Eosuchus lerichei is a gavialoid crocodylian from late Paleocene marine deposits of northwestern Europe, known from a skull and lower jaws, as well as postcrania. Its sister taxon relationship with the approximately contemporaneous species Eosuchus minor from the east coast of the USA has been explained through transoceanic dispersal, indicating a capability for salt excretion that is absent in extant gavialoids. However, there is currently no anatomical evidence to support marine adaptation in extinct gavialoids. Furthermore, the placement of Eosuchus within Gavialoidea is labile, with some analyses supporting affinities with the Late Cretaceous to early Paleogene “thoracosaurs.” Here we present novel data on the internal and external anatomy of the skull of E. lerichei that enables a revised diagnosis, with 6 autapormorphies identified for the genus and 10 features that enable differentiation of the species from Eosuchus minor. Our phylogenetic analyses recover Eosuchus as an early diverging gavialid gavialoid that is not part of the “thoracosaur” group. In addition to thickened semi-circular canal walls of the endosseous labyrinth and paratympanic sinus reduction, we identify potential osteological correlates for salt glands in the internal surface of the prefrontal and lacrimal bones of E. lerichei. These salt glands potentially provide anatomical evidence for the capability of transoceanic dispersal within Eosuchus, and we also identify them in the Late Cretaceous “thoracosaur” Portugalosuchus. Given that the earliest diverging and stratigraphically oldest gavialoids either have evidence for a nasal salt gland and/or have been recovered from marine deposits, this suggests the capacity for salt excretion might be ancestral for Gavialoidea. Mapping osteological and geological evidence for marine adaptation onto a phylogeny indicates that there was probably more than one independent loss/reduction in the capacity for salt excretion in gavialoids.
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RBINS Staff Publications 2024 OA
Marine Isotope Stage 3 (MIS 3, ca. 60–30 ka) is characterized by dynamic alternations of forest expansion with semi-arid area expansion in accordance with the warming and cooling, respectively, of the sea-surface temperatures in Northern Europe. It was in this context of rapid fluctuations that the terrestrial sequence of Caverne Marie-Jeanne (Hastière-Lavaux, Belgium) in northwestern Europe was formed. The habitat weighting method and the bioclimaticmodel, as well as the Simpson diversity index, are applied to the small-mammal assemblage of CaverneMarie-Jeanne in order to reconstruct the environmental and climatic fluctuations that are reflected in the MIS 3 sequence of the cave. Revision of the small-mammal fossil material deposited in the collections of the Royal Belgian Institute of Natural Sciences (RBINS, Brussels, Belgium) shows that the lower layers (6 to 4) of the cave, pertaining to MIS 3 (ca. 50–40 ka), underwent cold, dry environmental and climatic conditions for these layers. This is indicated by temperatures lower than at present and precipitation slightly higher than at present, together with an environment dominated by openwoodland formations and open dry meadows. Our results are consistent with the available chronological, large-mammal, herpetofaunal and mollusc datasets for this lower part of the sequence. They are also consistentwith regional loess studies in Belgium andwith previouswork performed on small mammals from MIS 3 in Belgium and elsewhere in Europe.
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RBINS Staff Publications 2017
