Elasmosaurus was a plesiosaur with 72 neck vertebrae, giving it a neck that was more than seven metres long. © Dotted Yeti/ Shutterstock
By James Ashworth
Plesiosaurs are a group of extinct marine reptiles that lived in the oceans while the dinosaurs lived on land.
Find out what plesiosaur fossils have revealed about these animals and how big they could grow.
Earth has had its fair share of unusual animals over its history, and the plesiosaurs are head and shoulders above the rest. From their body shape to their swimming style, they’re a quirky chapter in the fossil record.
Firstly, they had a unique way of swimming, with four flipper-like limbs to push them through the water. While some evolved incredibly long necks with a small head perched on the front, others had much shorter necks but very large jaws and teeth instead.
Either way, plesiosaurs are some of the most iconic and successful marine reptiles to have ever lived on Earth. They survived for more than 140 million years, making it through multiple extinctions that saw other groups get wiped out.
Dr Marc Jones, our curator of fossil reptiles and amphibians, reveals more about these marine marvels.
The original Plesiosaurus dolicodeirus skeleton described by William Stukeley represents a turning point in our understanding of life on Earth.
A plesiosaur isn’t the same as a dinosaur. Plesiosaurs are ancient marine reptiles that lived in the oceans and are only distantly related to dinosaurs, which lived on land. In fact, scientists started studying and understanding plesiosaurs long before dinosaurs got the same kind of attention.
While it’s likely that people have been stumbling across fossil plesiosaurs for centuries, the first recorded discovery was in 1605 by Richard Verstegan. He believed that the backbones were part of a giant fish, and other plesiosaur fossils were often described the same way.
This only changed around a century later in 1718, when the first substantially intact plesiosaur skeleton was discovered in Nottinghamshire, UK. Charles Darwin’s great-grandfather, Robert, wrote about it in a letter to archaeologist William Stukeley, who then purchased it for the Royal Society.
Though the fossil was initially identified as human, William noticed clear differences from any known skeleton. Instead, he argued that it was more likely to come from a crocodile or porpoise instead. This skeleton was eventually donated to us and is now on display in our Fossil Marine Reptiles Gallery.
As more and more plesiosaur parts were recognised over the following century, interest in these animals continued to grow. The group eventually got its name after Plesiosaurus fossils were described in 1821 by William Conybeare and Henry Thomas De la Beche. The name translates as ‘near reptile’.
Mary Anning then discovered an almost complete Plesiosaurus skeleton two years later, finally revealing what these animals would have looked like in life. Plesiosaurus’ size is around 3.5 metres in length, with its skull structure and teeth suggesting it would have eaten fairly small prey such as fish and crustaceans.
This skeleton was introduced to the scientific community at a meeting of the Geological Society in 1824 – the same one where Megalosaurus, the first named dinosaur, was also presented. These species helped scientists to confirm the idea that animals could go extinct.
This Plesiosaurus specimen, found by Mary Anning on the south coast of England, was the first articulated skeleton of a complete plesiosaur ever discovered.
Plesiosaurs belong to a group of marine reptiles known as the sauropterygians, which first appeared in the Early Triassic Period. They’re divided into five main groups – pachypleurosaurs, placodonts, nothosaurs, pistosaurs and the plesiosaurs themselves.
All these groups were the descendants of reptiles that returned to the ocean from land, but it’s uncertain exactly what part of the family tree sauropterygians are related to.
“The early fossil record of sauropterygians isn’t very good, so they’re already very specialised when they first appear in the fossil record,” Marc explains. “This makes it difficult to know where they come from.”
“We know that they’re more closely related to modern reptiles than mammals, but beyond that it’s difficult to say. It’s possible they might be more closely related to crocodiles and dinosaurs than they are to lizards, snakes and tuatara, but it’s not really clear without finding more early fossils.”
The sauropterygians had very different lifestyles, with some adapting to life in the water more than others. It’s likely that the long, lizard-like pachypleurosaurs could still move around on land, for example, as their limbs aren’t as specialised as other members of the group.
