Why Do Turtles Have Shells?

It seems obvious that turtles' shells would have evolved to protect them from predators, but researchers have found that they probably first evolved to help turtles dig. Learn what that's all about in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain stuff. From how stuff works, Hey, brain stuff,
Lauren boge obamb here. Many of us have seen a
turtle tucking itself into its protective shell, pulling in not
only its four legs, but its head and tail as well.
But while its shell shields a turtle's tender innards, it
wasn't originally designed for that purpose, as an international group

(00:22):
of scientists has found. Instead, they say the turtle shell
most likely began as a digging tool. Tyler Lison, who
is the lead author of the studying question and a
paleontologist at the Denver Museum of Nature and Science, writes
that when turtles first began developing shells a major evolutionary transition.
The process included a broadening of the ribs. Broadened ribs

(00:44):
made the turtles thoraxic region, or chest, more rigid. But
the puzzling thing is that a rigid chest means it's
harder for the animal to breathe easily or move swiftly,
hardly changes that appear to enhance protective capabilities. But that's
because initially, turtle shells made of more than fifty fused
bones and their broad ribs were developed not for protection

(01:06):
but to lend stability to the turtles so it could
forcefully dig with its forelimbs. The researchers found being able
to dig into the ground for food and shelter, they
theorize allowed the turtles to move from the land into
the water. This may have saved them in their early
evolutionary history, specifically during the Permian Triassic extinction, the greatest
mass extinction ever and one which was spurred by hot,

(01:29):
dry weather. These insights came after two of the studies
co authors discovered several specimens of a two hundred and
sixty million year old turtle named Untosaurus africanus, the oldest
to known partially shelled proto turtle. These specimens, along with
another partially shelled turtle found by a young boy in
South Africa, indicate turtles developed shells for use as digging aids.

(01:51):
The scientists found similarities between these turtles and gopher tortoises,
which use their heads and next to brace themselves while
they dig with their forelimbs. The authors believe that the
turtle's shell as a protective enclosure is an x adaption,
meaning an evolutionary trait that originally served one function, but
winds up serving another. Scientists have been arguing for at

(02:13):
least two hundred years over whether the turtle's shell evolved
from bony scales like those on an armadillo or certain lizards,
or as part of its ribs broadening. Thanks to recent
discovery of partially shelled stem turtles, it now seems that
the latter theory is more likely. Today's episode was written

(02:34):
by Melanie red Zeki McManus and produced by Tyler Clang.
For more on this and lots of other evolutionarily advantageous topics,
visit our home planet, how Stuff Works dot com