Abstract
BackgroundIchthyosauria is a diverse clade of marine amniotes that spanned most of the Mesozoic. Until recently, most authors interpreted the fossil record as showing that three major extinction events affected this group during its history: one during the latest Triassic, one at the Jurassic–Cretaceous boundary (JCB), and one (resulting in total extinction) at the Cenomanian-Turonian boundary. The JCB was believed to eradicate most of the peculiar morphotypes found in the Late Jurassic, in favor of apparently less specialized forms in the Cretaceous. However, the record of ichthyosaurs from the Berriasian–Barremian interval is extremely limited, and the effects of the end-Jurassic extinction event on ichthyosaurs remains poorly understood.Methodology/Principal FindingsBased on new material from the Hauterivian of England and Germany and on abundant material from the Cambridge Greensand Formation, we name a new ophthalmosaurid, Acamptonectes densus gen. et sp. nov. This taxon shares numerous features with Ophthalmosaurus, a genus now restricted to the Callovian–Berriasian interval. Our phylogenetic analysis indicates that Ophthalmosauridae diverged early in its history into two markedly distinct clades, Ophthalmosaurinae and Platypterygiinae, both of which cross the JCB and persist to the late Albian at least. To evaluate the effect of the JCB extinction event on ichthyosaurs, we calculated cladogenesis, extinction, and survival rates for each stage of the Oxfordian–Barremian interval, under different scenarios. The extinction rate during the JCB never surpasses the background extinction rate for the Oxfordian–Barremian interval and the JCB records one of the highest survival rates of the interval.Conclusions/SignificanceThere is currently no evidence that ichthyosaurs were affected by the JCB extinction event, in contrast to many other marine groups. Ophthalmosaurid ichthyosaurs remained diverse from their rapid radiation in the Middle Jurassic to their total extinction at the beginning of the Late Cretaceous.
Highlights
The thunnosaurian ichthyosaur Ophthalmosaurus Seeley 1874 [1] is known from abundant material, most of it from the Oxford and Kimmeridge Clay formations of England and the Sundance Formation of the USA [2]
The presence of Ophthalmosaurus in Lower Cretaceous sediments has been claimed twice in the modern literature: McGowan [5] figured and discussed a humerus with three large distal facets from the Lower Cretaceous of Prince Patrick Island (Canada) that he referred to Ophthalmosaurus sp. and McGowan & Motani [2] mentioned the presence of isolated basioccipitals and humeri referable to Ophthalmosaurus in the early Cenomanian Cambridge Greensand Formation
Until recently [11], all Middle and Late Jurassic ichthyosaurs were thought to have become extinct at the end of the Jurassic, at the Jurassic–Cretaceous boundary [12,13,14,15,16], or during a more protracted extinction event that started during the Middle Jurassic [17]
Summary
The thunnosaurian ichthyosaur Ophthalmosaurus Seeley 1874 [1] (family Ophthalmosauridae) is known from abundant material, most of it from the Oxford and Kimmeridge Clay formations of England and the Sundance Formation of the USA [2]. McGowan & Motani [2] mentioned the presence of isolated basioccipitals and humeri referable to Ophthalmosaurus in the early Cenomanian Cambridge Greensand Formation (which includes a reworked late Albian fauna from the top of the Gault formation [6,7]). These claims are, based on isolated material, and other ophthalmosaurid ichthyosaurs with three large distal humeral facets have been described from Cretaceous sediments since including Caypullisaurus [8,9] and Maiaspondylus [10]. The record of ichthyosaurs from the Berriasian–Barremian interval is extremely limited, and the effects of the end-Jurassic extinction event on ichthyosaurs remains poorly understood
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