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Re: What are the biggest highlights of modern paleontology?
Just to make it clear again - this is my text, NOT Reisz's, and I didn't mean
to imply that it represented his views on anything other than Eunotosaurus (I
discuss Lee's paraeiasaur ideas further on). I'm inclined to think he is
leaning towards the archosaur link but I certainly cannot speak for him.
I go into a fair bit of detail on turtle rib embryology earlier in the book
(bear in mind that the book is a popular natural history, not a scentific
treatise!). Here, though, is some more of what I said about Odontochelys, and
once again, if it contains errors they are mine, not Reisz's! Mind you if you
can find the book (authors always appreciate this), I'd be curious to know what
you think of the whole palaeo chapter:
>From its discovery in the 1880s until the first decade of the 21st century,
>Proganochelys was the oldest turtle known. Then, in 2008, scientists in China
>announced the discovery of a still older Triassic turtle, about 40–50 cm
>(16–20 in) long, dating from roughly 215–220 million years ago. It has been
>named Odontochelys semitestacea, and if you saw one alive you might well be
>forgiven for wondering just what sort of an animal it was. The name
>semitestacea means “half-shelled,” and though Odontochelys had a fully formed
>plastron and oddly broadened ribs that resemble miniature
cricket bats, the known specimens show almost no trace of a bony carapace.
Odontochelys represents an earlier stage in turtle evolution than
that of any other fossil turtle so far discovered. Proganochelys had only
palatal teeth, situated not where we have our teeth but on the roof its mouth
(though there are vestiges of teeth on the premaxillary bone at the front of
the jaw). Odontochelys, by contrast (Odontochelys means “toothed turtle”),
still has a reptilian snout lined, like our own jaws,with marginal teeth.
Odontochelys’ ribs join up with its spinal column in a manner typical of other
reptiles rather than later turtles. Though a series of bony neural plates run
with the vertebrae as they do in almost all other turtles. Instead of radiating
forward over the shoulder girdle, the first few ribs appear to point backward
as they do in a turtle embryo before its carapace begins to develop. Does
Odontochelys represent a transitional stage, before turtles had evolved fully
Its discoverers thought so, but other paleontologists are not so
sure. Robert Reisz, who has examined the specimens, thinks that Odontochelys
represents an animal, like the modern leatherback (Dermochelys coriacea), in
the process of losing a carapace its ancestors once had. He believes that the
anterior ribs may point backward because they became damaged and distorted
after death. The rest of the ribs splay outward as they do in other turtles,
suggesting that the Odontochelys embryo had a carapacial ridge (see page XX
Chapter 1)—something that could hardly have been present if there had been no
carapace (though other scientists point out that the carapacial ridge may have
been present but not, as in modern turtles, complete). Its plastron extends to
the sides between the front and hind limbs, as though it was connecting to, or
bracing, something—perhaps, Reisz suggests, a leathery carapace that once had
been supported by bone.
Based on its limb structure,Odontochelys appears to have been an
animal that spent most of its time in the water (though its toe bones are
rather short, a feature recalling more terrestrial turtles). Did turtles evolve
from swimming creatures that did not need extensive bony armor? Or did they
develop from armoured land dwellers, making Odontochelys the first known turtle
to return to the sea? For those searching for the
ultimate turtle ancestor, these questions may be crucial.
1825 Shady Creek Court
Mississauga, ON L5L 3W2
From: David Marjanovic <email@example.com>
To: DML <firstname.lastname@example.org>
Sent: Monday, February 4, 2013 4:55:39 AM
Subject: Re: What are the biggest highlights of modern paleontology?
Now, Reisz is definitely something of an authority on turtle origins. However:
> "[..] For years, however, biologists dismissed Eunotosaurus on the
> grounds that turtle ribs are not expanded, but fused to plates of
> dermal bone. A careful study of Eunotosaurus showed that in almost
> every feature it was nothing like a turtle. Eunotosaurus was
> accordingly dropped from the list of suspects until the discovery of
> Odontochelys, which has expanded ribs of its own, created a brief
> flurry of renewed interest in it. The ribs of Eunotosaurus are,
> however, not really like those of Odontochelys (their shape is
> different, and they wrap around the chest cavity in non-turtle
*Odontochelys* doesn't have expanded ribs either, it has ribs fused to bone
plates of unknown origin (their histology hasn't been studied).
> Besides, Eunotosaurus lived a very long time ago—some 45 million
> years before even Odontochelys, more than the distance separating
> turtles from another candidate group, the pareiasaurs. Its role in
> turtle evolution seems, despite superficial similarities, unlikely."
The time gap is of course real, but that's the fossil record for you. *Triadoba
lived some 60 Ma before the next two described lissamphibians, *Prosalirus*
(stem-frog) and *Eocaecilia* (stem-caecilian).
I'm surprised Reisz brings up the pareiasaurs. Has he ever supported that
hypothesis in print? I thought his latest publications on the topic are the
ones that propose the procolophonoids as the turtle sister-group? Anyway, the
pareiasaurs are quite tempting, but they come with their own problems: off the
top of my head, they have sutures between the parasphenoid and the pterygoids
in the palate, making the basipterygoid articulations immobile, and so do
post-Triassic turtles -- but both *Progano-* and *Odontochelys* retain actual
synovial joints there. Also, now that we know what turtle teeth look like,
they're plesiomorphic, not elaborated with a long row of extra denticles for
cutting leaves like in pareiasaurs... unless *O.* has undergone a reversal.