The group of marine reptiles known as the Plesiosauria dominated the oceans of the Mesozoic for about 140 million years, up until the last days of the dinosaurs. Nested within the Plesiosauria were the subgroups Plesiosauroidea, the long-necked plesiosaurs, and the Pliosauroidea, the short-necked plesiosaurs. The Nothosauria, also discussed here, were a sister group of long-necked marine reptiles that evolved before the plesiosaurs. It is interesting to note, however, that at just about the time that the nothosaurs became extinct, at the end of the Late Triassic Epoch, the plesiosaurs were on the rise. It is likely that the plesiosaurs knocked the nothosaurs out of their ecological niches, taking over their role as a chief predator of the oceans.
Although they were not as fishlike as the ichthyosaurs, plesio-saurs and pliosaurs were adapted remarkably well for swimming. This can be seen in the design of their large, long paddles. The paddles were shaped like airplane wings: The front edge was thick and rounded and then tapered back to a narrower rear edge. They had evolved extra finger and toe bones to make the paddles longer. The parts of the shoulder and hip bones to which the paddles were attached were enormous compared with those of land-dwelling reptiles. This provided more surface area for the attachment of the huge muscles needed to propel these creatures through the water. The winglike shape of their paddles, combined with extensions of their unique shoulders and hips, made it possible for the Plesiosauria to "fly" underwater. They actually moved their paddles in graceful vertical strokes very much like the flapping wings of a bird in flight. A similar kind of swimming technique is seen in sea turtles, sea lions, and penguins today. Having two sets of "wings"—the front and rear paddles—gave plesiosaurs and pliosaurs power, speed, and maneuverability.
Nothosaurs are known only from the Triassic Period. Their fossils are almost exclusively limited to ancient ocean deposits in Europe, although a few remains have been found in China and Israel. Plesiosaurs and pliosaurs existed from the Late Triassic to the Late Cretaceous, and their remains are distributed on many continents, including Europe, North America, South America, Africa, Australia, and Antarctica.
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