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

[David Marjanovic <david.marjanovic@gmx.at>]

----- 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.