Among the extant crocodylians are two species with long, narrow snouts: Gavialis gangeticus,the Indian gharial and Tomistoma schlegelii, the "false" gharial. These enigmatic species are considered by the IUCN red list as critically endangered and vulnerable, respectively. However, despite this, knowledge of their evolutionary history is lacking. Extensive debate has surrounded the gharials for over four decades and remains unsolved today: the so-called gharial problem. Whereas molecular studies consistently indicate that these two species are sister taxa, morphological studies of both living and fossil taxa find that they belong to distantly related lineages. Moreover, molecular clock estimates indicate a shallow divergence time of 18-31 million years ago. This entirely contradicts the rich fossil record of gharials: in contrast to the modern gharials, these fossil taxa comprise a huge diversity and suggest that tomistomines and gavialines have diverged from each other at least 70 million years ago, prior to the K/Pg mass extinction. European museums, and especially the Royal Belgian Institute of Natural Sciences, Brussels, comprise rich collections containing many of the oldest fossil gavialoids, crucial to solving the gharial problem. Nevertheless, few modern morphological studies have been performed on these specimens, and their stratigraphic age is often poorly constrained. Therefore, in a new project we will use a multidisciplinary approach to study these specimens, combining morphological study and biostratigraphic analyses using dinoflagellate cysts. Moreover, we will revise the classical methods used by paleontologists to study fossil crocodylians, devising a new phylogenetic framework that makes use of both morphological, molecular, and biostratigraphic data. Here, we will present some of the first preliminary results of this project.
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RBINS Staff Publications 2023
Caecilians are elongate, limbless and annulated amphibians that, with the exception of one aquatic family, all have an at least partly fossorial lifestyle. It has been suggested that caecilian evolution resulted in sturdy and compact skulls with fused bones and tight sutures, as an adaptation to their head-first burrowing habits. However, although their cranial osteology is well described, relationships between form and function remain poorly understood. In the present study, we explored the relationship between cranial shape and in vivo burrowing forces. Using micro-computed tomography (µCT) data, we performed 3D geometric morphometrics to explore whether cranial and mandibular shapes reflected patterns that might be associated with maximal push forces. The results highlight important differences in maximal push forces, with the aquatic Typhlonectes producing a lower force for a given size compared with other species. Despite substantial differences in head morphology across species, no relationship between overall skull shape and push force could be detected. Although a strong phylogenetic signal may partly obscure the results, our conclusions confirm previous studies using biomechanical models and suggest that differences in the degree of fossoriality do not appear to be driving the evolution of head shape.
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RBINS Staff Publications 2021
