Plesiosaurs were a group of marine reptiles that evolved from land-living reptiles in the Triassic period about 210 million years ago. They too, like ichthyosaurs had four flippers, a remnant of their terrestrial ancestors. However, unlike the dolphin-looking ichthyosaurs plesiosaurs used all four paddles in propulsion. No other aquatic animal known to science use this mode of locomotion. Plesiosaurs came in a variety of shapes, especially when it came to relative length of neck and size of their head. Some had very short necks and enormous heads, others had extremely long necks - up to 72 vertebrae in some - and small heads, and then there were a large number of intermediates. Traditionally plesiosaurs were divided into two groups; the short-necked pliosaurs and the long-necked plesiosaurs. However, studies of plesiosaur interrelationships have shown this dichotomy to be artificial, and that some long-necked plesiosaurs are in fact more related to the so called pliosauroid short-necked plesiosaurs than to other long-necks. The smaller plesiosaurs had a diet of different kinds of squid and fish similar to many ichthyosaurs. Some are even thought to have been bottom-feeders, sieving sediment through their teeth. The larger more robust pliosaurs probably ate whatever they could get a hold of. Some had heads as long as 3 meters and a mouth filled with cucumber-sized teeth that could rip apart almost anything they sunk into. The large pliosaurs appeared in the Early Jurassic, but disappeared by the Middle Cretaceous, maybe outcompeted by the new arriving mosasaurs; another group of marine reptiles related to snakes and monitor-lizards. Unlike the ichthyosaurs which went extinct prior to the Cretaceous-Tertiary extinction, the plesiosaurs met their downfall along with the dinosaurs 65 million years ago.
