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奎斯特采石场的牛津黏土组（Oxford Clay Formation）中，产出了属于结节壳菊石（Sigiloceras enodatum）菊石亚带（中侏罗世卡洛夫阶早期）的部分骨骼标本，该标本代表了目前已知最基干的中侏罗世上龙科（Pliosauridae）类群之一。新属新种安德鲁玛尔泳龙（Marmornectes candrewi gen. et sp. nov.）具有7项自衍征，且拥有长吻型吻部。对上龙超科系统发育关系的重新评估显示，霍金斯海怪龙（Thalassiodracon hawkinsii）、豪夫泳龙属（Hauffiosaurus）各物种以及康尼贝亚滕伯龙（Attenborosaurus conybeari）依次为演化程度逐步升高的基干上龙科类群代表。这表明长吻型吻部曾在早侏罗世上龙类类群中演化出现（如豪夫泳龙属物种），但相较于同期躯体粗壮的大型顶级捕食者菱龙科（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。上龙科演化支仅在菱龙科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。