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Description

A selection of plesiosauroids known as elasmosaurids, to scale.

Popular perceptions of plesiosauroids (traditionally referred to as plesiosaurs, however the Plesiosauria include the pliosauroids as well) shows them floating at the surface with heads held aloft on long serpentine necks. Modern understanding is at odds with this view as these long-necked reptiles were unable to lift their head high above the water surface. The neck was relatively stiff and held straight in front with only sideway movements. Confusion around the long necks goes way back. In 1868 American palaeontologist Edward Cope (one half of the Bone Wars) infamously reconstructed Elasmosaurus with the skull on the short tail. Embarassingly for Cope it was his hated rival, Othniel Marsh who pointed out the mistake.

Overall plesiosauroids have the same general body plan. They are often referred to (in jest) as resembling 'a snake threaded into a turtle'. The body was barrel-shaped with four slender paddle-shaped flippers. Of course the iconic feature for most plesiosauroids was the elongated neck. It was the late-living elasmosaurids where the neck length reached it's maximum size. With 76 individual cervical vertebrae spread across 7 metres, Albertonectes had the longest neck of all plesiosauroids (longer than the rest of the body combined). Musculature stiffened the neck vertebrae which aided swimming, but reduced flexibility and turning circles, creating alot of drag in water. It is generally thought that the advantage of a long neck was to ambush prey well away from the body. Most plesiosauroids were specialised for hunting small fast-moving prey (fish and belemnites), but also targeted bottom dwelling invertebrates (bivalves, gastropods, crinoids and crustaceans). Whilst they are considered relatively slow swimmers, elasmosaurids were evidently long-distance travellers based on the swallowed gastroliths which originated far from the fossilised skeleton.
Although considered a conservative group, one subfamily deviates from the norm. The aristonectines have only been found in the Southern Hemisphere and had distinctively shorter necks. Some features are typical of sieve feeders with a shortened but wide skull and numerous reduced dentition. It isn't certain exactly what aristonectines were hunting as the teeth were not capable of consuming tiny prey. Large eyes indicate that vision was important for detecting prey, possibly at depths. Recently an unnamed giant species of Aristonectes was discovered in Antarctica. Based on the stocky morphology and at 11 metres long it was the largest known plesiosauroid.
Cretaceous seas were dangerous places and some species show evidence that plesiosaurs were not at the top of the chain. Only known from a single specimen Eromangasaurus had a crushed skull and toothmarks attributed to the giant Kronosaurus. Likewise the bones of Futabasaurus show signs of predation or possibly scavenging by sharks.

Elasmosaurids were some of the last plesiosaurs, surviving up to the K-Pg extinction.

Description

A selection of plesiosauroids known as elasmosaurids, to scale.

Popular perceptions of plesiosauroids (traditionally referred to as plesiosaurs, however the Plesiosauria include the pliosauroids as well) shows them floating at the surface with heads held aloft on long serpentine necks. Modern understanding is at odds with this view as these long-necked reptiles were unable to lift their head high above the water surface. The neck was relatively stiff and held straight in front with only sideway movements. Confusion around the long necks goes way back. In 1868 American palaeontologist Edward Cope (one half of the Bone Wars) infamously reconstructed Elasmosaurus with the skull on the short tail. Embarassingly for Cope it was his hated rival, Othniel Marsh who pointed out the mistake.

Overall plesiosauroids have the same general body plan. They are often referred to (in jest) as resembling 'a snake threaded into a turtle'. The body was barrel-shaped with four slender paddle-shaped flippers. Of course the iconic feature for most plesiosauroids was the elongated neck. It was the late-living elasmosaurids where the neck length reached it's maximum size. With 76 individual cervical vertebrae spread across 7 metres, Albertonectes had the longest neck of all plesiosauroids (longer than the rest of the body combined). Musculature stiffened the neck vertebrae which aided swimming, but reduced flexibility and turning circles, creating alot of drag in water. It is generally thought that the advantage of a long neck was to ambush prey well away from the body. Most plesiosauroids were specialised for hunting small fast-moving prey (fish and belemnites), but also targeted bottom dwelling invertebrates (bivalves, gastropods, crinoids and crustaceans). Whilst they are considered relatively slow swimmers, elasmosaurids were evidently long-distance travellers based on the swallowed gastroliths which originated far from the fossilised skeleton.
Although considered a conservative group, one subfamily deviates from the norm. The aristonectines have only been found in the Southern Hemisphere and had distinctively shorter necks. Some features are typical of sieve feeders with a shortened but wide skull and numerous reduced dentition. It isn't certain exactly what aristonectines were hunting as the teeth were not capable of consuming tiny prey. Large eyes indicate that vision was important for detecting prey, possibly at depths. Recently an unnamed giant species of Aristonectes was discovered in Antarctica. Based on the stocky morphology and at 11 metres long it was the largest known plesiosauroid.
Cretaceous seas were dangerous places and some species show evidence that plesiosaurs were not at the top of the chain. Only known from a single specimen Eromangasaurus had a crushed skull and toothmarks attributed to the giant Kronosaurus. Likewise the bones of Futabasaurus show signs of predation or possibly scavenging by sharks.

Elasmosaurids were some of the last plesiosaurs, surviving up to the K-Pg extinction.