BrainStuff Classics: 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 classic episode of BrainStuff, based on this article: https://animals.howstuffworks.com/reptiles/real-reason-turtles-have-shells-hint-its-not-protection.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio. Hey
brain Stuff Lauren Bogebam here with a classic episode from
our archives. This one deals with one of the weird
quirks of evolution. Okay, so most of evolution is pretty weird,
but this is about how turtles developed their shells. You'd
think it was for protection, but it turns out that

(00:23):
that was a side benefit of their original purpose. Hey
brain stuff, Lauren Bogebam here. Many of us have seen
a turtle tucking itself into its protective shell, pulling in
not only it's 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

(00:44):
group 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 to veloping shells, a
major evolutionary transition. The process included a broadening of the ribs.

(01:06):
Broadened ribs 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,

(01:28):
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,

(01:51):
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 name you notice, Saurus africanus,
the oldest 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

(02:12):
digging aids. The scientists found similarities between these turtles and
gopher tortoises, which use their heads and next to brace
themselves while they dig with their fore limbs. The authors
believe that the turtle's shell as a protective enclosure is
an ex adaption, meaning an evolutionary trait that originally served
one function but winds up serving another. Scientists have been

(02:34):
arguing for at 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

(02:58):
episode is based on the article the You'll reason turtles
have shells and It's not for protection on how staff
works dot com, written by Melanie red Seki McManus. Brain
Stuff is production of i Heart Radio and partnership with
how stuff works dot Com, and it is produced by
Tyler Klang. Four more podcasts from my heart Radio, visit
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