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的奎斯特采石场（Quest Pit）牛津黏土组（Oxford Clay Formation）中，产自雕菊石（Sigiloceras enodatum）带（中侏罗世（Middle Jurassic）卡洛维阶（Callovian）早期）的部分骨架，代表了迄今所知最基干的中侏罗世上龙科（Pliosauridae）类群之一。新属新种安德鲁长喙龙（Marmornectes candrewi gen. et sp. nov.）具有7项独有衍征，且拥有长吻型头骨。对上龙形类系统发育关系的重新评估显示，霍金斯海神龙（Thalassiodracon hawkinsii）、豪夫龙属各种（Hauffiosaurus spp.）以及康尼比尔阿滕伯勒龙（Attenborosaurus conybeari）是依次更具衍征的基干上龙科代表类群。这表明长吻型头骨早在早侏罗世（Early Jurassic）的上龙科类群（如豪夫龙属）中就已出现，但这些早期类群的体型较小、颈部较长，且头骨与四肢骨骼相较于同时代的菱龙科（Rhomaleosauridae）更为纤细——后者为体型粗壮的大型掠食类群。上龙科的演化支仅在中侏罗世晚期菱龙科灭绝之后，才演化出其标志性的大体型、粗壮头骨与短颈部特征。