BackgroundIchthyosaurs are reptiles that inhabited the marine realm during most of the Mesozoic. Their Cretaceous representatives have traditionally been considered as the last survivors of a group declining since the Jurassic. Recently, however, an unexpected diversity has been described in Upper Jurassic–Lower Cretaceous deposits, but is widely spread across time and space, giving small clues on the adaptive potential and ecosystem control of the last ichthyosaurs. The famous but little studied English Gault Formation and ‘greensands’ deposits (the Upper Greensand Formation and the Cambridge Greensand Member of the Lower Chalk Formation) offer an unprecedented opportunity to investigate this topic, containing thousands of ichthyosaur remains spanning the Early–Late Cretaceous boundary.Methodology/Principal FindingsTo assess the diversity of the ichthyosaur assemblage from these sedimentary bodies, we recognized morphotypes within each type of bones. We grouped these morphotypes together, when possible, by using articulated specimens from the same formations and from new localities in the Vocontian Basin (France); a revised taxonomic scheme is proposed. We recognize the following taxa in the ‘greensands’: the platypterygiines ‘Platypterygius’ sp. and Sisteronia seeleyi gen. et sp. nov., indeterminate ophthalmosaurines and the rare incertae sedis Cetarthrosaurus walkeri. The taxonomic diversity of late Albian ichthyosaurs now matches that of older, well-known intervals such as the Toarcian or the Tithonian. Contrasting tooth shapes and wear patterns suggest that these ichthyosaurs colonized three distinct feeding guilds, despite the presence of numerous plesiosaur taxa.Conclusion/SignificanceWestern Europe was a diversity hot-spot for ichthyosaurs a few million years prior to their final extinction. By contrast, the low diversity in Australia and U.S.A. suggests strong geographical disparities in the diversity pattern of Albian–early Cenomanian ichthyosaurs. This provides a whole new context to investigate the extinction of these successful marine reptiles, at the end of the Cenomanian.

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A complete ichthyosaur rostrum, with 124 associated teeth, was recently discovered in LauxMontaux locality, department of Drôme, southeastern France. The associated belemnites and ammonites indicate a late Valanginian age (Neocomites peregrinus Zone, Olcostephanus nicklesi Subzone) for this fossil, which consequently represents the first diagnostic ichthyosaur ever reported from Valanginian strata. This specimen also represents the first occurrence of Aegirosaurus outside the Tithonian (Upper Jurassic) lithographic limestones of Bavaria (southern Germany). Tooth morphology and wear pattern suggest that Aegirosaurus belonged to the "Pierce II/ Generalist" feeding guild, which was hitherto not represented in post-Liassic ichthyosaurs. Most Late Jurassic ichthyosaurs actually crossed the Jurassic-Cretaceous boundary.

The latest Domerian to late Toarcian sedimentary series (from −190 Ma up to −180 Ma) from the « Réserve Géologique de Haute-Provence » (southeastern France) yields two kinds of remarkable fossiliferous beds. The greatest interest of the early Toarcian type is the occurrence of ichtyosaur remains (at least in six sites) among many other fossils such as ammonites, belemnites, bivalves, wood. The middle Toarcian type is specifically rich in ammonites and nautiluses. Litho- and biostratigraphical, palaeontological, sedimentological and geochemical analyses allow us to determine whether these fossiliferous beds are the results of mass mortalities, linked or not to biological crisis, or of exceptional fossilizations of organisms after normal mortality. The early Toarcian accumulations of fossils have been accurately dated from the middle part of the Serpentinum zone, Exaratum sub-zone pro-parte and Falciferum sub-zone pro-parte, strangewaysi and falciferum horizons. Thus, they were not connected to the so called Toarcian biological crisis which occurred previously (during the Semicelatum sub-zone). Organisms likely died according to a normal mortality rate during a time span that lasted around 700 to 800 k.y. All organisms appear to have been removed from their life environments and buried within siliciclastic sediments. Organisms and sediments were trapped owing to the creation of hemigrabens in an extensional tectonic regime, as evidenced by synsedimentary normal faults. Burying and good preservation, i. e. exceptional fossilization, were favoured by the hypoxic or anoxic conditions which prevailed at the sea floor, specifically during these times. High organic carbon content (up to 2.49 per cent) in the fossiliferous silty-quartzose marls proves hypoxia if not anoxia. All these facts were linked to a deepening and transgressive systems tract which succeeded a relative sea-level lowstand at the Pliensbachian-Toarcian boundary. The marine transgression probably reworked and shifted basinwards the siliciclastic sediments and wood remains which formed previously on the exposed land. Thus, continental remains were mixed with the marine fossils. Small scale hummocky cross stratifications in the terrigenous deposits show episodic occurrence of high hydrodynamics events. Such events, likely storms and associated currents, may have provoked the accumulations of dead marine organisms. The middle Toarcian accumulations of cephalopods are dated from the upper part of the Bifrons zone, Bifrons sub-zone pro-parte, lusitanicum and bifrons horizons. They are contained within 3 to 4 calcareous beds, 60 up to 85 centimetres thick in the whole. Since the corresponding deposition time span lasted around 850 k.y. up to 1 m.y., the sedimentation rate was very weak : about 1 centimetre or less each 1 k.y. Subsequently, these accumulations are regarded firstly as faunal condensations. The weak sedimentation rate is considered as linked to a major deepening and flooding event which led to sedimentary starvation in the involved Vocontian basin. Moreover, this phenomenon was probably favoured by a well known crisis of the carbonate production at that time. High values of manganese at the base of these condensed limestones are useful in correlating lithostratigraphic units of the same age in the entire basin. Previous studies gave rise to similar interpretations of these units, i. e. records of major flooding and wide connection to the oceanic domain. In addition, the taphonomic study indicates that many fossils were reworked due to episodes of high hydrodynamism, as indicated by erosion of internal molds of ammonites, and in addition to bioturbation. So, the accumulations may come also from faunal concentrations. Finally, this study shows that the inventory of the local and general conditions that govern fossilization must be done before interpreting all exceptional and widespread fossiliferous beds in terms of mass mortality or extinction.

The Vocontian Basin (SE France) was formed along the northwestern border of Tethys during Mesozoic times. Mainly known for its rich ammonite fauna, this basin has also yielded several Lower Jurassic ichthyosaurs. The specimens discussed here were discovered in lower Toarcian limestone and marl successions in the vicinity of Digne-les-Bains, High-Provence Alps. The bestpreserved specimen is identified as Suevoleviathan sp., a rare taxon previously reported only in southern Germany. Along with this specimen, premaxillae and paddle elements of Eurhinosaurus sp. and probable Stenopterygiidae centra were found in neighbouring localities. These specimens were preserved thanks to the deposition of soft anoxic marls or calcarodetritic sediments, coeval with other anoxic shales in Europe (the Toarcian Oceanic Anoxic Event or T-OAE), which allows faunal comparisons between these basins. The localities from the Vocontian Basin are closer to the Tethys than any other sites where identifiable Toarcian ichthyosaurs have been found in Europe. Nevertheless, the Vocontian ichthyosaur assemblage is strikingly similar to those in other basins across Europe. It suggests that Toarcian ichthyosaurs had a wide palaeobiogeographical distribution, reflecting their anatomical adaptations as highly mobile swimmers.