Animal Bodies Rearranged

On Pharyngula, PZ Myers considers a computer model which posits that bones are simply exoskeletons turned inside-out. Myers writes “We know from the homology of the patterning molecules involved that vertebrates and invertebrates are upside-down relative to each other, so at some point an ancestor flipped.” Such major differences in body plan arise during embryonic development, driven by highly evolved genetic instruction. But the growth of internal and external skeletons depends on distinct biological mechanisms, leading PZ to call the dataless computer model “abiological and ahistorical bollocks.” PZ proves his point with the turtle, a vertebrate that also has an insect-like shell. He explains, “The ribs and vertebrae are ‘endoskeletal’, formed by chondrogenesis and ossification, while the scutes or plates of the shell are dermal bone,” and where they meet “represents the fusion of two kinds of bone.” But how did the turtle’s shoulder blades get inside its rib cage? Allow PZ to explain…