Plesiosauroids (Plesiosauroidea)

1. Thalassomedon hanningtoni 2. Futabasaurus suzukii 3. Plesiosaurus dolichodeirus 4. Elasmosaurus platyurus
The plesiosauroids were sea-going reptiles with long neck and small heads filled with long sharp curved teeth. They had a flattened, barrel-shaped body and large paddle-shaped flippers. The two front flippers were always somewhat larger than the hind flippers. They ranged in size from 2.5 to 20 metres.
The plesiosauroids seem to have evolved during the Triassic, as supposed fragments occur in the Upper Keuper and Rhaetic of Europe. But it is only during the Early Jurassic that the group became well established. Beautiful skeletons are known from shallow marine deposits of Europe. Early Jurassic forms were generally quite small, about 3 meters in length, with a relatively short neck of about 32 vertebrae. As time progressed plesiosaurs developed longer necks and, by the middle Cretaceous, increased greatly in body size as well. The largest and most derived forms are known from the Late Cretaceous. These had long, snake-like necks and overall lengths of up to 13 meters. Ironically, the paddles are relatively smaller in these late forms then they were in the Jurassic types, indicating less emphasis on overall movement and more on capturing prey by stealth.
The long flexible necks enabled them to dart their small heads and catch fish in their toothy jaws. The neck became longer -- and perhaps more flexible with the addition of additional, shorter vertebrae -- as the group evolved through time.Plesiosauroids, unlike ichthyosaurs, used stomach stones for ballast to offset the buoyancy of their lungs. Plesiosauroids may have rested on the bottom, striking at prey with their long necks.

Unlike their pliosauroid cousins, plesiosauroids (with the exception of the Polycotylidae) were probably slow swimmers.It is likely that they cruised slowly below the surface of the water, using their long flexible neck to move their head into position to snap up unwary fish or cephalopods. Their four-flippered swimming adaptation may have given them exceptional maneuverability, so that they could swiftly rotate their bodies as an aid to catching prey.
Contrary to many reconstructions of plesiosauroids, it would have been impossible for them to lift their head and long neck above the surface, in the "swan-like" pose that is often shown {Everhart, 2005}.Even if they had been able to bend their necks upward to that degree (which they could not), gravity would have tipped their body forward and kept most of the heavy neck in the water.
On 12 August 2011, researchers from the U.S. described a fossil of a pregnant plesiosaur found on a Kansas ranch in 1987.The plesiosauroid, Polycotylus latippinus, has confirmed that these predatory marine reptiles gave birth to single, large, live offspring - contrary to other marine reptile reproduction which typically involves a large number of small babies. Before this study, plesiosauroids had sometimes been portrayed crawling out of water to lay eggs in the manner of sea turtles, but experts had long suspected that their anatomy was not compatible to movement on land. The adult plesiosaur measures 4 m long and the baby is 1.5 m long.
Plesiosaur classification has traditionally been a mess. One of the confounding factors has been the change in plesiosaur osteology during the course of development.

The superfamily Plesiosauroidea is one of the two major traditional divisions of plesiosaurs, the group that typically have long necks. The other major group (superfamily) of plesiosaurs is the Pliosauroidea (often just called pliosaurs). During the 1990s and 2000s, this simple division of plesiosaurs into two superfamilies was called into question and multiple studies have demonstrated that the picture is more complicated, with at least some ‘true’ plesiosaurs being located outside of these two clades. Within the Plesiosauroidea, there are a number of families including the long-necked Plesiosauridae, Cryptoclididae, Elasmosauridae, and the short-necked Polycotylidae also originated from this group.


The longest plesiosauroid was Mauisaurus, possibly reaching 20 meters in length-

Mauisaurus ("Maui reptile") is a genus of plesiosaur that lived during the Late Cretaceous period around 80 to 69 million years ago in what is now New Zealand. It was the largest plesiosaur, and perhaps the largest marine reptile in New Zealand waters at the time. Mauisaurus haasti is the only known species of the genus. A handful of specimens have been found, although only a few are well preserved and mostly complete. The only other established New Zealand plesiosaur, Tuarangisaurus keyesi, cannot be linked as a direct relative to Mauisaurus.
At 68 cervical vertebrae, Mauisaurus is notable for having one of the longest necks (in terms of vertebra number) of any plesiosaur. Mauisaurus was fairly large, reaching over 8 metres (26 ft) in length, though estimates of 20 metres (66 ft) have been estimated. Like other plesiosaurs, it had a long slender body, with numerous vertebrae, allowing flexible movement. On its underside, Mauisaurus had two sets of large flippers. These aided in swimming at high speeds, but may have also allowed the plesiosaur to venture onto shorelines for short amounts of time. Mauisaurus was a carnivore, with sharp jagged teeth that would have been used to grip fish or squid.

