Even though a handful of long-lived reptilian clades dominated Mesozoic marine ecosystems, several biotic turnovers drastically changed the taxonomic composition of these communities. A seemingly slow paced, within-geological period turnover took place across the Early–Middle Jurassic transition. This turnover saw the demise of early neoichthyosaurians, rhomaleosaurid plesiosaurians and early plesiosauroids in favour of ophthalmosaurid ichthyosaurians and cryptoclidid and pliosaurid plesiosaurians, clades that will dominate the Late Jurassic and, for two of them, the entire Early Cretaceous as well. The fossil record of this turnover is however extremely poor and this change of dominance appears to be spread across the entire middle Toarcian–Bathonian interval. We describe a series of ichthyosaurian and plesiosaurian specimens from successive geological formations in Luxembourg and Belgium that detail the evolution of marine reptile assemblages across the Early–Middle Jurassic transition within a single area, the Belgo–Luxembourgian sub-basin. These fossils reveal the continuing dominance of large rhomaleosaurid plesiosaurians, microcleidid plesiosaurians and Temnodontosaurus-like ichthyosaurians up to the latest Toarcian, indicating that the structuration of the upper tier of Western Europe marine ecosystems remained essentially constant up to the very end of the Early Jurassic. These fossils also suddenly record ophthalmosaurid ichthyosaurians and cryptoclidid plesiosaurians by the early Bajocian. These results from a geographically-restricted area provide a clearer picture of the shape of the marine reptile turnover occurring at the early–Middle Jurassic transition. This event appears restricted to the sole Aalenian stage, reducing the uncertainty of its duration, at least for ichthyosaurians and plesiosaurians, to 4 instead of 14 million years.

中文翻译：

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完善侏罗纪早期到中期的海洋爬行动物的转换

尽管少数长寿的爬虫类进化支配着中生代海洋生态系统，但一些生物更新却极大地改变了这些群落的分类学组成。在侏罗纪早期到中期的过渡期间，发生了一个缓慢的，地质时期内的周转。这种转变使早期新鱼龙科，菱角龙蛇颈龙和早期蛇龙类动物的灭绝，转而支持眼龙龙鱼龙和隐斜角蛇颈龙和胸龙，这些进化支将在侏罗纪晚期以及其中的两个早白垩世时期占主导地位。然而，这种转变的化石记录非常差，这种主导地位的变化似乎遍及整个Toarcian-Bathonian间隔。我们描述了一系列来自卢森堡和比利时陆续地质构造的鱼龙类和胸骨龙标本，详细描述了单个区域（贝尔戈-卢森堡次级盆地）中早侏罗世过渡期海洋爬行动物组合的演化。这些化石揭示了直到最近的Toarcian为止，大型假龙类胸骨龙，微梭类胸骨龙和Temnodontosaurus样鱼龙类一直在占主导地位，这表明直到侏罗纪早期，西欧上层海洋生态系统的结构基本上保持恒定。这些化石还突然记录了早期巴约西亚人的眼龙科鱼龙科和隐性蛇颈龙科。这些来自地理限制区域的结果更清晰地描绘了侏罗纪中期至中期过渡期发生的海洋爬行动物更新的形状。该事件似乎仅限于单一的亚伦时期，将其持续时间的不确定性（至少对于鱼龙类和胸骨类患者而言）降低至4年而不是1400万年。