Plesiosaur

Plesiosaurs, also called plesiosaurs in the group Plesiosauria, are an extinct group of marine reptiles that lived in the Mesozoic Era. They belong to the Sauropterygia and form an order or clade. They first appeared in the late Triassic Period, possibly in the Rhaetian stage, about 203 million years ago. They became especially common during the Jurassic Period. They disappeared at the end of the Cretaceous Period, about 66 million years ago, due to the Cretaceous–Paleogene extinction event. They lived in oceans all over the world, and some kinds lived at least partly in freshwater.

Plesiosaurs were among the first fossil reptiles ever found. In the early 1800s, scientists saw how unusual their body shape was, and they were placed in their own order in 1835. The first plesiosaur genus, the one that gives the group its name, Plesiosaurus, was named in 1821. Since then, more than a hundred valid species have been described. In the early twenty-first century, new discoveries helped scientists learn more about their bodies, relationships and way of life.

Plesiosaurs had a broad, flat body and a short tail. Their limbs had changed into four long flippers. Strong muscles attached to wide bony plates in the shoulders and hips powered these flippers. The flippers moved in a flying motion through the water. Plesiosaurs breathed air and gave birth to live young. There are signs that they were warm-blooded.

Plesiosaurs had two main morphological types. Some had long necks, sometimes extremely long, and small heads. These were relatively slow and caught small sea animals. Others had a short neck and a large head. These were apex predators, fast hunters of large prey. For a long time, the group was divided into two suborders: the long-necked Plesiosauroidea and the short-necked Pliosauroidea. Newer research shows that some "long-necked" groups had short-necked members, or the other way around. So the simpler terms "plesiosauromorph" and "pliosauromorph" were introduced. Today, "plesiosaur" properly refers to the Plesiosauria as a whole.

Other ancient marine reptiles, such as the Ichthyosauria and the Mosasauria, are not part of the Dinosauria group. The genera in Plesiosauria are also not dinosaurs.

History of discovery

Main article: Timeline of plesiosaur research

Parts of plesiosaur skeletons are among the first fossils of extinct reptiles that people recognised as reptile remains. In 1605, Richard Verstegen of Antwerp drew plesiosaur bones. He thought they came from fish, and he used them as proof that Great Britain was once joined to the European continent. The Welshman Edward Lhuyd also showed similar bones in 1699, again thinking they were fish bones. John Woodward kept similar remains, which were only much later understood to be plesiosaur. Some of these are now kept in the Sedgwick Museum.

In 1719, William Stukeley described a partial skeleton. It had been shown to him by Robert Darwin of Elston, the great-grandfather of Charles Darwin. The stone holding the bones came from a quarry at Fulbeck in Lincolnshire. It had been used to strengthen a watering-hole in Elston. Stukeley called its origin "diluvial" and thought it came from a sea creature, perhaps a crocodile or dolphin. The specimen is now on display at the Natural History Museum. It is the earliest discovered, more or less complete, fossil reptile skeleton in a museum collection. It may be called Plesiosaurus dolichodeirus.

During the eighteenth century, many more plesiosaur finds were made in England, though most were incomplete. Important collectors were the reverends William Mounsey and Baptist Noel Turner, who were active in the Vale of Belvoir. Their collections were described in 1795 by John Nicholls. One of Turner's skeletons is now called Thalassiodracon.

In the early nineteenth century, people still knew little about plesiosaurs, and their unusual body shape was not understood. Sometimes they were mixed up with ichthyosaurs to make more complete specimens. In 1821, William Conybeare and Henry Thomas De la Beche described a partial skeleton. They recognised it as a special group and named the new genus Plesiosaurus. The name comes from Ancient Greek words meaning "closer to" and "saurian". It was seen as closer to the Sauria, especially the crocodile, than to Ichthyosaurus, which looked more like a fish. The specimen parts are still kept at the Oxford University Museum of Natural History.

