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.

英国贝德福德郡斯图尔特比附近Quest采石场的牛津黏土组（Oxford Clay Formation）中，产自Sigiloceras enodatum菊石亚带（中侏罗世早卡洛夫期）的部分骨骼化石，代表了目前已知最基干的中侏罗世上龙科（Pliosauridae）类群之一。新属新种Marmornectes candrewi gen. et sp. nov.共具备7项自近裔性状（autapomorphy），且拥有长吻型头骨。对上龙超科（Pliosauroidea）系统发育关系的重新评估表明，Thalassiodracon hawkinsii、Hauffiosaurus属各类群以及Attenborosaurus conybeari依次为上龙科（Pliosauridae）中衍化程度更高的基干代表类群。这一结果显示，长吻型头骨在早侏罗世上龙类（如Hauffiosaurus）中便已演化形成；但相较于同时期的菱龙科（Rhomaleosauridae）类群，这些早期类群体型更小、颈部更长，头骨与肢骨更为纤细，而菱龙科类群均为体型粗壮的大型宏食性掠食者。上龙科演化支仅在菱龙科（Rhomaleosauridae）灭绝后的中侏罗世晚期，才演化出其标志性的大体型、粗壮头骨与短颈部特征。