Mysteries of the Turtle's Shell

The anatomy of a turtle differs so much from that of other vertebrates that scientists have long pondered how turtles evolved from their primitive reptilian ancestors. One aspect of the turtle's anatomy that never fails to capture attention is its shell.

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This unique structure is formed by the animal's ribs which are flattened and fused to its backbone. Additionally, the placement of the shoulder blades of the turtle is odd. Instead of being located on the turtle's back (as they are in all other vertebrates), the turtle's shoulder blades lie within its ribcage. To accommodate this skeletal arrangement, the muscles that connect the turtle's shoulder blades to the trunk have been twisted and folded.

Photo © Shigeru Kuratani and Hiroshi Nagashima / Science AAAS.

How did this bizarre arrangement of the turtle's shoulder blades and supporting musculature evolve? Until recently, no one really knew. The fossil record offered few insights, since no transitional forms had been unearthed that show the shoulder blade migrating, gradually over generations, towards the inside of the ribcage. But now a team of scientists led by Hiroshi Nagashima from the RIKEN Center for Developmental Biology in Japan have found some clues as to how the turtle's unique bone structure could have evolved. These clues have surfaced not from examining the fossil record but from studying the embryos of turtles and other vertebrates. By comparing the embryos of Chinese soft-shelled turtles (Pelodiscus sinensi) at various stages of development to those of mice and chickens, Nagashima's team was able to visualize just how the turtle's shell develops and how the shoulder blade ends up on the underside of its shell.

During the early stages of development, the embryos of all three species have shoulder blades that sit on their back, outside of the ribcage. As the mouse and chicken embryos develop, the shoulder blades remain on the animal's back. The ribs become embedded in a layer of muscle know as the muscle plate. In contrast, as the turtle embryo develops, the second pair of ribs swings forward and grows over the shoulder blades. Meanwhile, the muscle plate tucks inward and forms the edge of the turtle's shell.

Photo © Shigeru Kuratani and Hiroshi Nagashima / Science AAAS.

Ngashima's work not only describes a possible mechanism for the turtle's odd anatomy, but it also provides context for a recently discovered fossil turtle, Odontochelys. Onontochelys was a turtle that lacked an upper shell but posessed a lower shell (also called a plastron). It had short ribs that did not fan out as the modern turtle's do. Consequently, Onontochelys' second pair of ribs did not swing over its shoulder blades during development.

Photo © Shigeru Kuratani and Hiroshi Nagashima / Science AAAS.

Refs:
Nagashima, H., Sugahara, F., Takechi, M., Ericsson, R., Kawashima-Ohya, Y., Narita, Y., & Kuratani, S. (2009). Evolution of the Turtle Body Plan by the Folding and Creation of New Muscle Connections Science, 325 (5937), 193-196 DOI: 10.1126/science.1173826