Evolutionary Origin of the Turtle Shell

Evolutionary Origin of the Turtle Shell


♪ [MUSIC] ♪ Hi, I’m Tyler Lyson. I’m in the
Vertebrate Zoology Department and I’m currently
in my second year of a Peter Buck
Postdoctoral Fellowship. And, broadly, I’m interested
in the interrelationship of amniotes as well as the
origin of their body plans. Specifically, I’m interested
in the origin of turtles and the origin
of its body plan. Now if we only
consider living groups, and analyses published
within the last ten years, we can see that
turtles have occupied every possible position on
the amniote tree of life. Obviously, this is a
problem and lots of work needs to be done to
resolve this issue. And I use a number of
research tools to get at these very complicated problems. I conduct fieldwork
in southwestern North Dakota and South Africa, I use computer tomography
scans to visualize anatomy, including soft
tissue anatomy. You can see in the
lower left with the lungs. Then I integrate these data,
the skeletal, the soft tissue, developmental,
and molecular data, all within a tree-based or
phylogenetic context in order to get at these
complicated issues. So now while I’m interested
in both of these questions, I’m only going
to talk to you about the origin of the
turtle body plan today, specifically the
origin of the turtle shell. Now the turtle shell is this
evolutionary novel structure that’s made up of
around 50 bones. And basically I’m
interested in the timing and then the
sequence in going from something like this
lizard here on the left, this kind of represents the
basal amniote body plan from which turtles evolved, to the highly modified body plan
that we see here on the right, represented by
the snapping turtle. And so there are two
competing hypotheses for the origin of the
main part of the top shell. There’s a composite
hypothesis, and this argues that the shell
formed by the acquisition of more and more
osteoderms, or ossified scales. And this was supported by
limited paleontological data, and this contrasts
with the de novo hypothesis, which argues that
the shell forms not from the acquisition of
more and more osteoderms but rather by
the broadening of the ribs and
vertebrae themselves. And this is supported by a
lot of developmental data. And so this was kind of fought
out over the past 100 years and was largely the result
of the lack of a fossil record. Now this changed in 2008
when this thing was found, Odontochelys, from
the late Triassic of China. This thing has a fully
developed belly shell, or plastron, and it only
has a partial top shell, or carapace, but
it lacks osteoderms and has distinctively
broadened ribs and vertebrae, and thus falsifying the
composite hypothesis and corroborating the
de novo hypothesis. So we can kind of add this
to the evolutionary lead-up to modern day turtles, we can still see that
there’s anywhere between a thirty- to fifty-million-year
gap between Odontochelys and the rest of the turtle stem or from when turtles
diverged from other reptiles. So what I was interested
in were these early fossil taxa: what did they look like
and what do they tell us about the origin
of the turtle shell? So I went down to
South Africa to look at this particular animal,
Eunotosaurus africanus, when this thing was first
described in the late 1800’s, it was described
as a putative turtle. And then this idea
largely fell out of favor in the mid-1900’s and
was largely forgotten about. So I went down and
collected the specimens and then I performed the first
detailed anatomical study of this animal, and I’m not going to go
through all the characters, but it does share many
features with turtles, including having
nine broadened ribs, has very elongate vertebrae. And I looked at the development
of the Eunotosaurus ribs, and I did some
histology studies to get at how
these ribs developed, and I found that
the broadened part or the horizontal
part of the rib is the result of an
outgrowth of bone from the rib. And this is a feature
that’s uniquely shared, at least among living
animals, with turtles. So only turtles have
this extreme outgrowth of bone, dermal or sub-dermal
bone, from the rib. Now, that’s a feature
shared between turtles and Eunotosaurus. And so, there are a number of
shared, derived features, ranging from the
skeleton, the development, as well as the
inferred soft-tissue musculature features
that Eunotosaurus shares with
Odontochelys and other turtles. And now if we incorporate these
data with developmental data, you kind of see an
interesting pattern, so those features
that are important in building the shell are
mirrored phylogenetically and throughout development. What do I mean by that? Well, the first thing to
appear in a developing turtle are broad ribs,
that’s what we see phylogenetically with
Milleretta and Eunotosaurus. The next thing, we have
broadened vertebrae, that’s what we see
with Odontochelys. And finally, in
late-stage embryos we have the full
acquisition of the shell and that’s what we see
with Proganochelys, at around 210
million years old. And so now if we animate
these data we can kind of get an idea of what
these transformations may have looked like. First we have the
broadening of the ribs, and we get an
animal like Milleretta, that’s from South Africa,
and then we have an exact halving of
the vertebrae and ribs, going from 18 down to nine. That’s a unique feature shared only between
Eunotosaurus and turtles. And then we lose some of the
soft tissue parts of the ribs, the cartilage part, and then
we broaden the belly ribs, or gastralia, to
form the plastron, and then we have
Odontochelys, from the late
Triassic of China. And then at 210 million
years we have Proganochelys, we’ve got the acquisition
of the perimeter bones or the peripheral bones, and we basically have
a fully developed turtle, at 210 million years ago. And then between it and a
modern-day snapping turtle we don’t have too many changes. We lose a couple
bones in the belly, we have some shape
changes, and that’s about it. So with that I’d just like to
say thank you for listening and leave you
with this image of a modern-day
South African turtle with the earliest known basal
turtle Eunotosaurus africanus. Thanks. [APPLAUSE]>>Question: What problem
are turtles trying to solve?>>With the
broadening of the ribs? The acquisition of the shell?
What do you mean by that?>>Question: That
which is turtles. What are they
trying to do? [LAUGHTER]>>They’re aliens now.
[laughs] I mean I think with the acquisition of the shell,
one of the first, why the ribs started to
broaden to begin with is because they’re
fossorial animals, they’re burrowing and
that’s one of the adaptations for burrowing,
it’s not for protection. It’s kind of like,
“why did the feather evolve?” It wasn’t for flight; it was for
a different reason initially. Thanks.

Randy Schultz

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