Plesiosaurus
Plesiosaurus was a large marine reptile that thrived during the Early Jurassic period, primarily along the shores of what is now Europe and North America. This creature was part of the sauropterygians, a diverse group of marine reptiles characterized by modified limbs that evolved into flippers. Plesiosaurus typically measured between 3 to 5 meters (9.8–16.4 ft) in length and weighed up to 90.7 kilograms (200 lbs), featuring a distinctive long neck, small head, and a compact body. Paleontologists believe Plesiosaurus primarily fed on small prey, utilizing its sharp teeth and long neck to capture fish and belemnites.
Reproductive behaviors remain a topic of discussion, with some evidence suggesting that Plesiosaurus may have come ashore to lay eggs, similar to modern sea turtles. Social behavior appears limited, as fossils indicate solitary living rather than group dynamics. The Jurassic seas were rich with diverse marine life, including various fish, mollusks, and other prehistoric reptiles like ichthyosaurs and crocodylomorphs. Notably, significant contributions to our understanding of Plesiosaurus come from early fossil discoveries by Mary Anning and subsequent research that further clarified its classification within the broader context of marine reptiles.
Plesiosaurus
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Plesiosauria
Family: Plesiosauridae
Genus:Plesiosaurus
Species:Plesiosaurus dolichodeirus
Introduction
Plesiosaurus was a large marine reptile that lived in the Early Jurassic period off the coasts of present-day Europe and North America. Plesiosaurus was a relatively common reptile and a member of a large group of marine reptiles, the sauropterygians, one of the most successful groups of Mesozoic reptiles.
In the latest Triassic period, the continents of the earth were united in a single land mass known as Pangaea. The eastern edge of Pangaea supported a vast gulf, into which flowed the enormous Paleo-Tethys Ocean. In the Early Jurassic period, Pangaea began a long period of tectonic shifting that would eventually split the land mass into northern and southern supercontinents.
The sauropterygians first evolved in the Early Triassic, accompanying the evolution of the dominant terrestrial reptiles that preceded the dinosaurs. By the Early Jurassic, the sauropterygians had divided into two basic groups, the long-necked plesiosaurs and the shorter, stockier nothosaurs. Plesiosaurus was one of several species of plesiosaur that lived along the coasts of what would later become the northern supercontinent of Laurasia.
Classification
Scientists rely on two methods to determine groupings for different organisms. The system known as Linnaean taxonomy, or “systematics,” groups organisms according to overall physical similarity, while an alternate system, cladistics, creates groups called “clades” in an effort to reflect the evolutionary relationships between species.
Systematic (Linnaean) analysis places Plesiosaurus into the superorder Sauropterygia, a group of marine reptiles whose front and/or back legs are modified into flippers. The order Plesiosauria is used for sauropterygians with shorter bodies, short tails, and smaller heads. Animals in the family Plesiosauridae had elongated necks and small heads. Cladistic analysis identifies the clade Lepidosauromorpha as the group of reptiles that gave rise to the sauropterygians. From there, plesiosaurs are placed into the clade Plesiosauria, aquatic reptiles with modified limbs and wide bodies. Plesiosaurus is further divided into the clade Plesiosauroidea, based largely on skull features.
Anatomy
Plesiosaurus was a large reptile, reaching lengths of between 3 and 5 meters (9.8–16.4 ft) and weighing as much as 90.7 kilograms (200 lbs). The animal had a thick, compact body, with all four limbs modified into muscular, broad flippers for propulsion. It possessed a small tail and a long, flexible neck.
Plesiosaurus had a small head with rows of sharp, spike-like teeth. It had large eyes, situated on the sides of its head, giving it a wide field of vision. The animal's nostrils were set on the top of the skull and were relatively broad for its skull size. The animal had a relatively short jaw that attached to the skull with a flexible hinge, allowing the animal to open its jaws at a wide angle.
Intelligence
Hopson did not measure the EQ value for Plesiosaurus, but it is reasonable to surmise that the prehistoric sauropterygians would have been less intelligent than modern marine mammals that display similar behavioral patterns. Plesiosaurus may have had similar intelligence to large fish or reptiles.
Reproduction and Population
The flippers of sauropterygians were organized like those of turtles or sea lions, which may have allowed plesiosaurs limited ability to maneuver on land. Some paleontologists suggest that Plesiosaurus may have come to land to give birth or lay eggs, similar to a sea turtle, perhaps burying its eggs in the sand near the shore. It is also possible that plesiosaurs gave live birth in the sea.
Plesiosaurus appears to have been a relatively common species and was widespread throughout its range. Plesiosaurus was an early representative of the plesiosaur group, and its ancestors would continue to dominate the oceanic environment until the end of the Cretaceous period.
Diet
Paleontologists believe that Plesiosaurus and other members of its group specialized in trapping relatively small prey. The rows of sharp teeth were probably used to pierce and kill fish and belemnites before passing them through its long neck. The animal's long neck and small head may have evolved to allow it to maneuver its head into the crevices of rocks along the shore while searching for prey.
Behavior
Some plesiosaur fossils reveal stones within the stomach, which may have helped to grind food. Alternatively, swallowing stones may have provided ballast, helping to combat buoyancy and allow for greater diving depth. Plesiosaurs had large bodies and presumably had large lungs, potentially allowing them to hold their breath for extended periods while diving to obtain food.
Paleontologists have discovered groups of plesiosaur fossils in some areas, suggesting that the animals may have congregated while eating or breeding. There is no evidence to suggest that plesiosaurs travelled in groups or displayed other types of social behavior.
Fossils of some plesiosaur specimens have been discovered with noticeable wounds to the bones, indicating attacks from predatory animals. Plesiosaurus and other members of the group would have been at risk from larger nothosaurs, sharks, and a variety of other oceanic predators. If the animals came on land to lay eggs, this may have placed them at risk from terrestrial predators as well.
Habitat and Other Life Forms
The Jurassic oceans off the coast of North America and Europe were a rich environment with hundreds of species of fish, mollusks, cephalopods, and marine reptiles. Modern sharks first appear during this period, including groups like Neoselachii and Galeomorphii. Other types of fish in the oceans included a variety of bony fish and the first teleost fish. One species of fish, Leedsichthys, was one of the largest fishes in history, reaching between 10 to 15 meters (33–49 ft) in length.
There were a variety of plesiosaurs in the Jurassic seas, including both the long-necked plesiosaurs, like Plesiosaurus, and the short-necked pliosaurs, like Archaeonectrus and Eurycledius. The other major group of prehistoric reptiles were the ichthyosaurs, which included species like Ichthyosaurus and Temnodontosaurus. Towards the later portion of the Early Jurassic, a variety of marine crocodylomorphs, relatives of modern crocodiles, were also present off the coasts of North America and Europe, including species like Dakosaurus.
Research
Many of the earliest prehistoric marine reptile fossils were found by amateur fossil hunter Mary Anning, who began finding fossils in limestone deposits of her native England in the 1820s. Among the fossils discovered by Anning in the early 1820s was the first specimen of Plesiosaurus, which was described by British paleontologist William Conybeare. Modern research into plesiosaur classification was furthered by Hilary Ketcham, whose research in 2009 to 2010 helped to elucidate the relationships between species.
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