Name: How turtle shells evolved... twice - Judy Cebra Thomas
Uploaded: 2019-11-07T10:52:58.000Z
Duration: 4 min 46 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

This little creature spends its days  splashing in Late Triassic swamps

Under the surface lies its best  defense against attack:

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

Both are extensions of the animal's  skeleton,

and together they are made  of almost 60 bones.

turtle embryos are made of  undifferentiated cells

through gene activity and communication between cells.

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

except for a bulge of cells called  the carapacial ridge.

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

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.

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,

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

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

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.

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

Most mammals and reptiles rely on a flexible rib cage

but turtles use abdominal muscles  attached to the shell instead:

one to breathe in,  and one to breathe out.

Though there are still gaps in the  fossil record,

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

a creature called Eunotosaurus africanus,

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

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

and belongs to a species called  Proganochelys quenstedti,

Proganochelys could move  between water and land.

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

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

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

have domed shells that can slip free of predators' jaws

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

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

making it easier for them to  squeeze into tight spaces.

Subtitles ListPlay Video

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

specific

eventually

obvious

tough