Libonectes

Abstract: Plesiosaurs were a diverse clade of marine reptiles that spanned nearly threequarters of the Mesozoic (earliest Jurassic to latest Cretaceous). They exhibit variation in head and neck morphology that presumably relates to functional differences in feeding habits. However, from a biomechanical standpoint, these marine creatures have a cranial organization shared with few reptile clades: the neodiapsid condition. Nevertheless, basic structural features in some derived clades, such as elasmosaurids, remain poorly understood, namely the presence of large supratemporal fenestrae, tall temporal bars, and high parietal crests. These features present biomechanical compromises with paleobiological implications for feeding habits. Here we test specific hypotheses regarding skull structure and mechanics in the elasmosaurid plesiosaur Libonectes morgani from the Late Cretaceous of Texas (USA). Using finite element analysis and loadings based on a detailed reconstruction of adductor chamber musculature, we provide estimates of stress and strain distributions for the Libonectes skull. We also digitally morphed different anatomical variants of the Libonectes skull, in order to assess the role of those traits in skull’s mechanical performance (e.g., height of the temporal bar). Our results show that a larger physiological cross-section of the adductor muscles is achieved by an enlarged supratemporal fenestra which although it reduces mechanical performance of the skull, it is offset by increased strength of a taller parietal crest and temporal bar, given the loading is largely symmetrical, the lateral components are offsetting yielding a vertical force vector. This arrangement also increases the length of the adductor musculature and thus the total muscle mass. We propose that the reduced pterygoid flange indicates a diminished role for the pterygoideus muscle, reflecting a shift of the majority of the bite force to the adductor mandibulae externus, pseudotemporalis, and adductor mandibulae posterior muscles. Reduction of the pterygoideus falsifies the dual adductor system hypothesis, in which kinetic inertia and static pressure coexist. Ricardo Araújo. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, Texas 75275, USA, rmaraujo@smu.edu and Museu da Lourinhã, Rua João Luís de Moura, 95, 2530-158 Lourinhã, Portugal Michael J. Polcyn. Department of Earth Sciences, Southern Methodist University, Dallas, Texas 75275, USA mpolcyn@smu.edu ARAÚJO AND POLCYN: PLESIOSAUR MASTICATION 2

Abstract: A partial exoccipital-opisthotic from the uppermost lower Campanian (Upper Cretaceous) of the Asen locality, Kristianstad Basin, southernmost Sweden, is described and illustrated. The fossil represents the first braincase element of a plesiosaur found in Sweden. It includes the chamber for the ampulla and utriculus, openings for the caudal vertical and horizontal semicircular canals, and four foramina for cranial nerves. The incomplete braincase can be referred to an elasmosaurid plesiosaur, and closely resembles the exoccipital-opisthotic of Libonectes morgani and a referred specimen of Aristonectes parvidens. Although we discuss putative postcranial material of the elasmosaurid subfamily Aristonectinae in the uppermost lower Campanian of southernmost Sweden, the exoccipital-opisthotic from Asen most likely belongs to a juvenile individual of a non-aristonectine elasmosaur. (Less)