In the Mesozoic era the reptiles established an early mastery of the sea. Among them were the ichthyosaurs, which were the most well-adapted sea reptiles of all, with their fish-shaped bodies and tails. These no longer exist, having become extinct and been replaced by the lizard-like mosasaurs during the Late Cretaceous period (mosasaurs still exist). The turtles, slow-moving shelled herbivores, were also successful adopting a marine existence (in general, cryptodires and side-necked turtles still live while Baenids died out in the Paleogene). The other main line of marine reptiles was the plesiosaurs, dating back to the Late Triassic. These quickly evolved into two main lines: the short-necked types, or pliosaurs; and the long-necked types, or plesiosauroids. Both types still exist in the modern oceans. The largest marine reptile alive today is the whulk, a pliosaur. At 20 meters (67 feet) long, the whulk cruises the oceans of the world, but where its ancestors fed on ammonites, squid and other cephalopods, this pliosaur consumes much smaller organisms. During the Cretaceous period the shallow seas over the continental shelves produced vast volumes of plankton, tiny animals and plants that drifted in the warm nutritious waters. The shallow seas are not so extensive nowadays, but the plankton is still there. The whulk feeds on it by swallowing great volumes of water and straining out the plankton through its thousands of tiny teeth.