Ptychodus
Name: Ptychodus
(Fold tooth).
Phonetic: Tie-coe-dus.
Named By: Louis Agassiz - 1835.
Classification: Chordata, Chondrichthyes,
Elasmobranchii, Selachimorpha, Ptychodontidae.
Species: P. mortoni (type), P.
anonymus, P. janewayii, P. polygurus, P. rugosus, P.
whippleyi.
Diet: Durophagus (shellfish/crustaceans).
Size: Estimated at 10 meters long.
Known locations: Known mostly from Europe, Canada
- Saskatchewan, and the USA - Alabama, Colorado, Georgia,
Kansas, New Mexico and South Dakota. Similar teeth are also known
from other parts of the world hinting at a possibly cosmopolitan
distribution.
Time period: Coniacian through to the Maastrichtian
of the Cretaceous.
Fossil representation: Mostly teeth, but scales,
vertebra and a jawbone fragment are also known.
Ptychodus
was one of the most specialist sharks of the late Cretaceous oceans,
as the teeth are adapted for crushing shells rather than tearing
through flesh. As such the teeth are rounded rather than being
triangular and pointed, and have a series of ridges that run across
the surface of the crown. These ridges would have increased the bite
pressure along the length of the ridges, something that would have
helped to hold onto smooth shelled shellfish while also focusing the
bite pressure onto key areas of the shell, making it easier for
Ptychodus to break it. It is actually these
ridges that give a
folded appearance to the teeth that were the inspiration for the name
of the shark which when broken down becomes a combination of ‘ptych’
which means ‘fold’ or ‘layer’, and ‘dus’ which means
‘tooth’.
When
first discovered the teeth of Ptychodus were
thought to have belonged
to a ray, a bottom dwelling type of cartilaginous fish that are
related to the sharks. Later it was thought to be hybodont, a shark
similar to Hybodus
that had crushing teeth at the back of its mouth but
more regular pointed teeth at the back. Even later analysis combined
with new fossil discoveries now suggests that Ptychodus
was a
neoselachian, the group of sharks that appeared during the Cretaceous
which are thought to be the direct ancestors to today’s modern sharks.
Analysis
of Ptychodus fossils has indicated that it was
possibly around ten
meters long, something that would actually make Ptychodus
one of the
largest sharks of the Cretaceous, even bigger than the hyper
carnivorous Cretoxyrhina
that grew up to seven meters long. However
Cretoxyrhina was a pelagic (open ocean) predator
of other marine
creatures whereas Ptychodus was probably more of a
benthic (bottom)
predator of shellfish and crustaceans which it could easily eat with
its specialised teeth. It might seem strange that a shark could grow
so large upon a diet of shellfish, but back in the Cretaceous waters
there were giant inoceramid bivalves such as the almost two meter wide
Inoceramus.
Large prey animals like this combined with a more
sedentary lifestyle compared to the pelagic sharks meant that Ptychodus
could easily afford to grow large.
Additionally
by being a benthic creature Ptychodus may not have
come into much
contact with the apex predators of the Cretaceous seas, namely the
mosasaurs. As marine reptiles the mosasaurs had to remain within
reach of the surface so that they could breathe in air, similar in
fashion to whales. Sharks do not need to do this however because they
have gills that can extract oxygen from the water, which meant that
if a Ptychodus was ever harassed by a mosasaur it
would just have to
stay down until the mosasaur couldn’t hold its breath any longer.
Despite the reduction in the numbers of predators that a full grown
Ptychodus faced, a small number of teeth from
another shark called
Squalicorax
have been found in association with Ptychodus
remains,
something that raises the possibility that a Squalicorax
fed upon a
Ptychodus. This does not reveal the cause of death
for the Ptychodus
however, as it may not have been killed by the Squalicorax,
but
just scavenged by it. Nonetheless this does help piece together a
part of the marine ecosystems of the Cretaceous where sharks like
Ptychodus kept the numbers of large bivalves in
check while other
sharks ‘cleaned up’ the remains of dead animals by eating them.
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