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  • Meet Odontochelys semitestacea.

  • This little creature spends its days splashing in Late Triassic swamps

  • with a host of other reptiles.

  • Under the surface lies its best defense against attack:

  • a hard shell on its belly.

  • Odontochelys is an early ancestor of the turtle.

  • Its half-shelled body illustrates an important point about the modern turtle:

  • it actually has two shells that develop totally separately

  • while the turtle is still an embryo.

  • Both are extensions of the animal's skeleton,

  • and together they are made of almost 60 bones.

  • Like other embryos,

  • turtle embryos are made of undifferentiated cells

  • that become specific cell types,

  • and then organs and tissues,

  • through gene activity and communication between cells.

  • At first, turtle embryos look very similar to those of other reptiles,

  • birds, and mammals,

  • except for a bulge of cells called the carapacial ridge.

  • The ridge expands around the body between the neck and lower back,

  • creating a disc shape.

  • It guides the formation of the upper part of the turtle's shell,

  • called the carapace, likely by attracting the cells that will become ribs.

  • Instead of curving downwards to make a regular rib cage,

  • the ribs move outwards towards the carapacial ridge.

  • They then secrete a signaling protein

  • that converts surrounding cells into bone-forming cells.

  • These fifty bones grow until they meet and connect with sutures.

  • A ring of bone solidifies the carapace's edges.

  • The outer layer of skin cells produces the scales, known as scutes,

  • that cover the carapace.

  • The development of the bottom half of the shell, the plastron,

  • is driven by neural crest cells,

  • which can produce a variety of different cell types including neurons,

  • cartilage and bone.

  • A thick shield of these cells spreads across the belly,

  • coming together in regions that produce nine plate-like bones.

  • Eventually, these connect to the carapace by sutures.

  • A turtle's shell has obvious advantages for guarding against predators,

  • but the rigid casing also presents some challenges.

  • As the turtle grows,

  • the sutures between the bones of the carapace and plastron spread.

  • Most mammals and reptiles rely on a flexible rib cage

  • that expands to allow them to breathe,

  • but turtles use abdominal muscles attached to the shell instead:

  • one to breathe in, and one to breathe out.

  • So how did the shell evolve?

  • Though there are still gaps in the fossil record,

  • the first step seems to have been a thickening of the ribs.

  • The oldest known turtle ancestor,

  • a creature called Eunotosaurus africanus,

  • lived 260 million years ago and looked almost nothing like a modern turtle,

  • but it had a set of broad, flat ribs

  • that anchored the muscles of its powerful forearms.

  • Eunotosaurus was likely a burrowing creature,

  • digging homes for itself in what's now southern Africa.

  • Odontochelys semitestacea illustrates another, later step in turtle evolution,

  • with thick ribs like Eunotosaurus plus a belly plate for protection.

  • Our first fossil evidence of the full shell characteristic of modern turtles

  • is about 210 million years old,

  • and belongs to a species called Proganochelys quenstedti,

  • whose ribs had fused.

  • Proganochelys could move between water and land.

  • Unlike modern turtles, it couldn't retract its head into its shell,

  • but had defensive spines on its neck.

  • Modern turtle shells are almost as diverse as the turtles themselves.

  • Sea turtles have flatter, lighter shells for streamlined gliding through the water.

  • Land-dwelling tortoises, meanwhile,

  • have domed shells that can slip free of predators' jaws

  • and help them turn right-side up if they fall on their backs.

  • Leatherback and softshell turtles

  • have shells without the ring of bone around the edge of the carapace

  • or the tough scutes covering it,

  • making it easier for them to squeeze into tight spaces.

Meet Odontochelys semitestacea.

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B2 US TED-Ed turtle shell ridge bone modern

How turtle shells evolved... twice - Judy Cebra Thomas

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    Sophie posted on 2019/11/07
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