A new leptocleidid (Sauropterygia, Plesiosauria) from the Vectis Formation (Early Barremian-early Aptian; Early Cretaceous) of the Isle of Wight and the evolution of Leptocleididae, a controversial clade

摘要

Among the most enigmatic and controversial plesiosaurian clades is the Early Cretaceous Leptocleididae, a small group of (mostly) short-necked taxa with 'intermediate' or 'pliosauromorph' body proportions. Leptocleidids have often been interpreted as basal members of Pliosauroidea, and their presence in marginal marine and even freshwater facies has led to suggestions that they might represent late-surviving relicts, perhaps related to the Lower Jurassic rhomaleosaurids. We describe a new leptocleidid, Vectocleidus pastorum gen. et sp. nov., from the late Barremian part of the Cowleaze Chine Member (Vectis Formation), Isle of Wight, UK, and undertake a detailed reassessment of leptocleidid anatomy and relationships. New data on the long-necked Brancasaurus gives extra support to a monophyletic Leptocleididae with taxa of 'intermediate' body plan and robust skulls, Leptocleidus superstes, Leptocleidus capensis, Nichollssaura and Vectocleidus. Thus, leptocleidids adopted a range of body plans on the pliosauromorph-plesiosauromorph spectrum. Support for a placement of Leptocleididae within Pliosauroidea is weak, and most proposed synapomorphies fail the test of similarity. However, numerous synapomorphies, including many new observations, support a derived position within Plesiosauroidea. Thus, the 'intermediate' body plan of many leptocleidids is not plesiomorphic, and plesiosaurian body plan evolution was complex and highly plastic. We also summarize the anatomy of'Cimoliasaurus' valdensis, a short-necked Early Cretaceous taxon. 'C.' valdensis is a valid taxon for which we erect the new monotypic genus Hastanectes. Hastanectes shows evidence of possible pliosaurid affinities. Thus, multiple plesiosaurian clades may have invaded margin and freshwater environments.