What were pliosaurs?
The pliosaurs were a group of marine reptiles that existed from the Late Triassic (around 228 million years ago) to the early Late Cretaceous (around 89 million years ago) and inhabited seas all over the world.
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They’re known from fossils found on six of Earth’s seven continents: Africa, Australia, Asia, Europe, North America, and South America, suggesting they had an incredibly wide range.
As a group, the pliosaurs are very closely related to another group of marine reptiles from the Mesozoic Era, the plesiosaurs, and together form an order known as Plesiosauria.
This order contains long-necked plesiosaurs and primitive, long-necked pliosaurs, as well as the most well-known sub-group of pliosaurs - the thalassophonean pliosaurs. This is the group that palaeontologists typically picture whenever the word ‘pliosaur’ is mentioned.
The thalassophonean pliosaurs looked like a cross between a crocodile and a whale, with short necks, huge heads, large, paddle-shaped flippers, and powerful, hydrodynamic bodies.
They were the largest members of Plesiosauria, reaching lengths of more than 11m in the case of Kronosaurus and Pliosaurus macromerus. They were also the largest marine reptiles for the majority of their existence, ruling the world’s oceans as apex predators for more than 80 million years.
How do pliosaurs, plesiosaurs, mosasaurs, and ichthyosaurs differ?
The Mesozoic Era (252 to 66 million years ago) is often nicknamed ‘The Age of the Dinosaurs’, but the dinosaurs weren’t the only reptiles that thrived during this time. The world’s oceans were home to several groups of marine reptiles in the Mesozoic, and each took a turn lording over this underwater realm.
The first to rise to ecological dominance were the ichthyosaurs in the Early Triassic (around 250 million years ago). They looked a lot like today’s dolphins and sported pretty much the same body plan - long, slender bodies, pointed snouts, and tall dorsal fins.
The only difference was that their tail fins were orientated vertically, rather than horizontally like those of dolphins. The ichthyosaurs were an incredibly diverse group, ranging in size from 1m to 26m, and they were particularly prevalent during the Triassic.
The plesiosaurs and the pliosaurs emerged in the Late Triassic, but it wasn’t until the following period - the Jurassic - that they really started to compete with the ichthyosaurs. The plesiosaurs and the primitive pliosaurs are known for having unusually long necks and looking very similar to the popular depictions of the infamous Loch Ness Monster.
Their heads, jaws, and flippers were also smaller than those of the more derived and generally larger thalassophonean pliosaurs, which appeared in the Early Jurassic and, until the early Late Cretaceous, were some of it not the most dominant marine reptiles in the world’s oceans.
The mosasaurs, on the other hand, differ from the other groups of Mesozoic-aged marine reptiles in that they’re part of the reptilian order known as Squamata, which comprises lizards and snakes. They were the last major group of marine reptiles to make an appearance ( around 94 million years ago) and while they weren’t quite as stocky as the thalassophonean pliosaurs, they did grow longer - the largest, Mosasaurus hoffmannii, reaching lengths of up to 17m. After ichthyosaurs and pliosaurs faced extinction in the early Late Cretaceous, mosasaurs took over and ruled the oceans up until 66 million years ago.
What was the largest pliosaur?
The thalassophonean pliosaurs were giants, dwarfing many of the ichthyosaurs, plesiosaurs, and primitive pliosaurs they lived alongside. The largest known pliosaurs are Kronosaurus and a species of Pliosaurus - the genus that gives its name to this group of marine reptiles - known as Pliosaurus macromerus. Both of these brutes measured roughly 11m in length and weighed in at approximately 11 tonnes, making them just a tad smaller than a humpback whale.
Another, so-far-unnamed species of pliosaur was recently found poking out of a cliff near Weymouth, Dorset, and may rival Kronosaurus and Pliosaurus macromerus as one of the biggest pliosaurs ever discovered. So far, only the skull of this enigmatic giant has been excavated, but already researchers have been able to point to several novel features that distinguish it from other known pliosaurs, potentially making it a new species.
