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I really enjoy PZ Myers's posts on science, and this is a particularly interesting one: it's about the evolution of turtles (and tortoises) shells. Turtles are fascinating not only because their shells are sort of exoskeleton and endoskeleton fused together. Turtle shells are made of dermal plates (sort of like an exoskeleton) and of ribs, and in addition they have the interesting feature of the scapula (shoulder blade) located below the ribs and not on top of the ribs like in most reptiles, birds, mammals, etc. PZ Myers dispels arguments by creationists that gradual change could not have given rise to such a radical change as the position of the shoulder blade with respect to the ribs. There actually is a transitional form which is ancestor to turtles, called Odontochelys, a toothed proto-turtle from the Triassic. It did not have a "proper" shell, but its body was broad with a flattened back, dermal bone between the limbs, and a ventral shell (called "plastron") The scapula is not under the ribs in this fossil, but the ribs were pulled back with respect to the scapula. In modern turtles, the ribs have further moved laterally and then forward, so that they now lie above the shoulder blade. These changes are not so huge if you think about them from the point of view of developmental biology.

How the turtle got its shell


In my post bashing that silly article claiming to have figured out how endoskeletons evolved from exoskeletons, there was a good question buried in the comments, and I thought I’d answer it.

Are there any models pulled out of arses which explain the turtle’s unique skeleton?

Yes! I mean, no, not pulled out of arses, but there is a lot of really good and persuasive research that uses evidence to show how the turtle skeleton evolved.

First, I can see how this question popped up in a discussion of the evolution of endo/exoskeletons: the turtle shell is an excellent example of an exoskeleton that evolved in a vertebrate lineage at some time in the Triassic, so it’s definitely relevant. Also, turtle skeletons are a bit weird. The shell is made up of the animal’s ribs fused to plates of dermal bone — that is, sheets of bone formed directly by the ossification of the dermis of the skin (an exoskeleton!) rather than by ossification of cartilaginous centers deeper in the body (endoskeletons). The ribs and vertebrae are ‘endoskeletal’, formed by chondrogenesis and ossification, while the scutes or plates of the shell are dermal bone, so this structure also represents the fusion of two kinds of bone. The confusing bit is the scapula, or shoulder blade; yours, as you can tell, is outside the rib cage, but in turtles, the scapula is located inside the ribs of the shell. So somehow the ribs and scapulae in turtles have flipped their relative positions, which sounds like a radical transformation, and it’s not at all clear how you could do that gradually in evolution.

Here is a comparative diagram of cross sections of a turtle and a chicken to illustrate the difference. The ribs (r) are in light green while the scapula (sc) is in dark red. Notice how the shell makes a kind of shield over the whole turtle, with the scapula and whole shoulder girdle underneath and the forelimbs attached to it? While in the chick the ribs are the deepest bones, with the scapulae on the outside? How did that that happen?

Comparison of deep (as, serratus anterior; lsr, levator scapulae and rhomboid complex) and superficial (ld, latissimus dorsi; p, pectoralis) muscles connecting the trunk and shoulder girdle/forelimb.

The answer, as you might guess, comes from looking at how it gets that way in the development of modern turtles, because as you all know by now, developmental biology rules.

The early turtle embryo looks like a generic tetrapod embryo. The first sign of a significant morphological difference is the appearance of a thickened ridge of skin between the limbs, which eventually expands to form a ring marking the margins of the shell. This structure is called the carapace ridge (CR), and you can see it in the cross sections of embryos at two different ages below.

Read the rest here.

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Replies to This Discussion

Turtle Evolution is Fascinating!  More diagrams here

    • Developmental biologyWerneburg and Sanchez-Villagra, 2009, Considered a totally new data source - the developmental sequence of organogenesis. Variations in developmental sequences across a wide range of terrestrial vertebrates were scored as a phylogenetic matrix. As with molecular data, impossible to score for fossil taxa. Result: Turtles group outside the saurian reptiles (lepidosaurs and archosaurs). Consistent with morphological analyses placing them in Anapsida. 

    • New potential sister taxaLyson et al. 2010. Revisited the Rieppel and DeBraga matrix of 1996. adding Odontochelys andEunotosaurus, an odd Permian reptile sometimes thought to be associated with turtle evolution. This addition resulted in the removal of turtles and Eunotosaurus to Anapsida. 

Thanks for those links! Yes, turtle evolution is fascinating! I would lov to have a tortoise as a pet, actually, but I worry my dogs may hurt it.

Yes! Turtle evolution is fascinating but then all evolutions are fascinating!


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