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Geosaurus in the Lower Kimmeridge Clay Formation (Late Jurassic) of England.

From: Ben Creisler

A new online paper:

Mark T. Young, Lorna Steel & Heather Middleton (2013)
Evidence of the metriorhynchid crocodylomorph genus Geosaurus in the
Lower Kimmeridge Clay Formation (Late Jurassic) of England.
Historical Biology (advance online publication)

We report the oldest known specimen of the metriorhynchid
crocodylomorph genus Geosaurus, and the first specimen discovered from
the Lower Kimmeridge Clay Formation of England (Kimmeridgian stage,
Rasenia cymodoce Sub-Boreal ammonite Zone). This specimen, an isolated
tooth, has the tri-facetted labial surface morphology that is unique
to Late Jurassic species of Geosaurus. Its dimensions suggest that it
is one of the posterior teeth. Macrowear on the labial surface (enamel
spalling) shows evidence of tooth–food abrasion. The presence of
Geosaurus near the base of the lower Kimmeridgian, along with the
recently re-described middle Oxfordian specimen attributable to
Torvoneustes, shows that the subclade Geosaurini evolved and
diversified prior to the Kimmeridgian. Members Geosaurini evolved a
diverse array of sophisticated feeding mechanisms, including
durophagous forms (Torvoneustes), barracuda-like forms with
‘scissor-like’ occlusion (Geosaurus), species reminiscent of false
killer whales/Type 1 North Atlantic killer whales (Dakosaurus) and a
large genus similar to extant Type 2 North Atlantic killer whales
(Plesiosuchus). With the description of the isolated Geosaurus tooth,
all four Geosaurini genera are now known from the Lower Kimmeridgian
Clay Formation. The craniodental plasticity of Geosaurini, and the
astonishing range of feeding mechanisms they evolved, is unparalleled