The Placodonts
When looking at artwork depicting ancient marine ecosystems where ichthyosaurs and nothosaurs are swimming and hunting you often see smaller reptiles, sometimes like turtles facing down and nibbling food off the sea floor. Usually this glimpse is all that is offered but in actual fact these marine reptiles are called placodonts and they make up their own very distinct and diverse group of marine reptiles.
The evolution and rise of the
early placodonts
As
members of the Sauropterygia the placodonts are thought to have had
their ancestral links in the land living diapsid reptiles (today
commonly known as lizards). This ancestry can be clearly seen in
the early placodonts by the overall lizard like body plan,
particularly the feet that still have toes. The immediate ancestors
to the placodonts were probably beach combing lizards that learnt how
to dig into the sand and mud for buried shellfish as well as raiding
tidal pools for other kinds of shellfish and possibly other creatures
like crustaceans. The reptiles that began relying upon this behaviour
would gradually begin to change to take better advantage of the supply.
Ancestral
forms probably would have started out with teeth similar to other
slightly larger reptiles, but over successive generations evolution
would favour those that had teeth better suited to a shellfish diet.
Specifically this meant that those that had forward facing teeth would
have an easier time picking shellfish out of crevices amongst the rocks
of tidal pools, while those that also had robust rounded teeth at the
back could crush the shells with a greatly reduced amount of wear and
damage to the teeth. Near constant exposure to water as well as
moving through it would have likely brought new adaptations such as
webbing between the toes so that the placodont ancestors could move
through the tidal pools more efficiently while still keeping the
ability to move across the land to reach different pools. These newer
forms would have steadily outcompeted the older forms to replace them
in the coastal ecosystems.
While
the placodont ancestors would have evolved to take better advantage of
a new food source, their increasingly aquatic adaptations would have
seen them able to venture out into the sea as well. This expansion
would have started quite casually as the distance between a tidal pool
and the ocean can be a barrier of just a few centimetres. Those
ancestral forms that did start exploring the sea near the coast would
have found themselves a much greater abundance of viable prey, and as
such would have started to adapt even further. This is probably where
the early known placodont forms such as Placodus
and Paraplacodus
began
to appear, with skeletal features that reveal a greater focus upon
aquatic rather than terrestrial life.
Placodonts
like Placodus probably used a lateral
(side-ways) undulation of the
tail to push themselves through the water, but the main concern here
is that shellfish and crustaceans typically live on the sea floor,
and as such the early placodonts would have had to swim down to get
them. As air breathing creatures reptiles have to hold their breath
when underwater, and this very fact means that air is always in the
body and trying to reach the surface, which in turn means greater
effort is needed to reach and stay on the bottom to feed. The
placodonts solution to this problem was probably the simplest one they
could come up with: they just grew heavier.
The
bodies of the early placodonts are quite large and rotund, something
that actually would have made them quite cumbersome on land, possibly
only able to push themselves along. This is suggested by the way that
the ribs bend backwards to cover the lower internal organs. Although
often speculated as a defence against predators, the ribs would have
protected the internal organs from near constant contact with objects
like shore rocks as well. This basic body shape may have allowed for
a greater amount of air to be taken in so that the early forms could
stay underwater for longer, but it is actually the size and density
of the ribs that created this shape that is most important here.
Body
tissues like muscle and fat are composed of billions of cells that are
essentially filled with water that results in these tissues being
roughly neutrally buoyant. However in comparison to these tissues
bone is relatively solid, and the bones, particularly key ones like
the ribs, of the early placodonts where not just larger and more
robust than necessary, they were denser too. If you were to compare
a bone of a placodont with an equal sized bone of a fully terrestrial
reptile, you would almost certainly find that the placodont bone was
heavier. These bones allowed the placodonts to achieve negative
buoyancy which means that when a placodont entered the water it would
through no effort of its own sink to the bottom.