Elasmosaurus at 14 metres (46 feet) long.

Elasmosaurus was about 14 m (46 ft) in length and weighed over 2,000 kg (2.2 short tons), making it among the largest plesiosaurs.It differs from all other plesiosaurs by having six teeth per premaxilla (the bones at the tip of the snout) and 71 neck (cervical) vertebrae.The skull was relatively flat, with a number of long pointed teeth. The lower jaws were joined at the tip to a point between the fourth and fifth teeth.The neck vertebrae immediately following the skull were long and low, and had longitudinal lateral crests.Like most elasmosaurids, Elasmosaurus had around three pectoral vertebrae. The tail included at least 18 vertebrae.
The pectoral girdle featured a long bar,not present in juveniles. The scapula had margins of approximately equal length for the joint with the coracoid and the articular surface for the upper arm.The anterior edge of the pelvic girdle was made up of three almost straight edges directed to the front and sides of the animal. The ischia, a pair of bones that formed the posterior part of the pelvis, were joined along their medial surfaces.The limbs of Elasmosaurus, like those of other plesiosaurs, were modified into approximately equally sized rigid paddles.
Certain aspects of the anatomy of Elasmosaurus were fairly derived among elasmosaurids, and plesiosaurs in general.As noted, Elasmosaurus can be distinguished by its six premaxillary teeth and 71 cervical vertebrae.Primitively, plesiosaurs and most elasmosaurids had five teeth per premaxilla. Some elasmosaurids had more: Terminonatator had nine and Aristonectes had 10 to 13.In addition, most plesiosaurs had fewer than 60 cervical vertebrae.Aside from Elasmosaurus, plesiosaurs that exceeded 60 cervicals include Styxosaurus, Hydralmosaurus, and Thalassomedon. Elasmosaurus is the only known plesiosaur with more than 70 cervicals. However, it had roughly the same neck length as Thalassomedon because the latter has proportionally longer vertebrae.Elasmosaurus had more vertebrae than any known animal. The presence of the pectoral bar is also considered an advanced feature.The long, low axis centrum differs from the condition seen in most other plesiosaurs, which have centra that are either shorter in length than height, or about equidimensional.Styxosaurus and Hydralmosaurus also have the condition present in Elasmosaurus.Another unusual feature of Elasmosaurus is the relatively equal lengths of the margins of the scapula, as mentioned above. Most plesiosaurs had longer margins for articulation with the coracoid than for articulation with the upper arm.
While numerous species of Elasmosaurus have been named since its discovery, a 1999 review by Ken Carpenter showed that only one, the type species Elasmosaurus platyurus, could be considered valid. Various other species assigned to the genus are either dubious or have been classified in other genera. For example, E. serpentinus has been reclassified as Hydralmosaurus, E. morgani as Libonectes, and E. snowii as Styxosaurus.By the early part of the Late Cretaceous, plesiosaurs have evolved (or have been reduced) into two distinct groups.Elasmosaurus is the type genus for one of these groups, the elasmosaurids which had extremely long necks with relatively short heads, in contrast to the polycotylids which had shorter necks and relatively larger heads. Late Cretaceous elasmosaurids from the Western Interior of North America have few features that separate them and are morphologically primitive.However, as noted above, Elasmosaurus and some others have some derived features. It has been suggested that Elasmosaurus was closely related to Hydralmosaurus and Styxosaurus due to these advanced features.
When E. D. Cope received the specimen in early March, 1868, he had a pre-conceived idea of what it should look like, and mistakenly placed the head on the wrong end (i.e. the tail). In his defense, at the time he was an expert on lizards, which have a short neck and a long tail, and no one had ever seen a plesiosaur the size of Elasmosaurus. Although popular legend notes that it was Othniel Charles Marsh who pointed out the error, there is no factual justification for this account (see below). However, this event is often cited as one of the causes of their long-lasting and acrimonious rivalry, known as the Bone Wars. In fact, although Marsh personally collected at least one plesiosaur from Kansas, and had several more from Kansas in the Yale Peabody collection, he never published a single paper on them.
Although Cope verbally announced the discovery of Elasmosaurus platyurus in March 1868, he did not publish the "preprint" of his erroneous reconstruction of Elasmosaurus until August 1869. While much smaller, long-necked plesiosaurs from the Jurassic of England were well known at the time, this was the first time anyone had ever seen a Cretaceous elasmosaur. Cope"s reconstruction showed it to have a long sinuous tail like a lizard or a mosasaur. Note that while O.C. Marsh claimed to have pointed out Cope"s error "20 years after the fact" in an 1890 newspaper article, it was actually Joseph Leidy who pointed out the problem in his Remarks on Elasmosaurus platyurus address at the Academy of Natural Sciences of Philadelphia meeting on March 8, 1870.
Elasmosaurus fossils have been found in the Campanian-age Upper Cretaceous Pierre Shale of western Kansas. The Pierre Shale represents a period of marine deposition from the Western Interior Seaway, a shallow continental sea that submerged much of central North America during the Cretaceous.
Like most plesiosaurs, Elasmosaurus was incapable of raising anything more than its head above the water as it is commonly depicted in art and media. The weight of its long neck placed the center of gravity behind the front flippers. Thus Elasmosaurus could only have raised its head and neck above the water if in shallow water, where it could rest its body on the bottom. The weight of the neck, the limited musculature, and the limited movement between the vertebrae would have prevented Elasmosaurus from raising its head and neck very high as well.Nevertheless one study found that the necks of elasmosaurs were capable of 75–177˚ of ventral movement, 87–155° of dorsal movement, and 94–176° of lateral movement, depending on the amount of tissue between the vertebrae."Swan-like" S-shape neck postures which required more than 360° of vertical flexion were not possible. The head and shoulders of the Elasmosaurus most likely acted as a rudder. If the animal moved the anterior part of the body in a certain direction, it would cause the rest of the body to move in that direction. Thus, Elasmosaurus could not have swum in one direction while moving its head and neck either horizontally or vertically in a different direction.
Elasmosaurus was a slow swimmer and may have stalked schools of fish. The long neck would allow Elasmosaurus to conceal itself below the school of fish. It then would have moved its head slowly and approached its prey from below. The eyes of the animal could have had stereoscopic vision, which would help it find small prey. Hunting from below would also have helped by silhouetting the prey in the sunlight while concealing Elasmosaurus in the dark waters below. Elasmosaurus probably ate small bony fish, belemnites (similar to squid), and ammonites (molluscs). It swallowed small stones to aid its digestion.Elasmosaurus is believed to have lived mostly in open ocean. The paddles of Elasmosaurus and other plesiosaurs are so rigid and specialized for swimming that they could not have come on land to lay eggs. Thus it most likely gave live birth to its young like modern sea snakes.While direct evidence of reproduction in Elasmosaurus is not yet known, the contemporaneous plesiosaur Polycotylus is known to have given birth to live young.

