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Description

A selection of plesiosaurians from the family Rhomaleosauridae, to scale.

As mentioned in another post: www.deviantart.com/artbyjrc/ar… plesiosaurians are generally divided into two main groups based on neck length: the plesiosaurs and pliosaurs. But nature has a habit of not working to human ideas of neat taxonomy! Instead both groups have a mish-mash of long and short necks within each clade. Early pliosaurs ie. the short-necked ones, had relatively long-necks compared to the later giants. The sister group to the pliosaurs were the also relatively long-necked rhomaelosaurs. They were the first of the pliosaurids with an early species, Atychodracon being found in Late Triassic and earliest Jurassic deposits. Due to a confusion of identity, the species name of macrocephalus refers to the relatively large head compared to the early plesiosaurs. This confusion saw a number of other species labelled as rhomaleosaurs, however a re-examination of morphological evidence has now relocated former members to other families: Simolestes is actually a small pliosaur while Leptocleidus is in a group of short-necked plesiosaurs! Most rhomaleosaurs are known from the Early Jurassic in Europe. At 7 metres long, Rhomaleosaurus was the largest species and one of the largest marine reptiles of the Early Jurassic. Sensory pits in the skull have suggested an ability to pick up scents underwater in a similar manner to sharks. Possibly due to increased competition in the seas from plesiosaurs and pliosaurs, rhomaleosaurs are known from only two species in the Middle Jurassic, one from Argentina and the other from Canada. During the Mesozoic, Arctic Canada was located at high latitudes ie. towards the pole. Few plesiosaurians are known from this region, however good remains of Borealonectes have shown that the waters were inhabited during the mid-Jurassic. As to how cold the water was, we don't know, but other polar plesiosaurians are known from southern polar seas in the Cretaceous.

Description

A selection of plesiosaurians from the family Rhomaleosauridae, to scale.

As mentioned in another post: www.deviantart.com/artbyjrc/ar… plesiosaurians are generally divided into two main groups based on neck length: the plesiosaurs and pliosaurs. But nature has a habit of not working to human ideas of neat taxonomy! Instead both groups have a mish-mash of long and short necks within each clade. Early pliosaurs ie. the short-necked ones, had relatively long-necks compared to the later giants. The sister group to the pliosaurs were the also relatively long-necked rhomaelosaurs. They were the first of the pliosaurids with an early species, Atychodracon being found in Late Triassic and earliest Jurassic deposits. Due to a confusion of identity, the species name of macrocephalus refers to the relatively large head compared to the early plesiosaurs. This confusion saw a number of other species labelled as rhomaleosaurs, however a re-examination of morphological evidence has now relocated former members to other families: Simolestes is actually a small pliosaur while Leptocleidus is in a group of short-necked plesiosaurs! Most rhomaleosaurs are known from the Early Jurassic in Europe. At 7 metres long, Rhomaleosaurus was the largest species and one of the largest marine reptiles of the Early Jurassic. Sensory pits in the skull have suggested an ability to pick up scents underwater in a similar manner to sharks. Possibly due to increased competition in the seas from plesiosaurs and pliosaurs, rhomaleosaurs are known from only two species in the Middle Jurassic, one from Argentina and the other from Canada. During the Mesozoic, Arctic Canada was located at high latitudes ie. towards the pole. Few plesiosaurians are known from this region, however good remains of Borealonectes have shown that the waters were inhabited during the mid-Jurassic. As to how cold the water was, we don't know, but other polar plesiosaurians are known from southern polar seas in the Cretaceous.