Page 101 - Prehistoric Animals

Pla-Kodus

Placodus

Placodus (flat tooth) was a genus of marine rep-
tiles belonging to the order Placodontia that lived in
the middle Triassic, some 240 million years ago. It
measured around 2 to 3 metres in length. Since its
naming by Louis Agassiz in 1833 it has been found
throughout central Europe, North Africa, the Middle
East and China.

Placodus was not particularly big. It had a short
skull, with its eyes and nostrils on the top, its jaws were
lined with flat crushing teeth while at the front it had
longer forward facing incisors. Its body was rounded
with a flat underside and it tapered at the back into a
long tail. It had four short limbs, with feet and toes.
The toes may have been webbed. Protruding along the
full length of its back was a line of bony plates.

Placodus had a very dense and heavy bone
structure; this weight would have slowed it down and
made movement awkward. Coupled with this, the ar-
moured plates on its back, and its short legs, it would
have found it difficult to stay afloat in deep water or to
walk around on dry land. Sediment found on fossils
suggests it stayed close to the coastline, possibly using
the shallow waters for buoyancy. Living near to shore
would have kept it close to a plentiful supply of its
main source of food; shellfish. It would have used its
front incisors to scrape up shells like, bivalves and
crustaceans, from the seabed, or pick them from the
rocks. It would then use its flat back-teeth for crushing
the shells.

As with most reptiles at that time, Placodus was
probably an egg layer, and, when it needed to lay its
eggs, be gripped with a powerful instinctive force, forc-
ing it to venture onto the land. However, once on land
it was very vulnerable. Its short legs would struggled
to carry the weight of its heavy body. If they failed, it
would have had to drag itself around. If caught by a
land predator, its only protection was the armoured
plates on its back. This vulnerability probably limited
Placodus visits and time spent on land to the bare min-
imum and only when absolutely necessary.