Placodus
Name: Placodus
(Flat tooth).
Phonetic: Plak-oh-dus.
Named By: Louis Agassiz - 1833.
Classification: Chordata, Reptilia,
Sauropterygia, Placodontia.
Species: P. gigas (type),
P.
inexpectatus.
Diet: Shellfish.
Size: Up to 2 meters long.
Known locations: Germany, France, Poland,
China.
Time period: Anisian of the Triassic.
Fossil representation: Many specimens.
Placodus
is one of the most often represented reptiles of the placodont
group,
and the most common of the ‘unarmoured’ variety that seemed to be
similar to marine iguanas. Unlike the algae eating marine iguana
however, Placodus was a dedicated durophagous
shellfish eater. In
its search for food it used specialised forward pointing incisors to
grip and pluck things like bivalves and crustaceans off the sea floor
before taking them into the back of its mouth. Here a battery of flat
crushing teeth that extended across the palate broke up the shell so
that Placodus could swallow the soft bodied
organism within.
As
a marine reptile Placodus seems to have been
adapted for swimming while
still retaining features that saw it still able to move about on land,
although it does not seem especially able to do one or the other.
In the water Placodus would have relied upon its
laterally
compressed (sideways flattened) tail for its main propulsion,
while the legs and possibly webbed feet could have been used for
steering by pushing out against the water to turn the body in the
opposite direction. This method meant that Placodus’s
body and
swimming method was not suitable for pelagic life in the open ocean,
but really it would not have had to swim out this far as the greatest
abundance of shell fish would be close to the coast and in the sunlit
layers.
Placodus
seems to have relied upon neutral buoyancy to take most of the effort
out of swimming. Evidence for this comes from the dense bones of its
skeleton as well as scutes (bony armour) that formed a ridge along
its back, all helping to increase its body weight and density. The
result of this is that Placodus could effectively
sink itself without
struggling to keep a light air filled body on the sea floor.
Feeding
behaviour is thought to have been fairly simple with Placodus
using its
legs to hover in the water above some shellfish as it picked them out
with its forward teeth. Placodus also had a
parietal eye (sometimes
called a pineal eye) on top of its head. A parietal eye does not
work like a regular optic eye that you see out of, but is instead a
photoreceptive (light sensitive) organ. Usually these trigger
specific behaviour in accordance with increasing and decreasing photo
periods (i.e. longer to shorter hours of daylight), but in
Placodus it has also been suggested to help it to
orient itself
correctly when it needed to return to the surface, something that
would have been important when hovering face down towards the sea floor.
The
body of Placodus was not especially flexible, in
part caused by the
way the vertebrae connected together which resulted in a semi rigid
spine that gave good support when in the water, but was very poor for
land movement. The large ribs of Placodus also
bent backwards to
provide additional cover for the lower body. Although often
envisioned as armour, this may have been for support of the body when
on land as opposed to just protection from predators. The same legs
that were used for swimming were not especially large, and given the
large round bulk of the body, Placodus may have
had to push itself
along the ground with the ribs helping to protect the lower organs like
the intestines from this ground contact. Altogether the aquatic
adaptations of Placodus meant that it would have
been a cumbersome
animal when compared to its exclusively terrestrial counterparts, and
as such it probably did not venture too far from the water line as
doing so would increase the amount of energy required going to and from
the water while increasing the chance of contact with larger land
predators.
For
a long time Placodus was thought to be an
exclusively European genus,
but the naming of the second species, P. inexpectatus,
in China
in 2008 shows that while Placodus was not a
strong ocean going
swimmer, it was perfectly capable of following Triassic coastlines to
colonise new parts of the globe.
Further reading
- Description of the Skull and Teeth of the Placodus laticeps,
Owen,
with Indications of Other New Species of Placodus, and Evidence of the
Saurian Nature of That Genus. - Philosophical Transactions of the Royal
Society of London, Volume 148. - Owen.
- On the skull of Placodus gigas and the
relationships of the
Placodontia. -Journal of Vertebrate Paleontology vol 7. - Hans-Dieter
Sues - 1987.
- Palaeoecology of Placodus gigas (Reptilia) and
other placodontids —
Middle Triassic macroalgae feeders in the Germanic Basin of central
Europe — and evidence for convergent evolution with Sirenia. -
Palaeogeography, Palaeoclimatology, Palaeoecology vol 285 - Cajus G.
Diedrich - 2010.
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