Turtle I’m Totally Lacking Part of My Day Shirt

The turtle shell is made up of rows of firmly connected bony plates, which are covered by horny plates, or scutes. The domed carapace covering the back of the animal is connected to the flat plastron on the underside of the animal by a bridge of bone. The resulting box encloses the shoulder and hip girdles, but is open at the front for the head, neck and forelegs, and at the back for the tail and hind legs.How did this unusual structure evolve? One of Aesop’s fables tells of the wedding of Zeus and Hera. Hermes had invited all the animals, but the turtle did not show up. When an angry Hermes demanded to know why it had not come to the feast, the turtle responded that it preferred its own home. Enraged, Hermes made the animal carry its house forever after.Last year my colleague Rainer Schoch and I published our own version of how the turtle got its shell under the less entertaining title “Osteology of the Middle Triassic stem-turtle Pappochelys rosinae and the early evolution of the turtle skeleton.”CarapaceWhile other animals, like the armadillo, evolved body armor, the turtle shell (above: the carapace of a snapping turtle) For many years, the oldest known members of the turtle lineage were Proterochersis and Proganochelys, which are best known from Germany and Poland. They are about 210 million years old. Although less advanced in many ways than present-day turtles, these two stem-turtles already had fully formed shells, providing little insight into the origin of the turtle shell.

Turtle I’m Totally Lacking Part of My Day Shirt

The origin of a biological structure as complex as the turtle shell confronted zoologists with a dilemma. Unless a complete shell suddenly evolved, its development would have taken place in a number of steps. This puzzled researchers because it was not clear what survival advantage each intermediate step would confer. Until just a few years ago, the major problem in searching for fossils of turtle precursors was that paleontologists could not easily imagine what something on the way to becoming a turtle might look like.ProganochelysAmong the oldest known members of the turtle lineage is the 210 million-year-old Proganochelys (above), which already had a shell. ( Wikimedia Commons)Several researchers suggested that bony plates embedded in the skin, called osteoderms, which are found in crocodilians and some other reptiles, had fused to the underlying bones of the turtle to form a solid bony shell. This idea was refuted by zoologists, who since the early 19th century, have studied modern turtle embryos to trace how the shell develops. Their work established that the various parts of the shell develop from different parts of the skeleton. Much of the carapace is derived from the backbone and ribs. Within a disk of connective tissue on the back of the turtle embryo, the trunk ribs broaden and grow straight out to the sides, rather than curving downwards to enclose the trunk as it does for most reptiles.