Pliosaurs were carnivorous, aquatic, Mesozoic-era (251-65 million years ago) reptiles comprising the suborder Pliosauroidea of the Plesiosauria order, characterized by a broad body, short tail, four paddle-shaped flipper limbs, and a shorter neck and more elongated head than the closely related "true plesiosaurs" (suborder Plesiosauroidea of the Plesiosauria order). They lived during the three periods—Triassic, Jurassic, and Cretaceous—of the Mesozoic era and, along with the true plesiosaurs, were the largest aquatic animals of their time. They were not dinosaurs.
Once dominant aquatic creatures, with sharp teeth, massive heads, and bodies up to 15 meters (49 feet) in length (Rincon 2008), the pliosaurs had disappeared by the end of the Mesozoic era, when a catastrophic extinction event also ended the dinosaurs' dominance on land. Furthermore, they may not have left any direct descendants from their lineage. However, in the step-by-step process of the development of the world, the pliosaurs played an important role in the food chains of their time, and helped prepare the environment for life today. Furthermore, they continue to live in the human imagination and add to the wonder of life.
|Mesozoic era (251 - 65 mya)|
The Plesiosauria order, to which the pliosaurs belonged, were aquatic, mostly marine, reptiles (superorder Sauroptrygia) with a broad body, short tail, and four paddle-shaped flipper limbs. Plesiosauria first appear in the fossil record in the Middle Triassic. They thrived until the K-T extinction, at the end of the Cretaceous period. Although they lived at the same time as dinosaurs, and though they are often lumped together with the "terrible lizards," they were not dinosaurs.
The limb arrangement of members of Plesiosauria is unusual in aquatic animals and it is thought that the limbs were used to propel the animal through the water by a combination of rowing movements and up-and-down movements. Members of Plesiosauria had 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.
As a group, the plesiosaurs were the largest aquatic animals of their time, and even the smallest were about two meters (6.5 feet) long. They grew to be considerably larger than the largest giant crocodiles, and were bigger than their successors, the mosasaurs. However, their predecessors as rulers of the sea, the dolphin-like ichthyosaurs, are known to have reached 23 meters (m) in length, and the modern whale shark (18 m), sperm whale (20 m), and especially the blue whale (30 m) have produced considerably larger specimens.
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.
The Plesiosauria order is divided into two suborders, Pliosauroidea and Plesiosauroidea. Members of the Plesiosauria order in general are sometimes called plesiosaurs, as are members of the suborder Plesiosauroidea. The term "true plesiosaurs" can be applied to those belonging to the suborder Plesiosauroidea in order to distinguish them. Members of the Pliosauroidea suborder are known as pliosaurs.
The Pliosauroidea and Plesiosauroidea are primarily distinguished by head and neck size. The Plesiosauroidea, such as Cryptoclididae, Elasmosauridae, and Plesiosauridae, had long necks and may have been bottom-feeders, in shallow waters. The Pliosauridae (pliosaurs) had a short neck with large, elongated head and may have been at home in deeper waters. However, in recent classifications, one short-necked and large-headed Cretaceous group, the Polycotylidae, are included under the Plesiosauroidea, rather than under the traditional Pliosauroidea.
In addition to the shorter neck and more elongated head, pliosaurs also were more crocodile-shaped than the members of Plesiosauroidea. The pliosaurs' long and powerful jaws carried many sharp, conical teeth. Pliosaur fossils indicated a range from four to fifteen meters in length, with an average of five to six meters (16 to 20 feet)(NHM 2007). In 2006, a fossil was found on the Arctic island chain of Svalbard of Norway of a Jurassic era pliosaur estimated at fifteen meters (50 feet) from nose to tail (Rincon 2008). Its flipper alone measured three meters in length. This fossil is 20 percent larger than the previously largest pliosaur (genus Kronosaurus) of 10 to 11 meters long found in Australia (NHM 2007; Rincon 2008). Pliosaurs are considered to have larger, more powerful muscles than a crocodile, and huge, robust jaws (Rincon 2008).
Pliosaurs were carnivorous, and their prey may have included fish, ichthyosaurs, and other plesiosaurs. Their carnivorous abilities were described by two paleontologist announcing the find of the largest pliosaur: "Think of an animal as long as a bus with teeth larger than cucumbers. Now think that those teeth are contained in a head that could swallow an adult whole" (Knight 2006). Another paleontologist stated "a large pliosaur was big enough to pick up a small car in its jaws and bite it in half" (Rincon 2008).
The adaptation of having four-flippers remains a mystery. No modern animals have this swimming adaptation, so there is considerable speculation about what kind of stroke they used. The short-necked pliosaurs (for example, Liopleurodon) may have been fast swimmers, unlike their pliosaurian cousins, the long-necked true plesiosaurs, which (with the exception of the Polycotylidae) were built more for maneuverability than for speed, and were probably relatively slow swimmers.
Plesiosaurs, in general, trace roughly from about 210 million years ago to about 65 million years ago (Knight 2006). Fossil specimens of pliosaurs have been found in England, Mexico, South America, Australia, and the Arctic region near Norway. The name is derived from the Greek πλειω from the verb meaning "to sail" or πλειων meaning "fin" and σαυρος meaning "lizard."
Pliosaurs originally included were reptiles of the family Pliosauridae, but several other genera and families are now also included with the number and details of which varying according to the classification used. Typical genera include Macroplata, Kronosaurus, Liopleurodon, Pliosaurus, and Peloneustes. Many very early (from the Rhaetian (Latest Triassic) and Early Jurassic) primitive pliosaurs were very like plesiosaurs in appearance and indeed used to be included in the family Plesiosauridae.
The taxonomy presented here is mainly based on the plesiosaur cladistic analysis proposed by O'Keefe in 2001.
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