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Re: On the subject of mysterious absences...The Answer



----- Original Message -----
From: "Tim Williams" <twilliams_alpha@hotmail.com>
Sent: Wednesday, April 23, 2008 1:31 AM

Yeah, sorry about that. David's response preserves most of my original message.

...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."

Cheers

Tim

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.