High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary

Invasion of the open ocean by tetrapods represents a major evolutionary transition that occurred independently in cetaceans, mosasauroids, chelonioids (sea turtles), ichthyosaurs and plesiosaurs. Plesiosaurian reptiles invaded pelagic ocean environments immediately following the Late Triassic extinctions. This diversification is recorded by three intensively-sampled European fossil faunas, spanning 20 million years (Ma). These provide an unparalleled opportunity to document changes in key macroevolutionary parameters associated with secondary adaptation to pelagic life in tetrapods. A comprehensive assessment focuses on the oldest fauna, from the Blue Lias Formation of Street, and nearby localities, in Somerset, UK (Earliest Jurassic: 200 Ma), identifying three new species representing two small-bodied rhomaleosaurids (Stratesaurus taylori gen et sp. nov.; Avalonnectes arturi gen. et sp. nov) and the most basal plesiosauroid, Eoplesiosaurus antiquior gen. et sp. nov. The initial radiation of plesiosaurs was characterised by high, but short-lived, diversity of an archaic clade, Rhomaleosauridae. Representatives of this initial radiation were replaced by derived, neoplesiosaurian plesiosaurs at small-medium body sizes during a more gradual accumulation of morphological disparity. This gradualistic modality suggests that adaptive radiations within tetrapod subclades are not always characterised by the initially high levels of disparity observed in the Paleozoic origins of major metazoan body plans, or in the origin of tetrapods. High rhomaleosaurid diversity immediately following the Triassic-Jurassic boundary supports the gradual model of Late Triassic extinctions, mostly predating the boundary itself. Increase in both maximum and minimum body length early in plesiosaurian history suggests a driven evolutionary trend. However, Maximum-likelihood models suggest only passive expansion into higher body size categories.

四足类（tetrapods）入侵开阔大洋是一次重大的演化转变事件，该事件在鲸类、沧龙类（mosasauroids）、海龟总科（chelonioids，即海龟）、鱼龙与蛇颈龙类（plesiosaurs）中独立发生。蛇颈龙类爬行动物在三叠纪末生物大灭绝后迅速入侵远洋海洋环境。这一演化辐射事件被三个采样极为详尽的欧洲化石群所记录，时间跨度达2000万年（Ma）。这些化石群为记录与四足类次生适应远洋生活相关的关键宏观演化参数变化提供了无与伦比的契机。

本次综合研究聚焦于最古老的化石群——产自英国萨默塞特郡斯特里特及其周边地区的蓝里亚斯组（Blue Lias Formation）地层（时代为早侏罗世早期，约200 Ma），鉴定出3个新属新种：2种小型菱龙科（rhomaleosaurids）类群——斯特里特龙·泰勒氏（Stratesaurus taylori gen. et sp. nov.，新属新种）与阿瓦隆龙·阿瑟氏（Avalonnectes arturi gen. et sp. nov.，新属新种），以及最基干的蛇颈龙亚目类群（plesiosauroid）古老始蛇颈龙（Eoplesiosaurus antiquior gen. et sp. nov.，新属新种）。

蛇颈龙类的初始辐射演化以古老演化支菱龙科的高但短暂的多样性为特征。在形态差异度更平缓积累的过程中，这类初始辐射的代表类群被中小型体型的衍生新蛇颈龙类（neoplesiosaurian）所取代。这种渐变式的演化模式表明，四足类亚支内的适应性辐射并非总是以大型后生动物躯体构型在古生代起源时，或是四足类起源时所观察到的初始高水平形态差异度为特征。

三叠纪-侏罗纪界线后迅速出现的高菱龙科多样性，支持了三叠纪末大灭绝的渐变模型——该灭绝事件大多发生于三叠纪-侏罗纪界线之前。蛇颈龙类演化早期的最大与最小体长均呈现增长趋势，这暗示存在受驱动的演化趋势。不过，最大似然模型（Maximum-likelihood models）仅支持体型向更大尺寸区间发生被动扩张。