The skull of this pliosaur measures almost 2m in length and its jaws are lined with 130 grooved, dagger-shaped teeth. From studies of this exceptionally preserved skull - perhaps the most complete pliosaur skull ever discovered - researchers have worked out that this species (and likely others) was capable of biting down at a force of 33,000 newtons.
That’s more than twice the bite force of a saltwater crocodile (16,000 newtons) and only 12,000 newtons less than the estimated bite force of prehistory’s most infamous chomper, the T.rex (45,000 newtons)!
The skull has also revealed that pliosaurs, like crocodiles, may have performed ‘death rolls’, twisting their bodies after biting down on prey in an attempt to tear off limbs and/or huge chunks of meat. This is evidenced by the expanded area at the back of the studied skull, coupled with the stout, muscular necks that are characteristic of thalassophonean pliosaurs in particular.
It’s thought that the rest of the aforementioned specimen may still be in the cliff where this giant, well-preserved skull was originally found, waiting to be excavated. It’s a race against time, however, as the cliff is rapidly eroding and large chunks of rock, potentially containing valuable fossils, are being swallowed by the sea.
What did pliosaurs eat?
For more than 80 million years, the pliosaurs were the apex predators of the world’s oceans, feasting on all manner of prey from large fish to other marine reptiles, including their close cousins the plesiosaurs. There’s also fossilised gut contents that show that some pliosaurs scavenged dinosaur carcasses that had washed out to sea.
The largest thalassophonean pliosaurs, such as Kronosaurus, Pliosaurus macromerus, and the unnamed species described above, are thought to have hunted by ambushing their prey from below, using their powerful jaws and incredibly sharp teeth to dismember prey, oftentimes in a single bite. This is how great whites hunt today and, like great whites, it’s thought that pliosaurs may have also used countershading as a form of camouflage to make themselves undetectable to prey.
The teeth of many pliosaurs were shaped like three-sided pyramids, with two sharp cutting edges and striated grooves on two of the three sides. The striated grooves are thought to have stopped vacuums from forming when the pliosaur’s teeth were plunged into its prey, allowing it to make several swift bites on the occasions where a single bite wasn’t quite enough to finish the job.
CT scans of pliosaur skulls have also revealed that their snouts were lined with lots of blood vessels and sensory nerves, suggesting they had a keen sense of smell and may have been capable of tracking prey for many miles.
As apex predators of the world’s oceans, the thalassophonean pliosaurs had no natural predators. Still, that probably didn’t stop the largest individuals from preying on smaller individuals, or even juveniles. In fact, there’s fossil evidence of cannibalism amongst some species of pliosaurs, as well as interspecific predation - distinct but related species preying on one another.
Why did pliosaurs become extinct?
While the order Plesiosauria persisted until the end of the Cretaceous (around 66 million years ago) and ultimately faced extinction as a result of the same asteroid impact that wiped out the dinosaurs, the pliosaurs disappeared a little earlier, roughly 89 million years ago in the early Late Cretaceous.
There are several different theories as to why pliosaurs became extinct, and why they may have died out before their long-necked cousins did. The most generally accepted theory is that a disturbance in the global carbon cycle triggered a period of anoxia, or lack of oxygen, in parts of the world’s oceans. This event hit pliosaurs, and ichthyosaurs, particularly hard, sending populations into rapid decline.
After this event, which is thought to have taken place roughly 94 million years ago, mosasaurs appeared, diversified, and started to take over the same niches that were once dominated by the pliosaurs. This increased the pliosaurs’ trajectory towards extinction until they eventually disappeared, leaving the mosasaurs as the apex predators of the world’s oceans.
The plesiosaurs were also impacted by this oceanic anoxic event, but as they occupied different niches to the pliosaurs they were able to avoid competition with the newly evolved mosasaurs and persisted a little longer, clinging on until the end of the Cretaceous.