Of
course the immediate problem with negative buoyancy is that it makes
swimming to the surface harder, a potentially serious problem for
animals that would periodically have to in order to breathe. For this
simple reason placodonts are thought to have been restricted to shallow
coastal waters, never venturing too far from land. However this is
really just a restriction on paper as you have to remember two things.
One is that their food supply would have been most plentiful in
coastal waters particularly in the sunlit zone near the surface. Two
is that by staying in shallow waters the placodonts could effectively
avoid contact with some of the larger predators that were beginning to
appear in the oceans.
The later armoured placodonts and
their defensive adaptations
The
placodonts were certainly not the first group of reptiles to adapt to a
marine existence. Fossils of early placodonts such as Placodus
and
Paraplacodus are dated back to the Anisian stage of
the Triassic, and
at this time primitive ichthyosaurs like Utatsusaurus
were already
swimming the oceans. In addition to these were Nothosaurs that lived
on the coast, diving into the sea to hunt for food, and even sharks
that were steadily growing bigger and becoming more and more of a
threat.
Sharing
the same ecosystems as these carnivores meant that contact was
inevitable, something that necessitated a new development for the
placodonts to survive in seas which were becoming more and more
dangerous. The answer is what is now termed a case of convergent
evolution with the turtles, in that the placodonts began to develop
tough shells to protect their bodies. Despite similarities to the
turtles, placodont shells were slightly different. While some like
Placochelys
had single shells, others had two shells, one for the
upper body, and one for the pelvic area.
The
armoured placodonts were also much flatter than the earlier unarmoured
varieties with low cross sections when viewed from the front compared
to the more squared box-like cross-section of earlier members like
Placodus. The differences in cross section may be
down to the weight
afforded by the shell. The early forms like Placodus
did not have a
shell and so developed denser and thicker internal bones to achieve
negative buoyancy. The shells of later placodonts would in themselves
have provided an additional weight boost, one that may have actually
made the body too heavy making getting off the sea floor more
difficult. Rather than sacrifice the new armour the internal bones
reduced in size to compensate.
A
fully grown and developed placodont would have been a difficult meal
for a nothosaur, even a larger one like Nothosaurus
itself. The
teeth of nothosaurs were long and thin, perfect for soft bodied prey
like fish, but susceptible to breakage when used on tough armour.
Also the limbs, head, and depending upon the genus tail, did not
stick out far from the extremities of the shell, which meant that
even if these parts were bitten off and eaten they would not provide
all that much sustenance to a larger predator. As such fully grown
placodonts, particularly larger ones like Psephoderma
which were
almost two meters long, were too heavily armoured to eat without the
risk of breaking teeth, while the softer parts were just not worth
the bother. This means that fully grown placodonts may have been able
to live with only occasional interference from predators that were more
curious than hungry.
Despite
the new armour adaptation however shelled placodonts were still
vulnerable to predators when young. Evidence for this vulnerability
comes from the presence of two small Cyamodus
juveniles inside the
stomach area of a Lariosaurus,
a small sixty centimetre Nothosaur.
Not only were juveniles small enough to potentially be swallowed,
the shells may have actually been quite soft at this stage in their
development. This is something commonly seen in shelled animals as
the softness ensures the full and correct development of the adult
shell, particularly during the early growth stages.
The
fact that two Cyamodus juveniles were found inside
the same Lariosaurus
specimen before one of them could be digested suggests that not much
time passed between them being swallowed. Assuming such feeding was
not unusual for a Lariosaurus, you have the
possibility that large
numbers of juvenile Cyamodus were active at the
same time. This would
suggest that Cyamodus, and possibly other
placodonts, had an
r-strategy to survival. This means that large numbers of young were
raised but they had to fend for themselves without aid from the
parents. This meant that juvenile mortality was high from predation
from carnivores, but the numbers of juveniles were so high at least a
few would survive long enough to become too large for other predators.
