Cranial anatomy of a newpliosaurid Acostasaurus pavachoquensis from the LowerCretaceous of Colombia, South America

Abstract

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 protoAtlantic 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.