How turtle shells evolved… twice – Judy Cebra Thomas

How turtle shells evolved… twice – Judy Cebra Thomas


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.

99 thoughts on “How turtle shells evolved… twice – Judy Cebra Thomas

  1. Absolutely gorgeous animation with good information.. I want more of these paleontologic animations. So much fun!

  2. Wow this narrator has an amazingly gentle yet academically authoritative voice. I could see her narrating the expository introduction of some epic sci-fi or fantasy movie.

  3. ohh wow did u know guys that turtle and u came from a rock the origin of life thats evolution teaches. look it up. evolution is a fairytale that humans and animals evolve from 1 ancestor and thats a ROCK.

  4. Ok, somebody please explain where all the missing links Are??
    If you can find bones of the first one that looks somewhat similar, why can't you find ones that are closer to the present? Wouldn't it be easier to find newer bones that are less likely to have been damaged and closer to ground level?

  5. That turtle embryo is so cute to me.. looking back at us while we look at his under development shell… awww❤️

  6. I read the title and I imagine the shouted-out "TWICE!" in every music video single of Twice after the ellipsis.

  7. Awesome! Just wish the illustrations were more lifelike and less cartoony so I could get a better idea of how the creatures really looked. Fascinating work in such a short and concise video 🙂

  8. Ye I know about scutes. I wait till baby turtles grow up so I can then obtain the scutes and craft a Turtle Shell.

  9. I have one question to ask u, were u there?!? Did you watch it evolve? Or did anybody watch it “evolve”

  10. I can’t wait to skin the outer layer of skin that covers the carapace to be able to breathe underwater longer

  11. Me, trying to make my pet turtles watch this : THIS IS YOUR STORY WHY WON'T YOU WATCHHHHHH

    (Btw I do have 2 turtles, one is Buffy and one is Tooter)

  12. Creationists have no problem of dogs coming from wolves, but can't accept the fact that turtles came from that.

  13. Wait, so what happen if you remove the shell when they are born, I mean you need a surgery for it but would be cool seeing one without one, cruel but cool

  14. how did they evolve from having shoulder blades on outside of ribs to shoulder blades on the inside. Seems impossible to have million years successfully surviving with one in and one out or any other implication of instant transformation of out to in.

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