Turtle morphology poses a unique puzzle in that the turtle's scapulae (shoulder blades), situated outside the ribs in other animals, are found inside its shell (which is formed from the bones equivalent to ribs in other species). To explain this inside-out skeletal morphology, researchers at the Laboratory for Evolutionary Morphology of the RIKEN Center for Developmental Biology compared embryonic development of the turtle to that of chicken and mice. While muscles and skeletons initially developed in a similar way, turtle embryo development diverged at a late stage, with the ventral part of the body wall folding inwards together with the scapula, a step made possible by the anatomical layout of the turtle embryo.
Their findings also indicate a resemblance between the early form of the turtle embryo and that of Odontochelys, a 220 million-year-old fossil species unearthed in China last year, believed to represent the ancestor of all modern turtles. Based on their results, the research group has concluded that modern turtle anatomy results from the late development of ribs in an Odontochelys-like ancestor, unraveling the long-standing mystery of the turtle and its shell.
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