GunnerJ

I've probebly started a thread on this subject in the past, but I wanted to post a link I found:

The Turtle Pages - Turtle Evolution

The site has a info on the early species and the history of the "kind" (if I may borrow a term.) One thing it doesn't have much info on, however, is the developemnt of these minature repltilian tanks from early reptiles. How much information is availble on the subject, and how good is the fossil reccord for turtle development?

theyeti

Bauplan! We call it a bauplan! (Plural: bauplanë.) Get your terminology straight -- no borrowing creationists words!

theyeti

lpetrich

The plural is baupläne -- the umlaut over the a, not the e.

I looked for stuff on Proganochelys, the earliest known turtle, which lived in the Triassic, and found this AMNH page, this UIUC page, this detailed page examining several origin hypotheses, etc.

It has a mixture of ancestral-reptile and turtle features:

Familiar bony shell
No teeth
Beak
Large ear opening
Sprawling limbs
Un-retractable neck protected by spines
(all present-day turtles have retractable necks)
Long tail with spikes and studs and a club on the end (present-day turtles have short tails)

There is also the contentious question of what the turtles' closest relatives are. From PubMed I find:

Gene. 2000 Dec 23;259(1-2):139-48.

Phylogenetic position of turtles among amniotes: evidence from mitochondrial and nuclear genes.

Cao Y, Sorenson MD, Kumazawa Y, Mindell DP, Hasegawa M.

The Institute of Statistical Mathematics, 4-6-7 Minami-Azabu, Minato-ku, Tokyo 106-8569, Japan.

Maximum likelihood analysis, accounting for site-heterogeneity in evolutionary rate with the Gamma-distribution model, was carried out with amino acid sequences of 12 mitochondrial proteins and nucleotide sequences of mitochondrial 12S and 16S rRNAs from three turtles, one squamate, one crocodile, and eight birds. The analysis strongly suggests that turtles are closely related to archosaurs (birds+crocodilians), and it supports both Tree-2: (((birds, crocodilians), turtles), squamates) and Tree-3: ((birds, (crocodilians, turtles)), squamates). A more traditional Tree-1: (((birds, crocodilians), squamates), turtles) and a tree in which turtles are basal to other amniotes were rejected with high statistical significance. Tree-3 has recently been proposed by Hedges and Poling [Science 283 (1999) 998-1001] based mainly on nuclear genes. Therefore, we re-analyzed their data using the maximum likelihood method, and evaluated the total evidence of the analyses of mitochondrial and nuclear data sets. Tree-1 was again rejected strongly. The most likely hypothesis was Tree-3, though Tree-2 remained a plausible candidate.

Conclusion: turtles are closest to either crocodilians or birds+crocodilians

Turtles as close to crocodilians may seem odd, because a common traditional view is that they are a survivor of late-Paleozoic anapsid reptiles. However, turtles share with crocodilians the distinctive feature of bony plates in their skins.

MrDarwin

Turtles are to other reptiles as birds were to other reptiles just 20 years ago, except that there is no chelonian version of Archaeopteryx. This gap gets less press because most people consider turtles to be just another kind of reptile, even though their specializations and differences from other reptiles are nearly as extreme as those of birds. One particular problem is how turtles got their shoulder blades inside their ribcage (a position necessitated by the shell) when all other vertebrates have their shoulder blades on the outside.

I'm just waiting for a series of fossils to elucidate the evolution of turtles just as fossils have recently (and spectacularly) elucidated the evolution of tetrapods from fish, birds from dinosaurs, and whales from terrestrial ungulates. I think it's only a matter of time because turtles are easily fossilized and live in environments conducive to fossilization (unless, of course, the transition happened on land and not in the water--a distinct possibility).

GunnerJ

lpetrich, I dodn't know that crocs had bony plates in their skin! But it actually makes a lot of sense to me: IIRC, crocs spend a lot of their time laying around doing jack-shit, which is also how many turtles spend their days.

GunnerJ

MrD- so you're saying that turtles could represent a whole different class from reptiles?

Oolon Colluphid

Yeah, but koalas and sloths similarly spend ages doing bugger all. And sporulated bacteria aren’t reknowned for their busy lifestyles either. Hell, do you think they may be all related?

Oolon

Coragyps

And don't leave out some of us here at IIDB........

lpetrich

Yes, crocodilians have rows of bony plates in their skins; these are sometimes called scutes or osteoderms.

But I don't think that turtles are as derived as birds; they keep several ancestral-reptilian features that birds do not:

They walk on all fours.
Their legs are short and sprawled.
They are cold-blooded.
Etc.

Doubting Didymus

Didn't Megatherium have some sort of bony plating under its skin? Still, I guess you wouldn't call megatherium "slothful", at least not to its face.

Yes, they certainly do. So do turtles. Most people I have met try to apply a common-sense based taxonomy, apparently using the primary criterion of "greenness". In reality, there's no way to squeeze all green scaly things into a monophyletic group.

The Center for North American Herpetology