The fossil record is our only direct means for evaluating shifts in biodiversity through Earth's history. However, analyses of fossil marine invertebrates have demonstrated that geological megabiases profoundly influence fossil preservation and discovery, obscuring true diversity signals. Comparable studies of vertebrate palaeodiversity patterns remain in their infancy. A new species-level dataset of Mesozoic marine tetrapod occurrences was compared with a proxy for temporal variation in the volume and facies diversity of fossiliferous rock (number of marine fossiliferous formations: FMF). A strong correlation between taxic diversity and FMF is present during the Cretaceous. Weak or no correlation of Jurassic data suggests a qualitatively different sampling regime resulting from five apparent peaks in Triassic -Jurassic diversity. These correspond to a small number of European formations that have been the subject of intensive collecting, and represent 'Lagerstätten effects'. Consideration of sampling biases allows re-evaluation of proposed mass extinction events. Marine tetrapod diversity declined during the Carnian or Norian. However, the proposed end-Triassic extinction event cannot be recognized with confidence. Some evidence supports an extinction event near the Jurassic/Cretaceous boundary, but the proposed end-Cenomanian extinction is probably an artefact of poor sampling. Marine tetrapod diversity underwent a long-term decline prior to the Cretaceous -Palaeogene extinction.

The predatory pliosaurs were among the largest creatures ever to inhabit the oceans, some reaching gigantic proportions greater than 15 m in length. Fossils of this subclade of plesiosaurs are known from sediments all over the world, ranging in age from the Hettangian (approximately 198 Myr) to the Turonian (approximately 92 Myr). However, due to a lack of detailed studies and because only incomplete specimens are usually reported, pliosaur evolution remains poorly understood. In this paper, we describe the three dimensionally preserved skull of the giant Jurassic pliosaur Rhomaleosaurus cramptoni. The first phylogenetic analysis dedicated to in-group relationships of pliosaurs allows us to hypothesise a number of well-supported lineages that correlate with marine biogeography and the palaeoecology of these reptiles. Rhomaleosaurids comprised a short-lived and early diverging lineage within pliosaurs, whose open-water top-predator niche was filled by other pliosaur taxa by the mid-late Jurassic.

Pliosaurids were a long-lived and cosmopolitan group of marine predators that spanned 110 million years and occupied the upper tiers of marine ecosystems from the Middle Jurassic until the early Late Cretaceous. A well-preserved giant pliosaurid skull from the Late Jurassic Kimmeridge Clay Formation of Dorset, United Kingdom, represents a new species, Pliosaurus kevani. This specimen is described in detail, and the taxonomy and systematics of Late Jurassic pliosaurids is revised. We name two additional new species, Pliosaurus carpenteri and Pliosaurus westburyensis, based on previously described relatively complete, well-preserved remains. Most or all Late Jurassic pliosaurids represent a globally distributed monophyletic group (the genus Pliosaurus, excluding ‘Pliosaurus’ andrewsi). Despite its high species diversity, and geographically widespread, temporally extensive occurrence, Pliosaurus shows relatively less morphological and ecological variation than is seen in earlier, multi-genus pliosaurid assemblages such as that of the Middle Jurassic Oxford Clay Formation. It also shows less ecological variation than the pliosaurid-like Cretaceous clade Polycotylidae. Species of Pliosaurus had robust skulls, large body sizes (with skull lengths of 1.7–2.1 metres), and trihedral or subtrihedral teeth suggesting macropredaceous habits. Our data support a trend of decreasing length of the mandibular symphysis through Late Jurassic time, as previously suggested. This may be correlated with increasing adaptation to feeding on large prey. Maximum body size of pliosaurids increased from their first appearance in the Early Jurassic until the Early Cretaceous (skull lengths up to 2360 mm). However, some reduction occurred before their final extinction in the early Late Cretaceous (skull lengths up to 1750 mm).

The exquisitely preserved holotype of the pliosaur 'Rhomaleosaurus' victor (SMNS 12478) is described from the Toarcian Posidonien-Schiefer (Upper Lias, Lower Jurassic) of Holzmaden (Baden-Württemberg), Germany. The specimen presents a novel combination of synapomorphies and unique morphometric proportions separating it from Rhomaleosaurus sensu stricto and warranting the erection of a new genus, Meyerasaurus gen. nov. Historically, the name 'Thaumatosaurus' has been interchangeable with Rhomaleosaurus and is frequently associated with SMNS 12478 in the literature. However, this is an invalid taxon and cannot be reinstated. The anatomy of Meyerasaurus victor is compared in detail with other pliosaurs, and its taxonomic affinity is reviewed. M. victor belongs to the family Rhomaleosauridae and shares several anatomical characters with Rhomaleosaurus including a short and robust premaxillary rostrum (lengthto-width ratio c. 1.0), parallel premaxilla-maxilla sutures anterior to the nares, vomers contacting the maxillae posterior to the internal nares, and c. 28 cervical vertebrae minus the atlas-axis. The known geographical distribution of Rhomaleosaurus, which previously extended across the German and English palaeobiogeographical zones, is reduced to the English zone as a consequence of the referral of SMNS 12478 to a new genus. This is significant because it contributes to an ongoing trend of increasing generic separation between the German and English zones, while increasing the generic diversity within the German zone itself.