The bodies of turtles and other chelonians are fundamentally different from those of all other animals. Their distinctive shells develop from the ribcage, which lies above, rather than below, the shoulder blades.
Shigeru Kuratani and his colleagues at the RIKEN Center for Developmental Biology in Kobe, Japan, compared the embryogenesis of the Chinese soft-shelled turtle (Pelodiscus sinensis, pictured) with that of other animals to understand how developmental changes may have resulted in the turtle's unusual form. The authors found that whereas the ribs of mice and chickens grow to encircle the body, those of turtles grow outwards and then stop, with the shoulder blades tucking under the edges of the ribs.
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Developmental biology: The turtle fold. Nature 460, 309 (2009). https://doi.org/10.1038/460309c
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DOI: https://doi.org/10.1038/460309c
