A new elasmosaurid (Plesiosauria: Sauropterygia) from the López de Bertodano Formation: new data on the evolution of the aristonectine morphology

A new elasmosaurid <i>Marambionectes molinai</i> gen. et sp. nov. from the upper Maastrichtian levels of the López de Bertodano Formation is here described. The new taxon is characterized by one autapomorphy: 48 cervical vertebrae with lateral keels in cervicals 5 to 29. The following combination of features also differentiates <i>Marambionectes molinai</i> gen. et sp. nov. from other elasmosaurids: three symphyseal alveoli, symphyseal angle of <i>c.</i> 100°, wide symphyseal sulcus, high and triangular coronoid process, retroarticular process horizontally oriented, circular atlantal cup, low and rounded hypapophyseal keel on atlas-axis complex, cervical centra longer than high (middle cervical with vertebral length index <i>c.</i> 97) and caudal centra without strongly laterally projected parapophyses. The phylogenetic analysis recovers <i>M. molinai</i> gen. et sp. nov. within the Weddellonectia and as a sister group of aristonectines, indicating that the process of decreasing cervical vertebrae numbers, wide symphyseal sulcus and atlas-axis complex with circular atlantal cup and low and rounded hypapophyseal keel started previous to the appearance of aristonectines. In this sense, we suggest that some of the skeletal characteristics of aristonectines were acquired through a process that began in non-aristonectine elasmosaurids, before the appearance of this clade. http://zoobank.org/urn:lsid:zoobank.org:pub:693E0AAA-D03A-42E7-B454-BBA4D481649A