An almost complete, three-dimensionally preserved plesiosaurian from the Hildoceras bifrons Zone of the Alum Shale Member (Whitby Limestone Formation; Lower Toarcian) of Yorkshire, UK, is described in detail. This represents a new species of Hauffiosaurus, H. tomistomimus, distinguished from H. zanoni (Harpoceras serpentinum Zone, Lower Toarcian, Germany) by the proportionally shorter neck and strongly concave preaxial margin of the tibia. It differs from H. longirostris (previously 'Macroplata' longirostris; Har. serpentinum Zone, Yorkshire) by the absence of prominent midline ridges on the dorsal surface of the premaxillae and ventral surface of the mandibular symphysis, and the absence of midline pterygoid contact ventral to the basioccipital. Several synapomorphies support a monophyletic Hauffiosaurus: broad longitudinal troughs occupy the dorsolateral surface of the maxilla and the posterior half of the lateral surface of the dentary; basicranial fontanelle bounded laterally by posterolaterally elongate projections of an undetermined ossification; and the neural arch contacts the rib facet in all postaxial cervical vertebrae. However, the systematic position of Hauffiosaurus, as a pliosauroid or basal plesiosauroid, remains uncertain. There is little evidence for geographic differentiation of Lower Toarcian plesiosaurian faunas in the United Kingdom and Germany as minor differences between abundant taxa may arise from temporal offset of fossils from these regions, and marked taxonomic differences are confined to rare taxa whose absence in one or other area may be attributable to incomplete sampling. Lack of consensus on the relationships of Lower Jurassic plesiosaurians requires further detailed description of Lower Jurassic taxa.

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Following the marine extinctions at the end of the Jurassic, only three clades of Plesiosauria passed into the Lower Cretaceous: Brachaucheninae (thalassophonean pliosaurids), Cryptoclididae, and Xenopsaria. However, these plesiosaur clades, and the Brachaucheninae in particular, are represented by a limited number of genera. The Berriasian to Albian thereby represents a considerable period of time (~45 Ma) lacking a plesiosaur-rich strata or Lagerstätte: a time period we here designate as the "Lower Cretaceous Gap" or LCG. One critical region for understanding LCG plesiosaurs is modern northern South America, which, during Early Cretaceous times acted as a crossover between the northern and southern hemispheres, and between the Pacific and proto-Atlantic oceans, as Gondwana gradually divided. Colombia preserves one of the most complete Lower Cretaceous sedimentary successions in the world. These strata, deposited in an epicontinental sea on the margin of Gondwana, are well-exposed close to Villa de Leyva, central Colombia. From the lower Barremian Arcillolitas Abigarradas Member of the Paja Formation, we describe a new genus and species of pliosaurid, Acostasaurus pavachoquensis. Acid preparation has exposed an exceptionally well-preserved three-dimensional pliosaurid skull with superb anatomical detail, allowing thorough description of previously poorly known areas of the plesiosaur skull such as the olfactory wings, otic capsules, sclerotic plates, basicranium and mandibular symphysis. Acostasaurus displays a unique suite of characters: short preorbital rostrum, stepped maxilla contacting the nasal and parietal posteriorly, large nasal in contact with the parietal, frontal with small exposure dorsally, deep notch in the dorsal surface of orbital margin, sagittal crest formed from the parietal and squamosal, dentition markedly heterodont, four pairs of premaxillary teeth, and a mandibular symphysial region containing five-and-a-half tooth pairs, which together differentiate Acostasaurus from all the other Cretaceous pliosaurid genera. Based on analysis of morphological characters used in recent phylogenetic studies, Acostasaurus nests firmly within the increasingly inclusive Pliosauroidea and Pliosauridae, almost certainly within Thalassophonea, and with much less certainty within Brachaucheninae, assuming Acostasaurus does not represent part of a new clade passing into the Early Cretaceous. As a brevirostrine taxon, Acostasaurus is distinctly different from the generally longer-snouted Brachauchenine genera Brachauchenius, Kronosaurus, Megacephalosaurus, Makhaira and Stenorhynchosaurus. The presence of Acostasaurus, together with other as yet undescribed plesiosaurs in central Colombia, firmly establishes northern South America as a key region for understanding of the taxonomy and phylogeny of Lower Cretaceous Gap pliosaurids.

From one of the most complete Lower Cretaceous rock sequences in the world (in Villa de Leiva region, central Colombia), we describe a new genus and species of pliosaurid plesiosaur Stenorhynchosaurus munozi. Stenorhynchosaurus displays a series of features which differentiate it from all the other Cretaceous pliosaurid genera: the anterior of the vomer, in ventral view, posterior of the palatal premaxilla-maxilla suture, contacting the posterior palatal process of the premaxilla level with the third maxillary alveolus; rostrum narrow and elongated with straight sides in dorsal view; lacrimal forming the anterior border and greater part of the ventral border of the orbit and broadly borders the maxilla anteriorly; anterior interpterygoid vacuity present; internal nares located between vomer and maxilla at the level of 13 th -15 th maxillary alveoli; anterior of rostrum and mandible without lateral expansion or marked increase in size of the functional alveoli; penultimate premaxillary alveolus slightly larger than adjacent premaxillary alveoli; homodont maxillary functional alveoli, with fourth tooth positions very slightly enlarged with respect to the neighboring alveoli; homodont dentary dentition; and epipodials extremely short. Based on morphological analysis of phylogenetic characters, Stenorhynchosaurus most likely nests within the increasingly inclusive Pliosauridae and Pliosauroidea, however, as currently defined, firm inference for referral of Stenorhynchosaurus to Thalassophonea is much more problematic, but ultimately seems likely based on characters shared with Pliosaurus and Brachaucheninae. However, this uncertainty indicates Thalassophonea requires rigorous redefinition. As to whether Stenorhynchosaurus is a member of the currently exclusively Jurassic genus Pliosaurus, or the Cretaceous sub-family Brachaucheninae, remains equivocal. ResumenSe describe un nuevo género y especie de pliosáurido, Stenorhynchosaurus munozi, procedente de una de las secuencias de rocas del Cretácico Inferior más completas del mundo (en la región de Villa de Leiva, centro de Colombia). Stenorhynchosaurus exhibe una serie de características que lo diferencian de los demás géneros de pliosáuridos cretácicos: extremo anterior del vómer en vista ventral en posición posterior a la sutura palatal premaxilar-maxilar, contactando al proceso palatal del premaxilar al nivel del tercer alveolo maxilar; rostro angosto y alargado con lados rectos en vista dorsal; lagrimal formando el borde anterior y gran parte del borde ventral de la órbita y limitando ampliamente el maxilar anteriormente; cavidad inter-pterigoidea presente; narinas internas ubicadas entre el vómer y el maxilar, al nivel de los alvéolos 13 y 15; parte anterior del rostro y de la mandíbula sin expansión lateral o incremento marcado en el tamaño de los alvéolos funcionales; penúltimo alvéolo premaxilar ligeramente más grande que los adyacentes; alvéolos funcionales maxilares homodontes, el cuarto apenas más grande que los adyacentes; dentición dentaria homod...

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