Soon after, scientists learned more about the morphology. In 1823, Thomas Clark reported an almost complete skull, which is now kept by the British Geological Survey. That same year, the fossil collector Mary Anning and her family uncovered an almost complete skeleton at Lyme Regis, on what is now called the Jurassic Coast. It was bought by the Duke of Buckingham and studied by William Buckland. Conybeare described it in 1824 to the Geological Society of London, at the same meeting where the dinosaur Megalosaurus was named. Conybeare also gave the specific name dolichodeirus, meaning "longneck." Later, Plesiosaurus giganteus was named as a short-necked form. It is now placed in the Pliosauroidea.

The collector Thomas Hawkins made plesiosaurs known to the public through richly illustrated books. He saw them as creatures from a pre-Adamitic time. During the first half of the nineteenth century, many more finds were made, especially at Lyme Regis. Richard Owen named nearly a hundred new species on his own, but many of these are now invalidated. Plesiosaurus became a wastebasket taxon, meaning many different fossils were placed in it. Other genera were gradually named, such as Pliosaurus in 1841, whose etymology referred to the earlier name. The Pliosauridae were later recognised as fundamentally different from the Plesiosauridae, a family named by John Edward Gray in 1825. In 1835, Henri Marie Ducrotay de Blainville named the order Plesiosauria.

In the second half of the nineteenth century, important finds were made in the American Western Interior Seaway, in the Niobrara Chalk. These finds started the Bone Wars, a rivalry between the scientists Edward Drinker Cope and Othniel Charles Marsh. In 1867, Theophilus Turner found a skeleton near Fort Wallace in Kansas, which he gave to Cope. Cope misread the skeleton's build and proposed a new group, the Streptosauria. Marsh and Joseph Leidy showed that the bones had been put in the wrong order. Cope was embarrassed, and this affair is seen as the start of the rivalry.

Around the turn of the century, most plesiosaur research was done by Samuel Wendell Williston, a former student of Marsh. In the middle of the twentieth century, Samuel Paul Welles led important work in the United States.

In recent times, plesiosaurs have been named at a faster pace. About half of the known species are new to science. Important finds have been made in new places, such as New Zealand, Argentina, Chile, Norway, Japan, China and Morocco. In 2002, the "Monster of Aramberri" was announced. It was found in 1982 in the Nuevo León region of Mexico, and it is a very large plesiosaur. In 2004, a nearly complete young plesiosaur was found at Bridgwater Bay in England. The ammonites found with it show that it lived about 180 million years ago. It may be related to Rhomaleosaurus. In 2005, remains from Canada near Herschel, Saskatchewan were found to be a new species, Dolichorhynchops herschelensis.

In 2006, a team from the Argentinian Antarctic Institute and the La Plata Museum found a young plesiosaur on Vega Island in Antarctica. It is now on display at the South Dakota School of Mines and Technology. In 2008, fossils named Predator X, now known as Pliosaurus funkei, were found in Svalbard. In December 2017, a large plesiosaur skeleton was found on the continent of Antarctica.

Evolution

The Plesiosauria came from the Sauropterygia, a group of reptiles. Their ancestors may have come from archelosaurian reptiles and then returned to the sea. During the Upper Triassic, an advanced group called the Eusauropterygia split into two branches. The meat-eating animals in this group had small heads and long necks. One branch, the Nothosauroidea, kept elbow and knee joints that could bend for walking. The other branch, the Pistosauria, became better suited to the sea. Their backbones became stiffer, and their limbs turned into flippers that did the swimming instead of the tail. The Pistosauria were viviparous, which means they gave birth to live young. Early, basal members, often called "pistosaurids", still lived mostly near the coast. Later, a more advanced group split off. These were the Plesiosauria. They had stronger shoulder bones, flatter pelves, and pointed flippers. Their stiffer limb joints, extra finger and toe bones, and shorter tails helped them spread into the open seas.

