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In the seas of the Triassic lived five major groups of marine reptiles: Pachypleurosauria, Placodontia, Nothosauria, Plesiosauria and Ichthyosauria. Each group is adapted to aquatic life in a very different way and altogether represent an important adaptive radiation to aquatic predators whose origins are probably independent.
Pachypleurosaurs are elongated animals with a small head, neck and tail long and limbs similar to paddles. They are mainly known in the Middle Triassic, found in abundant numbers in marine sediments of Central Europe. These animals were long from 20 cm to 1 m and were clearly adapted for a fundamentally aquatic life, wherein the locomotion was entrusted to oscillations of the expanded tails that provided the impetus for swimming. The forelimbs could serve in part for pushing that for direction. The hind limbs were probably held close to the sides of the body to reduce water resistance. The pelvis is very small and is weakly connected to the trunk, so that they could not support the weight of the animal on land. The skull is long and light with large orbits and nostrils, but with small temporal bone windows. The thin and sharp teeth are spaced and protruding forward, which suggests a diet of fish, that the agile Pachypleurosaurs could chase and capture with a shots of the long neck.
The bigger Nothosauria (the length of which varies from 1 to 4 m) possessed elongated heads and large temporal windows, but
otherwise were similar to Pachypleurosaurs in their skeletal adaptations. These reptiles appear to be closely related to the Plesiosaurs.
Placodontia are a group of marine reptiles with a very different structure from that of the two previous groups. Placodus at first sight it would seem a massive land animal, but his remains were found in sediments of shallow sea. The tail is flattened and limbs are not transformed into paddles; However the hip bones are not firmly connected to the trunk as you would expect in a land animal. The dense array of gastralia that covers the belly is a character that is common to Pachypleurosaurs and Nothosaurs.
Placodus possesses teeth characteristic of the group, broad, flattened and covered with a thick coating of enamel. They were used to crush hard-shelled prey, such as that of mollusks, which were torn from the rocks through chisel incisor and then crushed by the teeth of the palate.
The main marine predators were the Ichthyosaurs and Plesiosaurs. After the first discoveries in the first half of 800, these reptiles were known as “sea dragons“.
Ichthyosauria (lizard-fish) were reptiles better adapted to aquatic life ever, with their bodies similar to those of the dolphins. They appeared in the Triassic and lived throughout the Mesozoic without substantial changes in their structure. There is considerable variability in size, with lengths ranging from 1 to 16 m. The profile of the body of Ichthyosaurs is well known thanks to the discovery of specimens in which one can observe a “footprint” of the contour of the skin. This has revealed that the flippers were made longer thanks to skin and connective tissue, and that the spine was bent down and that there was a high dorsal fin consists only of tissues. The Ichthyosaurs swam definitely efficiently moving sideways the tail and using the flippers to change direction and control pitching and rolling, like fish do. The Ichthyosaurs’ tail heterocercal inverse (that is with ventral lobe bigger than dorsal) would produce a propulsive force upward that would offset the tendency to sink during the deep motion. The front fins contrasted the effect of buoyancy force near the surface.
The first true Plesiosauria are known from the Upper Triassic and their sizes ranged from 2 to 14 m in length overall. Were highly adapted to locomotion under water due to the presence of powerful paddle-shaped limbs and pelvis extremely reinforced. We know four families of plesiosaurs, some with long neck, others with short neck.
Plesiosauridae owned small skulls and long necks. The Cryptoclididae owned necks even longer (30 cervical vertebrae) and a skull with elongated snout; long pointed teeth will fitted in the closure of the jaws to better hold fish in their mouth.
Elasmosauridae as Muraenosaurus (Upper Jurassic) possessed very long necks and some forms of the Upper Cretaceous boasted no fewer than 76 cervical vertebrae.
Pliosauroidea as Liopleurodon, over 12 m long, possessed a very long and heavy skull with a relatively short neck. Pliosauroids possessed enormous flattened heads, armed with powerful jaws, however, it was kept a hydrodynamic shape to not hinder the fast swimming. The neck was shorter than in other groups of Plesiosaurs, but nevertheless the Pliosauroids possessed a high number of cervical vertebrae.
The locomotion of Plesiosaurs has been subject of interesting speculations. A solution was proposed by Robinson (1975) assuming the “underwater flight” as in today’s turtles and penguins. The tip of the dorsal fin describes a figure 8 and each phase of the cycle produces a forward movement. Godfrey (1984) proposed a modified version, in which the dorsal fin describes a crescent. This assumption is based on the anatomy of the skeleton of Plesiosaur: the shoulder girdle and the pelvis were heavy structures, flattened and too weak to perform a powerful movement of elevation of the limbs.
Bibliography:
– “Paleontologia dei vertebrati” Michael J. Benton (2000)
– “Ancient Marine Reptiles” J.M. Callaway & E.L. Nicholls (1996)
– Disegni tratti da “The World Encyclopedia of Dinosaurs” D. Dixon
– Prehistoric Marine Life Art Print Poster by EuroGraphics
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