Placodonts such as Henodus grew thick shells that helped to protect them from predators. Public domain image by Daderot via Wikimedia Commons.
Nothosaurs, meanwhile, became ferocious predators that ate everything from fish and squid to other marine reptiles. This included the placodonts, which eventually developed turtle-like shells to protect themselves through the process of convergent evolution.
The pistosaurs, meanwhile, were adapting for life in the wider ocean. These animals looked very plesiosaur-like, with streamlined bodies and limbs that were being used for swimming.
Plesiosaurs eventually evolved from the pistosaurs, and the earliest known plesiosaur is Rhaeticosaurus from around 204 million years ago. Rhaeticosaurus is already a distinct plesiosaur, meaning that the group must have evolved earlier – perhaps as early as 240 million years ago – but more fossils will be needed to confirm this.
Just after plesiosaurs first appear in the fossil record, widespread volcanic eruptions brought the Triassic Period to an end. All of the sauropterygians were wiped out in the mass extinction except for the plesiosaurs, who may have been better able to adapt to the changes.
“Other sauropterygians are thought to have lived closer to land than the plesiosaurs, making them less able to move to less affected areas,” says Marc. “In contrast, the plesiosaurs were adapting to life in the open ocean, so could cover greater distances to find food and survive the mass extinction.”
Rhomaleosaurus was a member of one of the earliest diverging plesiosaur groups. It was first found in Yorkshire, UK, and later brought to Dublin by Sir Philip Crampton. It received its current name from geologist Harry Govier Seeley in 1874.
With their closest relatives and many of their competitors driven to extinction, plesiosaurs were able to rapidly evolve into many new forms during the Jurassic Period and the Cretaceous Period. Despite their differences, these reptiles were still united by certain characteristics.
All known plesiosaurs are fully adapted to life in the water, having evolved wing-shaped flippers that made them efficient swimmers. Experiments using robot flippers showed that plesiosaurs could generate 60% more thrust from their back flipper by carefully timing the movement of both limbs. Some plesiosaurs also likely had tail fins that could have helped them to steer.
This combination of flippers and fins also meant that plesiosaurs were unable to return to land. As a result, it’s very likely that these marine reptiles gave birth to live young in open water, and a preserved foetus discovered in a specimen of Polycotylus supports this theory.
A well-preserved plesiosaur fossil from Germany tells us more about plesiosaurs’ appearance. It suggests they had a mixture of different skin types, with flippers covered in hard, scaly skin similar to a sea turtle’s that would have helped them to swim. In contrast, the tail, and perhaps other parts of the body, were smooth and scaleless to reduce drag.
Some of the most ancient plesiosaurs are the rhomaleosaurs, which were already quite diverse by the Early Jurassic Period. They get their name from Rhomaleosaurus cramptoni, a seven-metre-long animal that was among the earliest giant plesiosaurs.
This plesiosaur’s diet is likely to have included large fish and, given its size, other marine reptiles. It’s also known to have swallowed stones, just like other plesiosaurs and many living animals, that stayed in its stomach. Some scientists think this might have helped to break up food, while others suggest that the stones could have helped it to maintain buoyancy.
Despite living all around the world, rhomaleosaurs were all extinct by the end of the Middle Jurassic Period around 161 million years ago. This left an opportunity for the biggest known plesiosaurs to evolve.
Pliosaur size varied by species and time, but Liopleurodon was among the biggest in the Jurassic Period. Image © Jonathan Chen via Wikimedia Commons, licensed under CC BY-SA 4.0.
While Late Triassic species such as Rhaeticosaurus were only a few metres long, plesiosaur size increased dramatically in the Jurassic Period. Some of the biggest were the pliosaurs, which grew to enormous sizes after the rhomaleosaurs disappeared.
Among the largest pliosaurs are Pliosaurus, which is estimated to have grown to as much as 13 metres long, and Liopleurodon, the biggest of which were probably around 10 metres long.