From the earliest Jurassic, in the Hettangian stage, many kinds of plesiosaurs are known. This means the group must already have spread out in the Late Triassic. Only a few very early forms from that time have been found. The most advanced is Rhaeticosaurus. Scientists still disagree about how plesiosaurs later changed over time. They were once divided into long-necked Plesiosauroidea and short-necked Pliosauroidea. However, some groups that were usually long-necked may have had short-necked members. To avoid confusion between phylogeny (evolutionary relationships) and morphology (body shape), long-necked forms are called "plesiosauromorph" and short-necked forms are called "pliosauromorph".

The latest common ancestor of the Plesiosauria was probably a small, short-necked animal. In the earliest Jurassic, the group with the most species was the Rhomaleosauridae, which also had short necks. By the Toarcian, about 180 million years ago, groups such as the Plesiosauridae had become more common. Some of these had longer necks, reaching up to ten metres (33 feet). In the middle of the Jurassic, very large Pliosauridae evolved. They had big heads and short necks. Examples are Liopleurodon and Simolestes. They grew up to seventeen metres (56 feet) long and were the strongest hunters in the sea at that time. The shorter Cryptoclididae had long necks and small heads. The Leptocleididae spread out in the Early Cretaceous. Later in that period, the Elasmosauridae appeared. They were among the longest plesiosaurs, reaching up to fifteen metres (fifty feet), with very long necks. Pliosaurids were still around as large hunters, as Kronosaurus shows. At the start of the Late Cretaceous, the pliosaurids disappeared and were replaced by the Polycotylidae. All plesiosaurs became extinct because of the K-T event at the end of the Cretaceous period, about 66 million years ago.

In modern phylogeny, clades are groups that contain all species belonging to a certain branch of the evolutionary tree. One kind of clade, called a "node clade", includes the last common ancestor of two species and all of its descendants. In 2008, Patrick Druckenmiller and Anthony Russell defined Plesiosauria this way. They used Plesiosaurus dolichocheirus and Peloneustes philarchus. Another kind, called a "stem clade", includes all species that are more closely related to one chosen species than to another species that is left out. In 2010, Hillary Ketchum and Roger Benson defined Plesiosauria this way. They described it as "all taxa more closely related to Plesiosaurus dolichodeirus and Pliosaurus brachydeirus than to Augustasaurus hagdorni". They also named Neoplesiosauria, which probably includes the same species. In 2012, Benson and colleagues found that the traditional Pliosauroidea were paraphyletic compared to Plesiosauroidea. This means they did not form one complete family group. The early Carnian pistosaur Bobosaurus was found to be the most basal known plesiosaur. A more detailed study in 2014 by Benson and Druckenmiller could not settle how the early lineages of Plesiosauria were related to each other.

The following cladogram follows an analysis by Benson & Druckenmiller (2014).

Description

Plesiosaurs were large sea reptiles. Adults ranged in length from about 1.5 meters (4.9 ft) to about 15 meters (49 ft). They were among the biggest marine apex predators in the fossil record, about as long as the longest ichthyosaurs, mosasaurids, sharks and toothed whales. The typical plesiosaur had a broad, flat body and a short tail. Its two pairs of limbs had become large flippers. Plesiosaurs were related to the earlier Nothosauridae, which had a more crocodile-like body. Unlike the ichthyosaurs and later mosasaurs, which mainly used their tails to swim, plesiosaurs probably used all four flippers to move through the water. The tail mainly helped them steer. The shoulder girdle and pelvis were changed to hold strong muscles that pulled the flippers downwards. The flippers were stiff and flat. They had more small finger and toe bones than usual, a feature called hyperphalangy.

Plesiosaurs had similar bodies, but their necks and heads were different. "Plesiosauromorphs", such as Cryptoclididae, Elasmosauridae, and Plesiosauridae, had long necks and small heads. "Pliosauromorphs", such as the Pliosauridae and the Rhomaleosauridae, had shorter necks and large heads. Long necks came from having more neck bones, not longer ones. Early researchers thought the neck could bend like a swan's, but newer research shows it was fairly stiff. The skull had large eye sockets and large openings at the sides. The teeth varied in number and shape.

Soft tissue remains of plesiosaurs are rare. Where they have been found, they show smooth skin with small wrinkles and a tail with a vertical fin. A 2025 study reported traces of scaly skin on one front flipper of a young Plesiopterys specimen.

