Plesiosauria

Plesiosauria Temporal range: Early Jurassic - Late Cretaceous, 199.6–65.5Ma PreЄ Є O S D C P T J K Pg N
Artist's reconstruction of Plesiosaurus.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Node:	†Pistosauria
Order:	†Plesiosauria de Blainville, 1835
Suborders
Plesiosauroidea Pliosauroidea

Plesiosauria (pron.: /ˌpliːsi.ɵˈsɔriə/; Greek: plesios meaning 'near to' and sauros meaning 'lizard') is an order of Mesozoic marine reptiles (sauropsida). Plesiosaurs first appeared in the Early Jurassic (and possibly Rhaetian, latest Triassic) Period and became especially common during the Jurassic Period, thriving until the Cretaceous–Paleogene extinction at the end of the Cretaceous Period. They had a worldwide distribution.

The name "plesiosaur" is used to refer to the order Plesiosauria as a whole, not only to the long-necked forms (suborder Plesiosauroidea). These latter constitute the plesiosaurs in the popular imagination ("Nessie", "Nahuelito").

Description

Plesiosaur skeleton (Meyerasaurus) in the Museum am Löwentor, Stuttgart

The typical plesiosaur had a broad body and a short tail. They retained their ancestral two pairs of limbs, which evolved into large flippers. Plesiosaurs evolved from the earlier nothosaurs, that had a more crocodile-like body; major types of plesiosaur are primarily distinguished by head and neck size. Members of Plesiosauroidea, such as Cryptoclididae, Elasmosauridae, and Plesiosauridae, had long necks and may have been 'bottom-feeders', in shallow waters. Most pliosaurs and rhomaleosaurs, however, had shorter necks with a large, elongated head and may have been at home in deeper waters.

All plesiosaurs had four paddle-shaped 'flipper' limbs. This is an unusual arrangement in aquatic animals and it is thought that they were used to propel the animal through the water by a combination of rowing movements and up-and-down movements. There appears to have been no tail fin and the tail was most likely used for helping in directional control. This arrangement is in contrast to that of the later mosasaurs and the earlier ichthyosaurs. There may be similarities with the method of swimming used by penguins and turtles, which respectively have two and four flipper-like limbs.

Elasmosaur cast, Canadian Museum of Nature

In general, plesiosaurians are among the largest marine predators in the fossil record, with even the smallest species around 2 m (6.5 ft) long. The largest pliosaurs were up to 15 m long, while the plesiosaurs grew even longer; Mauisaurus was 20 meters long. However, the late Triassic ichthyosaurs, such as shastasaurids, are known to have reached 21 m in length, and modern large marine animals, such as the sperm whale (20 m), and especially the blue whale (~30 m) are much larger than any currently known plesiosaurians.

The anteriorly placed internal nostrils have palatal grooves to channel water, the flow of which would be maintained by hydrodynamic pressure over the posteriorly placed external nares during locomotion. During its passage through the nasal ducts, the water would have been 'tasted' by olfactory epithelia.

History of discovery

First published plesiosaur skeleton, 1719

Skeletal elements of plesiosaurs are among the first fossils of extinct reptiles recognised as such, along with Mosasaurus, and the Iguanodon. In 1605, Richard Verstegen of Antwerp illustrated in his A Restitution of Decayed Intelligence plesiosaur vertebrae that he referred to fishes and saw as proof that Great Britain was once connected to the European continent. The Welshman Edward Lhuyd in his Lithophylacii Brittannici Ichnographia from 1699 also included depictions of plesiosaur vertebrae that again were considered fish vertebrae or Ichthyospondyli. Other naturalists during the seventeenth century added plesiosaur remains to their collections, such as John Woodward; these were only much later understood to be of a plesiosaurian nature and are today partly preserved in the Sedgwick Museum.

In 1719, William Stukeley described a partial skeleton of a plesiosaur, which had been brought to his attention by the great-grandfather of Charles Darwin, Robert Darwin of Elston. The stone plate came from a quarry at Fulbeck and had been used, with the fossil at its underside, to reinforce the slope of a watering-hole in Elston. After the strange bones it contained had been discovered, it was displayed in the local vicarage as the remains of a sinner drowned in the Great Flood. Stukely affirmed its "diluvial" nature but understood it represented some sea creature, perhaps a crocodile or dolphin. The specimen is today preserved in the Natural History Museum, its inventory number being BMNH R.1330. It is the earliest discovered more or less complete fossil reptile skeleton in a museum collection. It can perhaps be referred to Plesiosaurus dolichodeirus.

