The way the body wall of the growing embryo folds inwards helps to explain how the reptiles achieve their unique body shape.
The origin of the turtle鈥檚 body plan has been a mystery. Unlike most vertebrates, its ribs are short and wide, like paddles. They do not wrap around the body like humans, but rather fuse with vertebrae and superficial bony plates in the skin of its back called osteoderms to form a carapace. Previously, it was unclear whether the flattened ribs or osteoderms formed first in the turtle鈥檚 evolutionary history.
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Last November, researchers in China uncovered a 220-million-year-old fossil that had a complete plastron 鈥� the portion of the shell on the turtle鈥檚 underside 鈥� but an incomplete carapace. Its ribs were short, wide, and flat like modern turtles, but the osteoderms were absent. Some took this as evidence that the ribs drove the evolution of the shell, rather than the skin.
Now Hiroshi Nagashima and colleagues at the in Kobe, Japan, have fresh evidence that supports the 鈥渞ibs first鈥� interpretation, by helping to explain a mystery surrounding the turtle鈥檚 oddly positioned shoulder blade.
Outside in
The turtle shoulder blade is unique among tetrapods 鈥� vertebrates with four limbs 鈥� because it sits inside the reptile鈥檚 ribcage, which runs beneath its shell. To understand how this came about, Nagashima鈥檚 team compared Chinese soft-shelled turtle embryos with mouse and chicken embryos.
They found that early on in development, the body wall folds down and in towards the centre of the turtle鈥檚 body. The fold defines the edge of the future carapace and draws the shoulder blade down beneath the ribcage (see image, top right).
鈥淲e have successfully explained why the scapula of the turtle is found inside the ribcage,鈥� says Shigeru Kuratani, director of the lab where Nagashima made the discovery. 鈥淚t鈥檚 a simple folding, but it doesn鈥檛 happen in other animals,鈥� he explains.
Turning turtle
The upshot of this folding is that it keeps the ribs from growing right around the animal鈥檚 belly. Hence rib growth is restricted to the upper part of the shell, where the bone fuses with other body parts to form the hard part of the shell.
The embryonic stage at which the folding occurs suggests that it happened very early in the evolutionary history of turtle ancestors. 鈥淚t鈥檚 this fundamental morphological event that characterises the turtle body plan,鈥� says , a curator of Evolutionary Biology at the Field Museum in Chicago, who was not involved with the study.
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