Data from: A new pliosaurid (Sauropterygia, Plesiosauria) from the Oxford Clay Formation (Middle Jurassic, Callovian) of England: evidence for a gracile, longirostrine grade of Early–Middle Jurassic pliosaurids

A partial skeleton from the Sigiloceras enodatum ammonite Subzone (early
Callovian, Middle Jurassic) of the Oxford Clay Formation of Quest Pit,
near Stewartby, Bedfordshire, UK respresents one of the basalmost Middle
Jurassic pliosaurids. Marmornectes candrewi gen. et sp. nov. possesses
seven autapomorphies and a longirostrine snout. Reassessment of
pliosauroid relationships demonstrates that Thalassiodracon hawkinsii,
Hauffiosaurus spp. and Attenborosaurus conybeari are successively more
derived basal representatives of Pliosauridae. This indicates that a
longirostrine snout was acquired among Early Jurassic pliosaurids (such as
Hauffiosaurus), but that these early taxa had a small body size, long
neck, and gracile skull and limb bones relative to the contemporaneous
rhomaleosaurids, which were robust, large-bodied macropredators. The
pliosaurid lineage only acquired its ‘characteristic’ large size, robust
skull and short neck in the late Middle Jurassic after the extinction of
Rhomaleosauridae.