REVIEWING THE PALAEOCENE CAIMANINE (CROCODYLIA) RECORD OF ARGENTINA: AN INTEGRATION OF TAXONOMIC REASSESSMENTS AND MACROEVOLUTIONARY ANALYSES

BONA, P.; BARRIOS, F.; EZCURRA, M. D.; FERNANDEZ BLACO, M. V.

Congreso; VI Congreso Latinoamericano de Paleontologia de Vertebrados; 2018

Modern crocodylian faunas of South America are dominated by several species ofcaimanine alligatorids that inhabit tropical and subtropical freshwater systems.Caimanines have a long history in the continent that spans most of the Cenozoic andachieved a considerably higher taxonomic diversity in some palaeoecosystems than today.This group is currently restricted to South and Central America and their oldestspecimens have been collected in the lower Palaeocene of Argentina and Brazil. ThePalaeogene Argentinian caimanine record is extremely relevant because it includes forms[i.e., Eocaiman cavernensis, E. palaeocenicus, Necrosuchus ionensis, Notocaiman stromeri and anew taxon under revision, Museo de La Plata (MLP) 80-X-10-1] that provide keymorphological information on the early evolutionary history of these crocodylians. Theoldest Argentinian caimanines are from the lower Palaeocene localities of the SalamancaFormation (Chubut Province, Patagonia). This crocodilian assemblage is composed ofdifferent lineages that are a consequence of independent dispersal events that occurredbetween North and South America, and within South America around theCretaceous−Palaeogene boundary. The new caimanine MLP 80-X-10-1 from theSalamanca Formation is represented by a skull roof, quadrate and a partial braincase andour phylogenetic analyses place it as the most basal member of the South Americancaimanines. Despite the presence of typical caimanine apomorphies, its skull tablemorphology resembles more that of latest Cretaceous alligatoroids, such asBrachychampsa and related forms, rather than species historically included withinCaimaninae. Indeed, the optimization of a two-dimensional morphogeometricconfiguration of the skull table in the phylogeny of Alligatoroidea shows that MLP 80-X-10-1 retains a skull table morphology only slightly transformed from that of thehypothetical ancestor of Caimaninae and more distinct transformations occur in morederived members of this clade (i.e. narrower supratemporal fenestrae and moreposteriomedially expanded posterior margin of the skull table). It is remarkable thatsimilar morphological transformations occur in extant jacarean caimanines during posthatchingontogeny, in which adult specimens possess a narrower skull table and smallersupratemporal fenestrae than juveniles. Contrasting with MLP 80-X-10-1, the other currently known Palaeocene caimanine species of South America are mostly representedby mandibular elements. Among them, No. stromeri [Paleontología de Vertebrados Lillo(PVL) 752, holotype: fragment of a left dentary], from the middle Palaeocene Las VioletasFormation (Chubut Province), possesses the same mandibular shape in lateral view as thespecies of Eocaiman and, like E. cavernensis, E. itaboraiensis and E. palaeocenicus, has a moreposteriorly extended mandibular symphysis (level to 5th alveolous) than in Ne. ionensis,the other known Palaeocene Patagonian caimanine. No. stromeri shares with E.palaeocenicus the same alveolar pattern and robustness of the lower jaw, and ourphylogenetic analyses recovered the former species as more closely related to the speciesof Eocaiman than to other caimanines because of the presence of dentary at level of 1stand 4th teeth lower than at level of 11th−12th teeth. Thus, these results suggest thatNotocaiman could be a junior synonym of Eocaiman. The origin and early diversificationof the caimanines is one of the least understood aspects of the crocodylian evolutionaryhistory. These new results about the Palaeogene Argentinian alligatorids improve ourunderstanding of the evolutionary and biogeographic history of these crocodylians.