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.