Placodont distribution &
behaviour
As
a group the placodonts have been traditionally associated with the
marine deposits of Europe from where the greatest number of remains are
known. However the discovery of a new species of Placodus
in 2008
from China proves that the placodonts had a much larger geographic
distribution than previously thought. The breakup of Pangaea during
the Triassic created large areas of island chains and shallow seas that
were the perfect environment to thrive in. All the placodonts had to
do was follow the coastlines. It’s also likely that the placodonts
did not just spread east but may have headed west towards the land
masses that would become North America, possibly even reaching
further afield than this.
Despite
their adaptions for aquatic life, the placodonts were still tied to
the land. Negative buoyancy meant that as long as a placodont was in
the water it was either holding its breath on the bottom or working to
keep its head above the water as it breathed in. By returning to the
land, even if it was just clinging to a rock face, it could both
rest and breathe easily. How much time placodonts stayed in the water
may not have just been down to how long they could hold their breath
but how long they could maintain body temperature. Marine iguanas
which have a similar coastal existence can be seen today basking in the
sun to raise their body temperature before diving into the ocean to
feed on algae. Placodonts would have evolved from cold blooded
ancestors, and even if they were not biologically cold blooded in the
sense that we use today, they still would have been susceptible to
the cooling effects of the ocean, although shallow seas nearer the
equator are usually warmer than those in the extreme Northern and
Southern latitudes. Still while their small bodies would have cooled
quickly, they would also warm up quite quickly as well. The sight
of placodonts climbing onto the beach and rocks to rest and bask in the
sun may have been quite a common one on Triassic shorelines.
No
one is sure for certain if placodonts gave birth to live young or laid
eggs. Their ability to return to the land means that it is possible
that they could have laid eggs, but this is by no means certain.
Nothosaurs which are in the same superorder as the placodonts also
seem to have returned to the land when not hunting, and fossil
evidence for them suggests that they may have given birth to live
young. Without fossil evidence such as confirmed placodont eggs or
developing juveniles inside the body of a mother placodont, it will
remain impossible to say for certain which theory is the correct one.
How
Placodonts moved through the water depends upon the kind in question.
Earlier forms had long tails which could have been laterally undulated
side to side to provide forward momentum (think of how crocodiles
move their tails when in the water). Armoured placodonts however
had tails that were either reduced in size or quite rigid, which
means that had no option but to ‘crawl’ through the water with
their legs. Given that all placodonts were negatively buoyant they
probably had a more casual approach to aquatic movement, relying upon
their weight to take the effort off reaching the bottom. Once on the
bottom they could use their short limbs to walk across the sea floor,
perhaps hovering just above it to get a better angle at the shellfish
on the bottom.
The
only real amount of energy expenditure would come when a placodont had
to get itself off the sea floor and to the surface to breathe.
However this may not have been a tremendous ordeal and placodonts may
have adapted their foraging behaviour to take this into account. It
would be quite easy for a placodont to use a criss-crossing system
where it could enter the water at one point of the coast and forage in
a line to emerge in another part of the coast. It could then work its
way back on a slightly different path to cover ground it went past but
did not investigate before. There is no real need for a placodont to
be more active than this as shellfish such as bivalves pretty much
stays in the same spot meaning that a placodont could afford to take
its time and be methodical. This would also remove the need for a
placodont to make a ‘mad dash’ to the surface in the middle of
foraging and expend unnecessary amounts of energy.
Exactly
where and how placodonts fed can found by looking at their mouths.
The early placodonts had forward facing teeth that projected out from
the mouth. Most of the later armoured placodonts lost these teeth but
had a pointed beak. Both of these were for plucking out shellfish
from the sea floor, particularly those lodged amongst rocks and other
shellfish such as those on muscle beds. A clear exception to this
however is the broad flat snout of the armoured placodont Henodus.