Paleobiology

Plesiosaurs probably ate different foods depending on their necks. Those with very long necks ("plesiosauromorphs") and those with short necks ("pliosauromorphs") likely hunted in different ways. Long-necked plesiosaurs may have surprised schools of fish by moving their heads toward them before the fish noticed. They likely hunted by sight. Their diet probably included cephalopods and bony fish (Osteichthyes). Some scientists think they ate benthos from the sea bottom. Others think some groups filtered plankton from the water, as Aristonectes may have done. Short-necked plesiosaurs were the top apex predators in their foodwebs. They had large heads and teeth for catching big animals. Plesiosaurs were also prey for other animals, such as sharks and mosasaurs. Many skeletons contain gastroliths, or stones. These may have helped them digest food or adjust their buoyancy.

Plesiosaurs swam with their four flippers. Scientists have debated how the flippers moved. Eberhard Fraas and Othenio Abel suggested in the early 1900s that they flew through the water like turtles and penguins. In 1975, Jane Ann Robinson revived this idea. In 2010, Kenneth Carpenter and Frank Sanders supported it with a study. Scientists still debate whether all four flippers moved together or one after another. This movement pattern is called the gait.

Plesiosaurs were probably not very fast. Their smooth skin may have reduced form drag. Their long necks may have caused skin friction. Scientists have changed their speed estimates over time as they corrected their calculations. The highest estimate is still slower than modern Cetacea.

Plesiosaurs could dive to considerable depths. Bone damage in some fossils suggests they rose to the surface too quickly after deep dives. This condition is called decompression sickness. Young plesiosaurs had very dense bones, which may have helped them sink. Some deep-diving groups, such as Abyssosaurus, had relatively large eyes. These may have helped them see in deep water.

Scientists are not sure whether plesiosaurs were warm-blooded or cold-blooded. Bone evidence from related groups, such as the Sauropterygia, suggests plesiosaurs may have had a fairly fast metabolism.

Like most reptiles, plesiosaurs were generally oviparous, meaning they laid eggs. Later research showed that they were viviparous and gave birth to live young. A fossil of a pregnant Polycotylus showed a single large baby. This suggests the mother cared for her young, somewhat like modern whales. Little is known about how they grew or how males and females differed. Their social behavior may have been fairly complex, but it is not known whether they hunted in groups. Sight and smell were their most important senses, and hearing was less important. Some fossils show signs of illness or old age. For example, a Pliosaurus jaw joint was affected by arthritis.

Distribution

Plesiosaur fossils have been found on every continent, including Antarctica.

Timeline of Plesiosauria Species

Stratigraphic distribution

Main article: List of plesiosaur-bearing stratigraphic units

Scientists have found plesiosaur fossils in many geologic formations. These are layers of rock that formed at different times.

In contemporary culture

Main article: Loch Ness Monster

See also: Sea monster

Many people think plesiosaurs were dinosaurs, but this is a common misconception. In popular culture, they are often erroneously depicted as dinosaurs.

Some people have suggested that legends of sea serpents and modern sightings of monsters in lakes or the sea could be plesiosaurs that survived until today. The scientific community has rejected this cryptozoological idea. Scientists say it is based on fantasy and pseudoscience. Animals once thought to be plesiosaurs were later shown to be partly decayed basking sharks.

The Loch Ness monster is often said to look like a plesiosaur, but it is also often described as looking completely different. There are several reasons it is unlikely to be one. The osteology of the plesiosaur's neck means it could not lift its head up like a swan out of the water, as the Loch Ness monster is said to do. Animals that breathe air would also be easy to see when they come up for air. The lake is also too small and has too little food for a group of large animals to live there. The lake formed only about 10,000 years ago, at the end of the last ice age. The latest plesiosaur fossils are much older than that. Common explanations for sightings include waves, floating objects, tricks of light, known animals swimming, and practical jokes. Even so, plesiosaurs have become closely linked with the Loch Ness monster in popular imagination, which has helped people know about them.