Origin, diversity and evolution of the most common crocodylians of Europe in the Paleogene greenhouse context

Crocodylia is an iconic reptile vertebrate group whose extant representatives inhabit most of the continental land masses. Modern Europe is a notable exception, as no crocodyliform populations are found in this region of the world, which is greatly contrasting with the past biogeographical history of the group. The early Cenozoic, specifically, shows an exceptionally diverse crocodyliform fauna mainly dominated by alligatoroids. Morphological disparity, complex biogeography or even stratigraphically young age of the earliest-branching forms have been difficult to reconcile with consistently inferred phylogenetic relationships to alligatorids, an otherwise freshwater and small-bodied group in the Paleogene. The European putative alligatoroid genus Diplocynodon is recognised as the most common and best sampled crocodyliform in the Paleogene of Europe, as demonstrated by the extensive fossil record spread across multiple occurrences throughout the Cenozoic. This record hints at an exceptional survivorship of the genus from the late Paleocene to the middle Miocene, that perplexes the comprehensive investigation of the taxon systematics and taxonomy, indication of an overdue revision. The thesis presents an expanded phylogeny with increased spatiotemporally coherence that reinterprets Diplocynodon spp. (recovered closely related to the North American Borealosuchus) as well as the North American Deinosuchus spp. and Leidyosuchus canadensis as stem-group crocodylians. The novel topology elucidates the evolution of osmoregulation in Crocodylia and its close relatives by inferring plesiomorphic saltwater tolerance for Deinosuchus and the crown-group, and secondary loss already in stem-group alligatorids. Divergence of Alligatoroidea coincided with extreme mid-Cretaceous sea level highs and the distribution of Deinosuchus across the American Western Interior Seaway can be best explained by marine dispersal. Phylogenetic body-length analysis using a head-width proxy reveals phyletic dwarfism early in alligatoroid evolution and a reasonable total length estimate for the most complete specimen of Deinosuchus riograndensis. Gigantism in crocodyliforms is suggested as being correlated with high-productive extensive aquatic ecosystems in the present and in the past. The second and third chapters tackle the complex ingroup taxonomy of Diplocynodon. The diagnoses of currently accepted Diplocynodon species commonly include shared and/or irreproducible characters, hampering specific delimitations. Based on the review of all currently known species, the first taxonomic revision of the group since its inclusion in modern phylogenetic works is presented. An identification key to assist researchers with the identification of the valid species is additionally provided. Furthermore, the present work quantitatively reviews the state of the entire fossil record of Diplocynodon (based on two openly available databases) and discusses the waste-basket status of the taxon with respect to better taxonomical practices. Within Diplocynodon, the Eocene species Diplocynodon darwini Ludwig, 1877 has the largest sample in the Paleogene period, consisting of tens of complete well-preserved specimens, but yet critically lacks a detailed osteological description. The taxon is here redescribed for the first time based on type specimens collected 150 years ago and abundant excellently preserved material from the Messel and Geiseltal Konservat-Lagerstätten. Insights into the intraspecific variation in the taxon are provided in a detailed morphological description on the skeletal elements, including ontogenetic variation in temporally restricted populations. The complex ingroup taxonomy of Diplocynodon is furthermore explored and discussed through a review of previously published and newly retrieved phylogenies.