Dakosaurus and Plesiosuchus are characteristic genera of aquatic, large-bodied, macrophagous metriorhynchid crocodylomorphs. Recent studies show that these genera were apex predators in marine ecosystems during the latter part of the Late Jurassic, with robust skulls and strong bite forces optimized for feeding on large prey. Here we present comprehensive osteological descriptions and systematic revisions of the type species of both genera, and in doing so we resurrect the genus Plesiosuchus for the species Dakosaurus manselii. Both species are diagnosed with numerous autapomorphies. Dakosaurus maximus has premaxillary ‘lateral plates’; strongly ornamented maxillae; macroziphodont dentition; tightly fitting tooth-to-tooth occlusion; and extensive macrowear on the mesial and distal margins. Plesiosuchus manselii is distinct in having: non-amblygnathous rostrum; long mandibular symphysis; microziphodont teeth; tooth-crown apices that lack spalled surfaces or breaks; and no evidence for occlusal wear facets. Our phylogenetic analysis finds Dakosaurus maximus to be the sister taxon of the South American Dakosaurus andiniensis, and Plesiosuchus manselii in a polytomy at the base of Geosaurini (the subclade of macrophagous metriorhynchids that includes Dakosaurus, Geosaurus and Torvoneustes). The sympatry of Dakosaurus and Plesiosuchus is curiously similar to North Atlantic killer whales, which have one larger ‘type’ that lacks tooth-crown breakage being sympatric with a smaller ‘type’ that has extensive crown breakage. Assuming this morphofunctional complex is indicative of diet, then Plesiosuchus would be a specialist feeding on other marine reptiles while Dakosaurus would be a generalist and possible suction-feeder. This hypothesis is supported by Plesiosuchus manselii having a very large optimum gape (gape at which multiple teeth come into contact with a prey-item), while Dakosaurus maximus possesses craniomandibular characteristics observed in extant suction-feeding odontocetes: shortened tooth-row, amblygnathous rostrum and a very short mandibular symphysis. We hypothesise that trophic specialisation enabled these two large-bodied species to coexist in the same ecosystem.

Aristonectes parvidens Cabrera, 1941, and Morturneria seymourensis (Chatterjee and Small, 1989), two plesiosaurs from the Maastrichtian of South America and Antarctica whose phylogenetical position is controversial, are reviewed and found to be congeneric and conspecific. Most differences between the two type specimens are interpreted as related to ontogenetic growth: Morturneria is based on an immature, whereas Aristonectes is based on an adult, probably an old individual. Aristonectes exhibits an unique set of characters among Plesiosauria: a low and wide ogival skull, a paired vomeronasal fenestra, a mandible high anteriorly with a very short and high symphysis, a homodont dentition composed of numerous, strongly outwardly directed and poorly ornamented teeth (dental formula: 10–13 premaxillary, at least 51 maxillary and probably 60–65 dentary teeth, depending upon individual ontogeny). Moreover, Aristonectes shares several synapomorphies with the elasmosaurid clade, mainly strongly binocular-...

Lomasuchus palpebrosus no v. gen. et sp., from the Upper Cretaceous of northwestern Patagonia (Argentina), is described. Lomasuchus is a “mesosuchian” crocodile with a relatively narrow snout and serrated teeth, two large supraorbital bones fused over the orbit, and a deeply downturned lateral margin of the squamosal. The skull of Lomasuchus displays some apomorphic conditions as a wedge-like anterior process of the maxilla and a very short premaxillary contribution in the palate, which are only shared by Peirosaurus. The cladistic analysis performed shows the family Peirosauridae (Peirosaurus and Lomasuchus) as a natural group. A synopsis of all described species of Cretaceous crocodiles of South America is presented. Peirosaurids, sebecosuchians, Uruguaysuchus, and the eusuchian dolichochampsids are endemic of South America. Both notosuchids and Araripesuchus occur in Africa and South America and represent examples of vicariance. The monophyly of trematochampsids is weakly supported and their vicariant distribution is questionable. The Dyrosauridae and Pholidosauridae have a wider distribution, being present in South America and Africa and on the northern continents. Unlike in the northern continents, eusuchians played a minor role in the Late Cretaceous crocodilian assemblages of South America, where “mesosuchians” were the principal components. This remarkable difference is another indication of the great influence that the geographic isolation of South America had on its terrestrial faunas during the Cretaceous.

The ophthalmosaurid Caypullisaurus from the Late Jurassic–Early Cretaceous of the Neuquén Basin, Argentina, is redescribed based on new information from recent discoveries. Cladistic analysis of Caypullisaurus, based on previous ichthyopterygian data sets and using NONA, yielded two most parsimonious trees. Two clades are recognized within the Ophthalmosauridae. Caypullisaurus is found to be nested with Brachypterygius and Platypterygius. Simultaneous, unconstrained analysis, using unambiguous character optimization, is suggested as the best way to analyze data sets with large amounts of missing data.

A large ichthyosaur from the Early Tithonian (Late Jurassic) of the Neuquén province, Argentina, represents a new genus and species, Caypullisaurus bonapartei, within the family Ichthyosauridae. The holotype is the most complete skeleton of a mature ichthyosaur known to date from the Jurassic of South America. In comparison with other Late Jurassic ichthyosaurs, the new taxon shares a long and slender rostrum, degenerate dentition in adults, and the enlargement of the orbit with Ophthalmosaurus. Unlike Ophthalmosaurus, the forefin of Caypullisaurus has four major digits composed of tightly packed, polygonal phalanges, and pre- and post-axial accessory digits. Caypullisaurus demonstrates that ichthyosaurs were more diverse during the Late Jurassic than thought previously.