Michael S. Y. Lee & Adam M. Yates (2018)
Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil record.
Proceedings of the Royal Society B 285 20181071Â
DOI: 10.1098/rspb.2018.1071
Simultaneously analysing morphological, molecular and stratigraphic data suggests a potential resolution to a major remaining inconsistency in crocodylian evolution. The ancient, long-snouted thoracosaurs have always been placed near the Indian gharial Gavialis, but their antiquity (ca 72 Ma) is highly incongruous with genomic evidence for the young age of the Gavialis lineage (ca 40 Ma). We reconcile this contradiction with an updated morphological dataset and novel analysis, and demonstrate that thoracosaurs are an ancient iteration of long-snouted stem crocodylians unrelated to modern gharials. The extensive similarities between thoracosaurs and Gavialis are shown to be an almost âperfect stormâ of homoplasy, combining convergent adaptions to fish-eating, as well resemblances between genuinely primitive traits (thoracosaurs) and atavisms (Gavialis). Phylogenetic methods that ignore stratigraphy (parsimony and undated Bayesian methods) are unable to tease apart these similarities and invariably unite thoracosaurs and Gavialis. However, tip-dated Bayesian approaches additionally consider the large temporal gap separating ancient (thoracosaurs) and modern (Gavialis) iterations of similar long-snouted crocodyliforms. These analyses robustly favour a phylogeny which places thoracosaurs basal to crocodylians, far removed from modern gharials, which accordingly are a very young radiation. This phylogenetic uncoupling of ancient and modern gharial-like crocs is more consistent with molecular clock divergence estimates, and also the bulk of the crocodylian fossil record (e.g. all unequivocal gharial fossils are very young). Provided that the priors and models attribute appropriate relative weights to the morphological and stratigraphic signalsâan issue that requires investigationâtip-dating approaches are potentially better able to detect homoplasy and improve inferences about phylogenetic relationships, character evolution and divergence dates.
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Jihed Dridi (2018)
New fossils of the giant pholidosaurid genus Sarcosuchus from the Early Cretaceous of Tunisia.
Journal of African Earth Sciences (advance online publication)
The Lower Cretaceous vertebrate assemblages from southeastern Tunisia contain one of the most diverse, unique and significant vertebrate faunas of this period in North Africa. The fossil record of crocodyliforms from these assemblages is becoming better understood with the increasing number of palaeontological discoveries that have been made over the past few years. However, very little data is available on the North Gondwanan pholidosaurid crocodyliform Sarcosuchus due to the paucity of material likely because of selective taphonomic factors. Here, I describe and figure new fossils of the pholidosaurid genus Sarcosuchus from Aptian-Albian deposits of the Tataouine Basin in southeastern Tunisia. These fossils include well-preserved dorsal osteoderms, which were found in anatomical connection, several isolated teeth and fragmentary remains mostly pertaining to the appendicular and axial skeletons. Anatomical studies and comparisons with other specimens have been performed, supporting the attribution of the material to the genus Sarcosuchus. I show that no attempt at a specific level can be made on the basis of the present fossils. The large size of the osteoderms and the stout conical teeth indicate that many Sarcosuchus individuals reached enormous body sizes.
Sarcosuchus would have prospered in a flood dominated river-delta system that characterized the Tethyan margin of Gondwana by the end of the Lower Cretaceous. The abundant remains of diverse fish taxa including actinopterygians, sarcopterygians and chondrichthyans are indicative of a productive ecosystem, and further suggest that the diet of the Tunisian taxon included large bodied fish. This discovery provides a substantial addition to the extremely poor record of the North African crocodylomorphs with new insights into their palaeobiogeographic distribution, palaeoecology and extinction.