The turtle has always been considered somewhat odd in evolutionary terms. In addition to lacking the hole in the skull—the temporal fenestra—that is characteristic of the egg-laying amniotes, the structure of its shell differs from that of other armored tetrapods such as the armadillo, and its shoulder blades are inside rather than outside the rib cage. An international research team led by Naoki Irie of the RIKEN Center for Developmental Biology has now shown through genomic analysis that turtles are most closely related to crocodiles and birds, and that their embryonic development follows the latest 'hourglass' model of vertebrate embryology1.
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