During the eighteenth century the number of English plesiosaur discoveries rapidly increased, although these were all of a more or less fragmentary nature. Important collectors were the reverends William Mounsey and Baptist Noel Turner, active in the Vale of Belvoir, whose collections were in 1795 described by John Nicholls in the first part of his The History and Antiquities of the County of Leicestershire. One of Turner's partial plesiosaur skeletons is still preserved as specimen BMNH R.45 in the British Museum of Natural History; this is today referred to Thalassiodracon.

Complete Plesiosaurus skeleton recovered by the Annings in 1823

In the early nineteenth century plesiosaurs were still poorly known and their special build was not understood. No systematic distinction was made with ichthyosaurs so the fossils of one group were sometimes combined with those of the other to obtain a more complete specimen. In 1821, a partial skeleton, found by the commercial fossil collector Mary Anning in the Jurassic marine fossil beds at Lyme Regis in Dorset, England, was described by William Conybeare and Henry Thomas De la Beche, and recognised as representing a distinctive group. A new genus was named, Plesiosaurus. The generic name was derived from the Greek πλήσιος, plèsios, "closer to" and the Latinised saurus, in the meaning of "saurian", to express that Plesiosaurus was in the Chain of Being more closely positioned to the Sauria than Ichthyosaurus which had the form of a more lowly fish. The name should thus be rather read as "approaching the Sauria" than as "near lizard". Parts of the specimen are still present in the Oxford University Museum of Natural History.

Soon afterwards, the morphology became much better known. In 1823, Thomas Clark reported an almost complete skull, probably belonging to Thalassiodracon, which is now preserved by the British Geological Survey as specimen BGS GSM 26035. The same year Mary Anning and her family uncovered an almost complete skeleton at Lyme Regis on what is today called the Jurassic Coast. It was acquired by the Duke of Buckingham who made it available to the geologist William Buckland. He in turn let it be described by Conybeare on 24 February 1824 in a lecture to the Geological Society of London, during the same meeting in which for the first time a dinosaur was named, Megalosaurus. The two finds revealed the unique and bizarre build of the animals, in 1832 by Professor Buckland likened to "a sea serpent run through a turtle". Conybeare in 1824 also provided a specific name to Plesiosaurus: dolichodeirus, "longneck". The skeleton was in 1848 bought by the British Museum of Natural History and catalogued as BMNH 22656. When the lecture was published Conybeare also named a second species: Plesiosaurus giganteus. This was a short-necked form later assigned to the Pliosauroidea.

The majority of plesiosaur finds discovered in the 19th Century were found in the sea cliffs of Lyme Regis. Sir Richard Owen alone named nearly 100 new species. Despite this, plesiosaurs were little known. The majority of the descriptions of new species were based on isolated bones, without sufficient diagnosis to be able to distinguish them from the other species that had previously been described. Many of the new species described at this time have subsequently been invalidated. The genus Plesiosaurus is particularly problematic as the majority of the new species were located in this genus so that it has become a "recyclable" taxon.

The classification of plesiosaurs is further complicated by two factors. Although they have been found on every continent, including Antarctica, nearly all the discoveries come either from the Upper Jurassic Oxford Clay in England or from the Middle Cretaceous Niobrara chalk formation in Kansas in the United States. As only two links of a long evolutionary chain are well known it is difficult to extrapolate the connection between the two. Another problem is that the traditional way of classifying plesiosaurs relates to the thickness of their bodies, but it appears that the same body shape has been developed on multiple occasions in an example of convergent evolution. Recent analysis shows that elasmosaurs, which have long necks, are in fact descended from at least three unrelated lineages, making the taxa polyphyletic. Some pliosaurs could also be more closely related to the long-necked species than to other short-necked species. The four major groups, although convenient, do not appear to be based on real evolutionary relationships.

