What is the significance of embryonic studies in understanding the evolutionary stages of turtles?

Two main elements characterise the skeletal morphology of turtles: the carapace, the upper shell, and the plastron, the lower shell. The upper shell distinguishes turtles and tortoises, the two genera belonging to the order Chelonia, from all other vertebrates. For a turtle, the carapacial ridge (CR) begins in the embryo as a bulge posterior to the limbs but on both sides of the body. Such outgrowths are the first indication of shell development in turtle embryos. While the exact mechanisms underpinning the formation of the CR are still not entirely known, some biologists argue that understanding these embryonic mechanisms is pivotal to explaining both the development of turtles and their evolutionary history.

The upper shell is composed both by the external shield visible from the outside and by the ribs and vertebrae ensnared underneath it. There is no other known vertebrae that posseses such a composite structure. The positioning of the ribs within the shell begins within the shell begins with the migration of the ribs into the CR. Several studies investigated the mechanisms responsible for the formation of the CR, and for the consequent migration of the ribs in the dermal armour.

In turtle embryos, the CR appears at stage 14 of the embryonic stages of turtle development. The CR forms on both sides of the embryo dorsal to the limbs and looks like an outgrowth of the body surface. At this first stage of development, the CR isn't yet a continuous margin across the neck to the base of the tail. In subsequent stages, it grows both anteriorly and posteriorly and becomes continuous over the base of the tail and the cervical region. When the formation of the embryo is complete, after stage 16 the CR forms a continuous margin delimiting the upper shell.

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