Without the ability to pluck out individual shellfish it’s feasible
that this broad snout could have been used to shovel through soft
sediment to find shellfish that had buried themselves. This would
infer niche specialisation which indicates that placodonts were
starting to adapt to different kinds of coastline as well as the
possibility that multiple kinds of placodont existed in the same
ecosystems without directly competing with one another.
All
placodonts had large and robust crushing teeth in the back and across
the palate of the mouth that made short work of shells that were
supposed to protect the invertebrates within. However even though the
purpose of the teeth was to crush the shell to get at the soft body,
the shells may have also been eaten. Shellfish shells could have
provided placodonts with an additional source of calcium, something
that would have been critically important for maintaining dense bone
growth as well as the shells of later armoured placodonts.
In
addition to the large crushing teeth, placodonts had skulls that were
well adapted for exerting strong downwards forces upon shellfish in the
mouth. First the skull attachments for the where the jaw closing
muscle attached to the skull bone are very well developed, something
that suggests the jaw closing muscles were inherently powerful. Also
by keeping the length of the jaws short, placodonts could more easily
pass shellfish to the back of the mouths nearer to the point of
articulation. When looking at placodont jaws like they are a
mechanism this places the shellfish near the fulcrum which allows for a
far more efficient transferal of force than what would happen if it
were at the tip of the jaws: in short, easier to crush shellfish.
Extinction of the placodonts
So
far no placodont fossils are known from after the Triassic,
something which suggests the placodonts did not live to see out the
end of the era. The exact reasons why remains unknown but the end of
the Triassic was a turbulent time for life on Earth with at least one
major extinction event taking place. The animals of the sea seem to
have been particularly affected by this, more so than the land
animals with as much as possibly half the known Triassic genera of sea
creatures disappearing from the fossil record for this time. Not
everyone is sure or can agree upon what exactly happened at this time,
other than it was one of the most devastating. Also fossil evidence
has been interpreted as representing a series of lesser events that
combined to have the same effect of a single cause, although again,
interpretations and opinions vary.
With
reference to the disappearance of the placodonts, it seems strange
that one group of marine reptiles should disappear when the
ichthyosaurs survived, while the nothosaurs seemingly evolved into
and were replaced by the plesiosaurs. Perhaps one of the root causes
for the extinction of the placodonts was actually loss of habitat.
The placodonts seemed to have spread with the breakup of Pangaea,
the continental drift creating new expanses of coastline and shallow
sea. However the very process that created this expanse of habitat
was still on-going with the distances between the landmasses ever
increasing. This would have reduced the amount of shallow sea habitat
as well as isolating the placodonts to smaller geographical areas.
Also
as predators, when other marine animals began disappearing in the
extinction event the knock on effect would eventually reach the
placodonts (assuming they were not already being directly affected by
the extinction events). This would affect the overall numbers,
and with populations now isolated from the wider world they may have
begun struggling to survive in sufficient numbers to consistently
reproduce every breeding season. All these effects probably combined
to bring a gradual but steady demise to the placodonts.
By
contrast the other two main groups of marine reptiles adapted and
thrived into the Jurassic. The ichthyosaurs that were very primitive
in the Triassic began developing new body shapes that saw them
resembling dolphins, something that allowed them to better survive in
the open oceans. The nothosaurs as a group also ended at the end of
the Triassic, but survived in the form of primitive plesiosaurs that
probably never had to return to the land. Both these lines continued
by adapting to new conditions, but because the placodonts were unable
to do so, their fate was ultimately sealed with their specialisation.
List of known Placodonts
Cyamodus
Henodus
Paraplacodus
Placochelys
Placodus
Protenodontosaurus
Psephoderma
Saurosphargis
Additional
Helveticosaurus
- Older texts refer to this reptile as a placodont, but new study
has revealed that it is actually a diapsid (possibly archosaur)
that merely resembles the early placodonts. Helveticosaurus still
remains a good example of convergent evolution where two evolutionary
lines evolve similar features for the same lifestyle.