtles are not part of this clade.

The main sources for the discrepancy?  Different characters and (probably
more importantly) different taxa used.  Lee does not include much in the way
of Triassic and later forms in his analysis; DeBraga & Lee have more
varieties of amniotes in their study, but use only three of the pareiasaurs.
To further test these hypotheses, someone is going to have to go through the
character and taxon lists, combine the lists, evaluate them to double check
the coding, and run the new matrix through.  I make no predictions as to
which (if either) of these hypotheses would be supported.

One problem with the DeBraga & Rieppel paper, though, is the fact that they
do not seem to be familiar (or at least don't use) the phylogenetic
taxonomic definition of previously established taxa.  For example, they
"redefine" Lepidosauromorpha as all descendants of the Lepidosauria,
Testudines, and Sauropterygia.  However, Lepidosauromorpha has already been
defined as Lepidosauria and all taxa sharing a more recent common ancestor
with lepidosaurs than with archosaurs.  By their analysis, turtles would
fall within that definition, even if Gauthier et al. did not include turtles
in this group when they named it.

A final thought: I'll have to look into ordering this issue as a single
unit.  Neither paper is short enough to be easily xeroxed...

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661