Mounted skeleton of the advanced elasmosaur Thalassomedon hanningtoni, in the American Museum of Natural History; late Cretaceous (Cenomanian) of North America (western Laurasia), length about 12 meters.
The Elasmosauridae were the largest most advanced of the long-necked plesiosaurs. The head is tiny in comparison to the rest of the body. The neck is extraordinarily long, with 32 to 71 cervical vertebrae, the number increasing in later species. The longer the neck the more flexible it is, thus giving a greater adaptive advantage in catching fish. It is known that these creatures swallowed stones for ballast. It is possible that they would rest on the bottom of shallow seas and wait for prey to come within reach.
Like the rest of the Plesiosauroidea, elasmosaurs had a barrel-shaped body with four paddle-shaped flippers. The two front flippers were always somewhat bigger than the hind flippers. There are up to 17 phalanges (finger bones) in each digit. The flippers are smaller in relation to the body then in other Plesiosaurus, indicating that these creatures were not fast swimmers.
Elasmosaurs have been found in North America, Europe, Australia and New Zealand. The group is diverse, and there is disagreement among different references over which genera to include in the family, but the current practice has both primitive Jurassic forms and advanced giant forms in the same family. In any case there are a number of distinct evolutionary trends in this group.
The trends in the evolution of the elasmosaur skull, in particular, have been taken up cladistically by Sachs (2005). He finds that the posterior maxilla and retroarticular process expand. Presumably the overall picture is that the elasmosaurs were trading length of tooth row for increased mass and mechanical advantage of jaw muscle. The axis centrum becomes longer and lower. It is unclear what this means, but may signify relatively restricted motion of the head with respect to the neck. This is consistent with increased size and length of neck, as it would be unwise to have one's head flapping around loose at the end of a powerful, flexible, and fast-moving neck.
The ensemble of these features suggests specialization for small, fast-moving prey animals, which probably lived in groups. The elasmosaur would move slowly into the vicinity of a school. Then a rapid movement of its neck would bring the quick, but relatively small and weak, jaws close enough to trap a prey animal. We might speculate that even the slow speed of an elasmosaur, relative to its size, could easily keep it in contact with a shoal of small fish or squid, which, however quick in the sprint, cannot move long distances continuously.
Late Cretaceous species attained over 12 metres in length and possibly several tonnes in weight. Elasmosaurids were highly successful creatures, despite their ungainly appearance, and they persisted until the very end of the Mesozoic.
Hydrotherosaurus - a giant elasmosaur (length over 12 meters) from the Late Cretaceous.
Longest, largest and last plesiosaurs; overall length from about 3 m to 14 m; occipital condyle formed from the basioccipital only, usually ringed by constricting groove; maximum five pairs of premaxillary teeth; upper tooth row of derived forms with enlarged premaxillary & anterior maxillary teeth separated by smaller teeth around maxillo-premaxillary suture; dentary teeth primitively 24 pairs, reducing to a minimum of 14 pairs in Cretaceous species; teeth ornamented with numerous longitudinal ridges; primitively ~32 cervical vertebrae, reaching a maximum of over 70; except for some primitive forms, cervical centra relatively elongated; cervical ribs primitively double-headed) are single-headed in Upper Jurassic and later species; ventral rami of the scapulae relatively broad and meeting in the ventral midline in 'adults'; Cretaceous species have strongly developed scapulae, permitting a strong backstroke and greater maneuverability; epipodials primitively longer than broad, becoming broader than long in later forms; 5th metapodial shifting proximally from the metapodial row in basal forms to the distal mesopodial row (see figure at metapodial); hyperphalangy of up to 17 phalanges in the longest digit.
The record for cervical vertebrae -- for any vertebrate -- appears to be held by the elasmosaurid Elasmosaurus platyurus at 76 cervicals.
Because of the drag caused by the long neck, as well as the relatively small paddles and weak limb girdles, it is supposed that elasmosaurids were very slow swimmers. Possible lifestyles include browsing on slowly moving schools of cephalopods (belemnites & squid), and ambush predation using quick movements of the neck only. Note that an elasmosaur might not need to surface to breath if it chose its spot carefully and simply raised its head above the water. However, the head is so small, that one suspects that most gas exchange was handled over the surface of the paddles, through the cloaca or in some similar fashion.
"In Cretaceous elasmosaurs the right and left coracoids meet along the animal's midline then separate toward the back to form a scooped-out, roughly heart-shaped opening--Jurassic elasmosaurids and other types of plesiosaurs lack this rounded separation in the front limb girdle."
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