Abstract:
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 headwidth
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.
Cenozoic