Archelon
Name: Archelon
(Ruling turtle).
Phonetic: Are-kell-on.
Named By: G.R. Wieland - 1895.
Classification: Chordata, Reptilia, Sauropsida,
Testudines, Cryptodira, Protostegidae.
Species: A. ischyros (type).
Diet: Probably soft bodied organisms like jellyfish
and cephalopods.
Size: 4 meter long, 4.9 meters wide.
Known locations: USA, South Dakota and Wyoming.
Time period: Late Campanian to early Maastrichtian
of the Cretaceous.
Fossil representation: Few specimens.
Although
a turtle, Archelon has become a staple species
that appears in almost
every book about dinosaurs and other prehistoric animals. This almost
mandatory inclusion comes from the fact that Archelon
is the largest
turtle ever known to exist and it lived at the end of the
Cretaceous. For the most part dinosaurs were probably not a threat,
as assuming Archelon laid eggs like turtles do
today, it would only
ever venture onto land to lay eggs. Conversely however these eggs
would eventually hatch and the young may have had to run a gauntlet of
predators on their way to the ocean. Back in the Cretaceous this
could mean everything from small beach combing dinosaurs, primitive
seabirds similar to Ichthyornis to even pterosaurs.
Archelon
is usually envisioned as being very similar to the leatherback turtle,
and likely had a similar preference for eating jellyfish and
cephalopods. The horned beak of the mouth which had a clear overbite
would have been very effective at snipping soft bodied animals like
these into bite sized portions that could be swallowed. Another
similarity between these two turtles is that neither one has a solid
shell. Instead a series of bony struts, that in Archelon
are
actually the ribs, create a framework of bone that an outer and
relatively thin carapace sits on top of.
Perhaps
the easiest explanation for Archelon’s shell design
is that of neutral
buoyancy. This is where a marine animal’s body adapts so that it
neither floats to the surface, nor sinks to the bottom. In order to
achieve this effect the body changes along such lines as denser thick
bones that counter the lifting effect of the air in the lungs so that
the animal does not bob around on the surface. Good examples for
marine adaptation in reptiles can be seen in the placodonts
from the
Triassic which were actually negatively buoyant so that they could sink
to the bottom where their preferred food was.
Archelon
however was not a bottom feeder, it had to stay active in the mid to
upper surface areas where its prey was most abundant, and as such it
needed to be neutrally buoyant so that it could adapt to different
depths wherever its prey was. As a large turtle Archelon
obviously
had a large shell, but if this shell was solid bone with a horn
covering like in smaller turtles it would be extremely heavy. It’s
feasible that a heavy solid shell would have tipped the scale into
negative buoyancy so that Archelon would have had
to spend a greater
amount of energy just to stay up in the areas its food supply was, as
well as rising up to breathe. This would not suit any animal,
especially large ones like Archelon.
A
shell composed of a framework with a relatively thin covering however
would actually be very lightweight as well as reducing the body density
so that Archelon did not readily sink as well as a
more solid
creature. This in turn would allow Archelon to
cruise the oceans at a
more leisurely pace and conserve energy so that it would not need as
high a calorie intake as it would have done if the shell were solid.
The
presence of large mosasaurs such as Tylosaurus
as well as possibly even
sharks like Cretoxyrhina
suggest that Archelon probably was not
invulnerable to predators, particular from attacks to the flippers.
Despite this the sheer physical size of the shell may have still been
enough to prevent some predators from being able to close their jaws
around the body. At the very least this would have made a fully grown
Archelon a difficult prey item compared to other
softer bodied marine
reptiles. Aside from this large size four star-shaped plates are
found on the underside of the shell which seems to reinforce it.
These may appear on the underside as a form of additional defence
from predators that hit it from below. It’s perhaps not implausible
that such attacks may have been initiated by mistake by predators that
confused Archelon for a different kind of animal,
possibly explaining
the presence of these plates only on the underside.
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