Hydrotherosaurus

Hydrotherosaurus is comfortably placed within the Elasmosauridae, a group of plesiosaurs noted for having proportionately long necks. Also like these other genera, Hydrotherosaurus had long pointed teeth, but these seem to have projected more to the sides rather than just up and down. This however is quite a common feature seen in piscivorous (fish eating) animals where the outward pointing teeth actually increase the available catch area for trapping prey (just think catchers mitt). Palaeontologists do need to be certain before making such claims though, since this idea was also proposed for a genus of pliosaur called Simolestes. When first described Simolestes had outward pointing teeth, but then it was realised that the teeth were actually pushed out by the weight of the sediment above pushing down on them during the fossilisation process.
Hydrotherosaurus was described from fossils discovered in the Moreno Formation of California, with other plesiosaur genera from here including Aphrosaurus, Morenosaurus and Fresnosaurus. Interesting facts concerning these four genera is that not have all four of these plesiosaurs come from the Moreno Formation, but they were all described by Samuel Paul Welles and all in 1943. Other marine reptiles include the mosasaurs Plesiotylosaurus and Plotosaurus.
At up to thirteen meters in length, Hydrotherosaurus was certainly one of the longest elasmosaurids, but still not as large as some other genera such as Elasmosaurus and Mauisaurus.

Thalassomedon

Thalassomedon is a genus of plesiosaur, named by Welles in 1943. Greek, thalassa, "sea" and Greek, medon, "lord, ruler": Sea lord.