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Colymbosaurus anatomy and high diversity among Late Jurassic plesiosauroids

Ben Creisler

In the new JVP:

Roger B. J. Benson & Timothy Bowdler (2014)
Anatomy of Colymbosaurus megadeirus (Reptilia, Plesiosauria) from the
Kimmeridge Clay Formation of the U.K., and high diversity among Late
Jurassic plesiosauroids.
Journal of Vertebrate Paleontology 34(5):  1053-1071

We describe the syntype postcranial skeletons of the cryptoclidid
plesiosauroid Colymbosaurus megadeirus. C. megadeirus is the type
species of Colymbosaurus, and one other valid species, C.
svalbardensis, is known. Although it is in widespread usage, C.
trochanterius is a not valid species, although the presence of an
anteroposteriorly oriented ridge bisecting the distal surfaces of the
holotype humerus allows its referral to Colymbosaurus sp. Some
specimens previously referred to C. trochanterius, including the
holotype of ‘Plesiosaurus’ manselii, lack this feature, or show other
differences from our hypodigm, suggesting that they are taxonomically
distinct from Colymbosaurus, and countering previous suggestions that
the Kimmeridge Clay Formation represents a depauperate plesiosauroid
fauna. Substantial differences between the atlas-axis complexes and
cervical vertebrae of Kimmerosaurus langhami, ‘Plesiosaurus’ manselii,
C. megadeirus, and specimens referred here to cf. Spitrasaurus
indicate that at least four plesiosauroid taxa were present in the
Kimmeridge Clay Formation. This proposition is also supported by
coracoid morphotypes, and demonstrates that Late Jurassic
plesiosauroids were taxonomically diverse. Most Late Jurassic–Early
Cretaceous cryptoclidids belong to a monophyletic Colymbosaurinae, but
Kimmerosaurus and its sister taxon Tatenectes originated independently
from Middle Jurassic cryptoclidids.