Recent discoveries

In 2002, the "Monster of Aramberri" was announced to the press. Discovered in 1982 at the village of Aramberri, in the northern Mexican state of Nuevo León, it was originally classified as a dinosaur. The specimen is actually a very large plesiosaur, possibly reaching 15 m (49 ft) in length. The media published exaggerated reports claiming it was 25 metres (82 ft) long, and weighed up to 150,000 kilograms (330,000 lb), which would have made it the largest predator of all time. This error was dramatically perpetuated in BBC's documentary series Walking with Dinosaurs, which also prematurely classified it as a Liopleurodon ferox.

In 2004, what appears to be a completely intact juvenile plesiosaur was discovered, by a local fisherman at Bridgwater Bay National Nature Reserve in Somerset, UK. The fossil, dated 180 Ma by the ammonites associated with it, measured 1.5 metres (4 ft 11 in) in length, and may be related to Rhomaleosaurus. It is probably the best preserved specimen of a plesiosaur yet discovered.

In 2006, a combined team of American and Argentinian investigators (the latter from the Argentinian Antarctic Institute and the La Plata Museum) found the skeleton of a juvenile plesiosaur measuring 1.5 metres (4 ft 11 in) in length on Vega Island in Antarctica. The fossil is currently on display as the geological museum of South Dakota School of Mines and Technology.

In 2008, fossil remains of an undescribed plesiosaur that was named Predator X, now known as Pliosaurus funkei, were unearthed in Svalbard. It had a length of 12 m (39 ft), and its bite force of 149 kilonewtons (33,000 lb_f) is one of the most powerful known.

Behavior

Painting of a plesiosaur on land, by Heinrich Harder

Plesiosaur gastroliths

Pliosaurs were top carnivores in their respective foodwebs. They were pursuit predators of various sized prey and opportunistic feeders, their teeth were used to pierce small soft-bodied prey, especially fish. Hard and soft-bodied cephalopods also formed part of the diet of all plesiosaurs. Plesiosaurs hunted visually and perhaps employed a directional sense of olfaction. They had powerful jaws, probably strong enough to bite through the hard shells of their prey. The bony fish (Osteichthyes), started to spread in the Jurassic, and were likely prey as well. Recent evidence seems to indicate that some plesiosaurs may have, in addition, been bottom feeders. Plesiosaurs were also prey for other carnivores as shown by bite marks left by a shark that have been discovered on a fossilized plesiosaur fin and the fossilized remains of a mosasaur’s stomach contents that are thought to be the remains of a plesiosaur.

It had been theorised that smaller plesiosaurs may have crawled up on a beach to lay their eggs, like the modern leatherback turtle, but it is now clear that plesiosaurs gave birth to live young: The fossil of a pregnant plesiosaur Polycotylus latippinus shows that these animals gave birth to one large juvenile and probably invested parental care in their offspring, similar to modern whales.

Another curiosity is their four-flippered design. No modern animals have this swimming adaptation (sea turtles only swim with their front flippers), so there is considerable speculation about what kind of stroke they used. While the short-necked pliosauroids (e.g. Liopleurodon) may have been fast swimmers, the long-necked varieties were built more for maneuverability than for speed. Skeletons have also been discovered with gastroliths in their stomachs, though whether to help break down food in a muscular gizzard, or to help with buoyancy, or both, has not been established. However, the total weight of the gastroliths found in various specimens appear to be insufficient to modify the buoyancy of these large reptiles.

Classification

Ilustration showing plesiosaurs

Ilustration showing of the pliosaurs Simolestes vorax

The closest related predecessor of plesiosaurs were the pistosaurids, which had small heads and long necks. They evolved approximately 220 million years ago during the Upper Triassic, and became extinct 175 million years ago at the beginning of the Jurassic period.

The next group of plesiosaurs were characterized by a large head and a short neck, and they are know as the pliosaurs, the largest specimens in this group included the kronosaurs, the megalneusaurs, plesiopleurodons, and pliosaurs. They had jaws up to 3 m long and could be more than 12–15 m long, weighing 10,000 kg. Isolated vertebrae and teeth found in England could belong to specimens that are thought to have been up to 22 m long. The pliosaurs had large, conical teeth and were the dominant carnivores of their time, pliosaur teeth marks have been discovered on other plesiosaurs, such as cryptoclidus. Pliosaurs evolved about 200 million years ago, during the Lower Jurassic and they became extinct approximately 80 million years ago in the Cretaceous period.