Both animals were top predators, with enormous skulls and sharp teeth that could be used to deliver powerful bites. Preserved stomach contents show they ate a variety of fish and squid, while pliosaur tooth marks have been found on the fossils of ichthyosaurs and other plesiosaurs. Some pliosaurs even ate dinosaurs, scavenging on corpses that washed out to sea.
While the biggest pliosaurs probably pursued their prey in open water, other species hunted differently. Preserved tracks discovered on Mount Conero, Italy, have been suggested as evidence of pliosaurs pushing off the sediment to follow prey along the seabed.
By timing the movement of their flippers, plesiosaurs such as Elasmosaurus could swim more efficiently. © Daniel Eskridge/ Shutterstock
By the start of the Cretaceous Period, 145 million years ago, pliosaurs were joined by another group of giant plesiosaurs – the elasmosaurs. In contrast to the pliosaurs, which have around 20 neck vertebrae, Elasmosaurus and its relatives have extremely long necks. These were often longer than their bodies and astonishingly could have more than 70 neck bones.
In fact, Elasmosaurus’ neck is so extreme that it caused confusion for more than a century. When the famous palaeontologist Edward Drinker Cope named Elasmosaurus, he believed that the neck was so long that it was actually the tail. This meant he ended up putting the skull on the wrong end of the animal!
He’s not the only scientist to be confused by plesiosaur necks. While species were historically divided based on the length of their neck, more recent research has shown that it’s not that simple. Short-necked plesiosaurs evolved multiple times from groups with long necks, which Marc says is possible because of how the genetics of reptiles are organised.
“Mammals can’t increase the number of vertebrae in their neck very easily because of how their chromosomes are organised,” he explains. “Reptiles, on the other hand, have much more flexibility around their body plan so some plesiosaurs ended up with extremely long necks by the Cretaceous.”
“Historically, plesiosaur necks were depicted as being very flexible, like a swan’s, but they were more rigid than that. We’re not sure why some of these animals have such long necks, but one suggestion is that moving the plesiosaur skull and flippers further apart meant they could sense their surroundings more clearly. The long neck might also have helped them sneak up on prey.”
This plesiosaur tooth was found near Peterborough, UK, where many marine reptile fossils have been discovered.
Our understanding of plesiosaurs is constantly changing as new fossils are discovered, and one of the best places in the world to do this is the UK. Even centuries after William Stukeley’s research, palaeontologists are still digging up and studying plesiosaur fossils.
“The Jurassic and Cretaceous rocks that contain plesiosaur fossils are only found in certain areas, and even then they might be buried so deeply that they can’t easily be accessed,” says Marc. “However, in the UK there are many areas that are near the surface, and often on the coast where they are constantly being eroded out of the cliffs.”
“This means many plesiosaur hotspots in this country have been studied for hundreds of years.”
One of the most well-known regions for plesiosaur fossils is the Jurassic Coast, which covers around 150 kilometres of England’s south coast. This is where Mary Anning’s Plesiosaurus fossil and many other marine reptiles were discovered, and spectacular new finds continue to turn up.
In 2022, a pliosaur fossil snout was discovered on a beach in Dorset. Palaeontologist Dr Steve Etches realised that the rest of the head was still in the cliff, and over 18 months managed to extract the pliosaur skull with the aid of a team of specialists.
Nicknamed the Sea-Rex due to the sharp teeth jutting out of the jaw, the well-preserved fossil hit the headlines after featuring in the documentary Attenborough and the Giant Sea Monster. Steve believes that more of the fossil is still in the cliffs and hopes to extract more of the Dorset pliosaur skeleton in the coming years.
It’s not just the UK’s south coast that’s famous for plesiosaur fossils. The Whitby Mudstone in Yorkshire are rocks renowned for species such as Rhomaleosaurus, while Cambridgeshire, Somerset and Nottinghamshire are also known for their plesiosaur fossils.
This Polycotylus specimen revealed that plesiosaurs could give birth to live young. The remains of its foetus are preserved between its front and back flippers. Image © Jonathan Chen via Wikimedia Commons, licensed under CC BY-SA 4.0.
