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Re: On the subject of mysterious absences...The Answer
----- Original Message -----
From: "Tim Williams" <email@example.com>
Sent: Wednesday, April 23, 2008 1:31 AM
Yeah, sorry about that. David's response preserves most of my original
...And _this_ message was in plain text without line breaks, so that most or
all people too cowardish to use Mucuswaft Outbreak Expose have got the
"truncated" message instead. So I repost the whole thing:
I'll add that the following paper offers a nice summary, despite being
nearly 10 years old. Although the authors recover a turtle+sauropterygian
clade (within Lepidosauromorpha), they do discuss some of the problems
associated with pinning down turtle relationships.
Rieppel O. & Reisz, R.R. (1999). The origin and early evolution of
turtles. Annual Review of Ecology and Systematics 30: 1-22.
Abstract: "A critical reexamination of turtle relationships continues to
support a sister-group relationship of turtles with a clade of marine
reptiles, Sauropterygia, within crown-group Diapsida (Sauria). The high
Homoplasy Index raises concerns about the phylogenetic information content
of various morphological characters in broad scale phylogenetic analyses.
Such analyses may also suffer from inadequate statements of primary
homology. Several such statements that have played an important role in
the analysis of turtle relationships (dermal armor, acromion,
astragalo-calcaneal complex, hooked fifth metatarsal) are reviewed in
detail. An evolutionary scenario for the origin of the turtle bauplan
suggests an aquatic origin of turtles, which is supported not only by
their sauropterygian relationships, but also by paleobiogeographic and
stratigraphic considerations. However, turtle relationships remain
labile, and further investigations of their relationships are required,
involving molecular and physiological data."
I'll just add two findings from recent papers: Although crown-group turtles
(Testudines) started out as amphibious (so that the tortoises are
secondarily terrestrial), turtles as a whole (Testudinata) started out as
terrestrial, judging from the hand and foot lengths and carapace shapes of
all non-testudine testudinates. Furthermore, all of the latter (where
known), as well as many fossil testudines, retain a bone called the
cleithrum in the shoulder girdle respectively plastron (one per side), even
though all crown-group diapsids and IIRC their closest relatives lack
cleithra, at least so far.