The third group also had a long neck and a small head and they are known as the cryptoclidus. They were shorter and more graceful that the plesiosaurs, but they had necks that were longer than their bodies. Their teeth were small and thin and were possible used to filter their food out of sediment in shallow coastal seas. This group first appeared approximately 160 million years ago at the end of the Jurassic period and survived up until the extinction event at the end of the Cretaceous period, approximately 66 million years ago.

The last group, the elasmosaurus also had a long neck and small head. They were the largest plesiosaurs reaching 13–17 m in length but the majority of this was the neck, they weighed less than the largest pliosaurs. They had more than 72 vertebrae in their necks, more than any other animal. They lived at the same time as the cryptoclidus, so they must have occupied a different ecological niche.

Taxonomy

Kronosaurus, a pliosaurid

Thalassomedon, an elasmosaurid

Dolichorhynchops, a polycotylid

The taxonomy presented here is mainly based on the plesiosaur cladistic analyses proposed by Ketchum and Benson, 2011 and Benson et al., 2012 unless otherwise noted.

• Node Pistosauria
  • Augustasaurus
  • Pistosaurus
  • Yunguisaurus
  • Order Plesiosauria
    • Bobosaurus
    • Suborder Plesiosauroidea
      • Family Elasmosauridae
      • Family Microcleididae
      • Family Plesiosauridae
      • Node Cryptoclidia
        • ? Family Aristonectidae
        • Family Cryptoclididae
        • Node Leptocleidia
          • Family Leptocleididae
          • Family Polycotylidae
    • Suborder Pliosauroidea
      • ? Family Rhomaleosauridae
      • Family Pliosauridae

Phylogeny

Plesiosauria is a stem-based taxon that was defined as "all taxa more closely related to Plesiosaurus dolichodeirus and Pliosaurus brachydeirus than to Augustasaurus hagdorni". Neoplesiosauria is a node-based taxon that was defined by Ketchum and Benson (2010) as "Plesiosaurus dolichodeirus, Pliosaurus brachydeirus, their most recent common ancestor and all of its descendants". Therefore, Neoplesiosauria has been considered to be a junior synonym of Plesiosauria. The following cladogram follows an analysis by Ketchum & Benson, 2011.

Benson et al. (2012) found Pliosauroidea to be paraphyletic in relation to Plesiosauroidea. Rhomaleosauridae was found to be outside Neoplesiosauria, but still within Plesiosauria. The early carnian pistosaur Bobosaurus was found to be more advanced then Augustasaurus in relation to the Plesiosauria and therefore it represents by definition the oldest known plesiosaur. However, the authors excluded Bobosaurus from this clade without any explanation, although that exclusion might be a typo in the article. This analysis focused on basal plesiosaurs and therefore only one derived pliosaurid and one cryptoclidian were included while elasmosaurids weren't included at all. The cladogram below follows the topology from Benson et al. (2012) analysis.

Distribution

Stratigraphic distribution

The following is a list of geologic formations that have produced plesiosaur fossils.

Click [show] to view a table of geologic formations 

Name	Age	Location	Notes
Allen Formation		Argentina
Mata Amarilla Formation		Argentina
Jagüel Formation		Argentina
Bearpaw Formation		Canada USA
Blue Lias Formation		UK
Britton Formation		USA
Carlile Formation		USA
Chichali Formation		Pakistan
Conway Formation		New Zealand
Favret Formation		USA
Franciscan Formation		USA
Guanling Formation		China
Hiccles Cove Formation		Canada
Horseshoe Canyon Formation		Canada
Jagua Formation		Cuba
Katiki Formation		New Zealand
Kimmeridge Clay		UK
Kiowa Shale		USA
Lake Waco Formation		USA
Maree Formation		Australia
Mooreville Chalk Formation		USA
Moreno Formation		USA
Muschelkalk		Germany
Naknek Formation		USA
Niobrara Formation		USA	The following genera have been reported from the formation: Brimosaurus, Dolichorhynchops, Elasmosaurus, Polycotylus, Styxosaurus, and Thalassonomosaurus.
Oxford Clay		UK
Paja Formation		Colombia
Paso del Sapo Formation		Argentina
Pierre Formation		USA
Sundance Formation		USA
Tahora Formation		New Zealand
Tamayama Formation		Japan
Toolebuc Formation		Australia
Vectis Formation		UK
Wallumbilla Formation		Australia
Weald Clay		UK
Zhenzhuchong Formation		China
Ziliujing Formation		China