Following early discoveries of plesiosaurs in England, their remains were subsequently discovered in southern Germany and the USA. Since then, further fossils have been uncovered all over the world, showing that plesiosaurs adapted to a range of conditions.
While most species lived in the sea, fossils found in China, Canada and Morocco suggest some were able to live in freshwater as well. Some species could have spent their entire lives in lakes and rivers, while others might have been able to move between the oceans and waterways.
Other plesiosaurs made the poles their home, moving into cold waters that modern reptiles couldn’t cope with. It’s likely that the plesiosaurs were warm blooded, making them better able to grow quickly and cope with a range of different temperatures.
Some of the species that lived in the polar regions were very unusual – Morturneria, from the Antarctic, is believed to have used its teeth like a sieve to filter prey out of the water. Meanwhile, the Arctic-dwelling Ophthalmothule has relatively large eyes that would have been better at detecting light, meaning they might have fed on animals living in deep water and on the seabed.
Plesiosaur fossils from Lightning Ridge in Australia are also strange, but in a different way – they’re made out of precious gems.
“Lightning Ridge is famous for its opalised fossils, where the original bone has been replaced by the mineral opal,” Marc explains. “The opal more commonly forms a mould of the bone’s original shape, but if it seeped into the bone itself it can also preserve the internal structure.”
Pliosaurs went extinct before the end of the Late Cretaceous Period, but have left behind a wealth of fossils including the Sea-Rex pliosaur skull. © David Winn-Morgan/ Shutterstock
Plesiosaurs were some of the dominant predators of the oceans for millions of years. However, their time at the top couldn’t last forever.
Following the extinction of the rhomaleosaurs in the Middle Jurassic Period, the pliosaurs were wiped out around 94 million years ago by severe climate change in the Late Cretaceous Period. It also caused the extinction of many other groups, including the ichthyosaurs, with major implications for ocean ecosystems.
Marine reptiles known as mosasaurs became prominent around this time, while the surviving plesiosaurs also took advantage of the changes. Polycotylus and its relatives, together known as the polycotylids, evolved into many new species in the extinction’s aftermath.
For the rest of the Cretaceous Period, plesiosaurs continued to thrive. However, 66 million years ago, they faced an extinction they couldn’t escape. A massive asteroid struck Earth and plunged the planet into a nuclear winter, leading to the extinction of the dinosaurs, mosasaurs and plesiosaurs as their food chains collapsed.
The Loch Ness Monster is often depicted as a plesiosaur-like animal. © Alinute Silzeviciute/ Shutterstock
While scientists are certain that the asteroid strike marked the end of the plesiosaurs’ time on Earth, rumours persist that they might still survive somewhere. The most famous potential plesiosaur is the Loch Ness Monster, said to live in the Scottish lake of the same name.
While stories of monsters living in Loch Ness and the nearby River Ness date back more than 1,000 years, Nessie’s reported appearance has changed over time. Initial reports suggest that it was more whale or eel-like. It only solidified into a plesiosaur-like shape in the 1930s, especially after the publication of the famous ‘Surgeon’s photograph’ hoax.
While searches are occasionally organised for the Loch Ness monster, none are yet to turn up a plesiosaur. Environmental DNA testing of the loch carried out in 2019 suggests that, if rumours of Nessie’s existence are true, it’s likely to be a giant eel.
Away from Loch Ness, some mysterious objects found floating in the ocean or washed up on beaches have also been suggested as plesiosaurs. However, these have turned out to be the badly decomposed remains of basking sharks or young whales after scientific investigation.
Ultimately, plesiosaurs only live on in our public consciousness. The strangeness of these animals means that they turn up time and time again in popular culture, from Jules Verne’s book Journey to the Centre of the Earth to Doctor Who.
It goes to show that even 66 million years after their extinction, plesiosaurs are still some of the strangest animals that ever lived.
This article includes information from the book Ancient Sea Reptiles: Plesiosaurs, Ichthyosaurs, Mosasaurs & More by Dr Darren Naish.
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