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
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 &
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
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.