Plesiosaurs in contemporary culture

Main article: Loch Ness Monster

See also: Sea monster

It has been suggested that legends of sea serpents and modern sightings of supposed monsters in lakes or the sea could be explained by the plesiosaur’s survival into modern times. The great majority of these theories have been rejected by the scientific community, which considers them to be fantasy and pseudoscience. However, the discovery of real examples of living fossils, such as the coelacanth, and of gigantic, previously unknown animals from the ocean’s depths, such as the giant squid, have continued to breathe fresh life into these myths.

The body of a marine animal with a long neck and small head was found in the nets of the Japanese fishing boat Zuiyo Maru that had been fishing in waters off New Zealand in 1977, creating a wave of plesiosaur mania in Japan. A panel of eminent Japanese marine biologists inspected the remains. The panel included professors Ikuo Obata and Hiroshi Ozaki of Japan's National Science Museum and Dr. Toshio Kasuya from the University of Center for Marine Research. These scientists came to a number of different conclusions. Professor Yoshinori Imaizumi from the National Museum of Nature and Science concluded that the specimen "was not a fish, whale or any other mammal". While others concluded that it was a basking shark, although Professor Toshio Kasuya stated "if it was a shark, the spine would be shorter … and its neck is too long … I think that we can exclude the fish theory". However, analysis of samples of body tissue taken from the animal before it was thrown back into the sea was carried out by Dr. Shigeru Kimura, a biochemist at the University of Tokyo, who concluded that the tissue samples contained a protein, called elastodin, that is only found in sharks and not in the other groups that it had been suggested that the animal belonged to. This meant that it could not be a reptile or a mammal.

The Loch Ness monster is normally reported as looking like a plesiosaur, although it is also often described as a creature that looks nothing like a plesiosaur. However, there are a number of reasons why it could not be a plesiosaur. These include the fact that the water in the lake is too cold for a cold blooded animal to be able to survive easily, air-breathing animals would be easy to see whenever they appear at the surface to breathe, the lake is too small and contains insufficient food to be able to support a breeding colony and finally the lake was only formed 10,000 years ago during the last ice age, while plesiosaurs became extinct over 65 million years ago. Frequent explanations for the sightings include waves, floating objects, tricks of the light, swimming animals (such as an otter, which can reach up to 2m in length, a sturgeon, etc.), a group of swimming birds seen from afar (such as a duck with its young), and practical jokes.

Bibliography

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• Carpenter, K. 1997. Comparative cranial anatomy of two North American Cretaceous plesiosaurs. Pp 91–216, in Calloway J. M. and E. L. Nicholls, (eds.), Ancient Marine Reptiles, Academic Press, San Diego.
• Carpenter, K. 1999. Revision of North American elasmosaurs from the Cretaceous of the western interior. Paludicola 2(2):148-173.
• Cope, E. D. 1868. Remarks on a new enaliosaurian, Elasmosaurus platyurus. Proceedings of the Academy of Natural Sciences of Philadelphia 20:92-93.
• Ellis, R. 2003: Sea Dragons' (Kansas University Press)
• Everhart, M. J. 2002. Where the elasmosaurs roamed Prehistoric Times 53: 24-27.
• Everhart, M.J. 2005. "Where the Elasmosaurs roamed," Chapter 7 in Oceans of Kansas: A Natural History of the Western Interior Sea, Indiana University Press, Bloomington, 322 p.
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External links

• The Plesiosaur Site. Richard Forrest.
• The Plesiosaur Directory. Adam Stuart Smith.
  • Plesiosauria technical definition at the Plesiosaur Directory
• Plesiosaur FAQ's. Raymond Thaddeus C. Ancog.
• Oceans of Kansas Paleontology. Mike Everhart.
• "Plesiosaur fossil found in Bridgwater Bay". Somersert Museums County Service. (best known fossil)
• "Fossil hunters turn up 50-ton monster of prehistoric deep". Allan Hall and Mark Henderson. Times Online, December 30, 2002. (Monster of Aramberri)
• Triassic reptiles had live young.
• Bridgwater Bay juvenile plesiosaur