Cranial anatomy and diversity of the Norian phytosaurs of southwestern Germany.
Phytosaurs are the most basal group of crurotarsan archosaurs. Superficially
resembling crocodiles in habit and probably also in their ecological requirements, they
form an important component of terrestrial vertebrate communities in the Late Triassic.
The phytosaurs from the Stubensandstein deposits (Norian) of southwestern Germany
are among the first representatives to become known of the group. However, our poor
knowledge of European Norian phytosaurs is well exemplified by many morphological
details of the cranium that in the course of this study were found to be hitherto unknown, or
to be at variance with the literature. Virtually all phytosaur specimens from these deposits
were examined to established their taxonomic status. The specimens comprise four valid
species, which are referred to the genera Nicrosaurus FRAAS, 1866 and Mystriosuchus
FRAAS, 1896. Both genera and all species are redefined on the basis of shared derived
characters. The aim of this thesis is to redescribe the cranial anatomy of each taxon, a
prerequisite to determine the variability and to test the validity of cranial characters that
have been utilised to establish phylogenetic relationships among phytosaurs.
A comprehensive skull osteology and a study of the variation in cranial characters of
Nicrosaurus kapffi (MEYER, 1860) forms the main part of the thesis. The complete upper
dentition is described in order to determine the positional variation of dental characters.
Nicrosaurus kapffi is characterised by two features of its prenarial crest. Two morphs are
recognised among the specimens based mainly on different skull width. The distinction is
congruent with the distribution of other dimorphic features of the postorbital part of the
skull and details of the crested rostrum, many of which are currently employed in
phytosaur taxonomy. The intraspecific variation is interpreted as sexual dimorphism. The
study demonstrates also that other characters are actually variable at species level and can
provide only limited taxonomic and phylogenetic information. These include, most
importantly, a reduction of the suborbital opening, a partly persisting parietal foramen, and,
varying individually, the configuration of a number of dermal skull bones.
The slender-snouted and gracile specimens previously referred to Belodon
plieningeri MEYER, 1844 actually represent a species of Nicrosaurus, here referred to as
Nicrosaurus species B. A hitherto undescribed skull provides important missing information
on the temporal region in this taxon. Nicrosaurus species B is more derived than
Nicrosaurus kapf in having, among more uncertain characters, a more elongated
squamosal with a pointed squamosal tip and a narrower supratemporal fenestra. Nicrosaurus
species B shows the same two intraspecific morphotypes regarding skull width, but,
additionally, a significant variation of the shorter prenarial crest can be observed.
Additional cranial data is provided to characterise the highly derived Mystriosuchus
planirostris (MEYER, 1863), the type species of the genus. Previous suggestions that a
particular skull represents a different species of Mystriosuchus are confirmed. The occipital
aspect of this skull is redescribed, and a new reconstruction of the braincase is presented.
The hitherto unnamed species is distinguished by numerous cranial characters, including
details of the temporal region, an orbitosphenoid, a supernumerary bone in the occipital
region, and possibly a premaxillary crest.
A phylogenetic analysis of 22 phytosaur taxa based on 49 characters using PAUP was
conducted. As a preliminary result, the Paleorhininae were found not to be monophyletic as
previously suggested, but to represent the paraphyletic stem-group of all other phytosaurs
(Phytosauridae). Secondly, Mystriosuchus is not closely related to any non-phytosaurid
phytosaur, but the most derived Phytosauridae forming a Glade with Pseudopalatus
pristinus and Arribasuchus buceros