Ontogenetic, macroevolutionary and morphofunctional patterns in archosaur skulls

The Archosauria represent the most successful clade within
tetrapods, having a large diversity in terms of species, diet
spectra, body plans and locomotion styles. This is also true for
the skull morphology, which shows a wide variety in shape and size,
as well as in the common formation of beaks, crests, domes or
horns. Archosaur skulls have been studied intensively in terms of
their morphology, ontogeny, function, ecology and behavior in the
past, but most of these studies have largely been restricted to
case studies of single species or only a small number of taxa. The
aim of the current thesis is to obtain better and comprehensive
insight into skull shape diversity of archosaurs by using a
two-dimensional geometric morphometric approach, with a special
focus on ontogenetic and macroevolutionary patterns and their
relation to function and ecology. Skull shape variation was
quantified for Crocodylomorpha (including an ontogenetic series of
the recent caimanine alligatorid Melanosuchus niger), Pterosauria,
Sauropodomorpha and Theropoda. The material used for the analyses
consists of skull reconstructions published in the scientific
literature and photographs of skull material. The most important
results of the thesis are summarized as follows: •The use of
different skull reconstructions of the same specimen from the
scientific literature has no significant influence on the results
of morphometric analyses. However, the results could be potentially
falsified by the use of reconstructions based on highly incomplete,
strongly deformed or pathologic specimens. •In some cases the
degree of intraspecific variation of one species can be as great as
the interspecific variation of closely related species with similar
ecological niches. Thus, species with great intraspecific diversity
could have an impact on the results of morphometric analyses. •The
skull shape of Archosauria is strongly correlated with function. A
closer examination within theropod skulls reveals that the shape of
the postrostrum is probably more affected by functional constraints
than the snout, but the greatest correlation to the function was
found in the orbital shape. The latter result supports previous
studies on the biomechanics of theropod skulls. A comparison of the
ontogenetic bite force performance with the cranial growth in the
alligatorid Melanosuchus and biomechanical studies on crocodile
skulls reveals that ontogenetic shape changes, especially in the
orbital and postorbital region, are functional constrained. •Both
ontogenetic and interspecific skull shape variation in archosaurs
is correlated to diet preferences and feeding behaviour. A
comparison between carnivorous and non-carnivorous (i.e. omnivorous
and herbivorous) theropods reveals that both ecological groups
occupy large areas within the morphospace without showing a
significant overlap. Furthermore, small-bodied theropods tend to
have a larger diet spectrum, suggesting that diet preferences
within theropods are probably size related. •The distribution of
taxa within the morphospace of Crocodylomorpha, Pterosauria,
Sauropodomorpha and Theropoda is strongly correlated with the
phylogenetic interrelationship of these clades: Closely related
taxa appear closer to one another within the morphospace than more
distantly related taxa. This result indicates that skull shape in
archosaurs is further constrained by phylogeny. •When inferred from
geometric morphometric data, disparity results proved to be similar
to those based on limb measurements and discrete characters from
phylogenetic analyses. This results justifies the use of geometric
morphometric data as a further and equally useful proxy for
addressing disparity. •Early archosaur hatchlings share features of
the skull shape, including short, pointed snouts, enlarged orbits
and large postorbital regions. However, ontogenetic shape changes
are only congruous in terms of a relative increase of the snout
length and a relative decrease of the orbit size. The degree of
these changes is not uniform, so that adult specimens of different
species can vary substantially in snout length or orbit shape.
Furthermore, archosaurs show a huge variability of changes in the
snout depth, the length of the postorbital region as well as the
relative size of the antorbital fenestra and the lateral temporal
fenestra during ontogeny. This variability in ontogenetic
trajectories probably causes the large skull shape diversity found
in archosaurs. •Due to the great variability in ontogenetic
trajectories, cranial evolution of archosaurs is strongly affected
by heterochronic events. Skull shape evolution of Crocodylomorpha,
Sauropodomorpha, basal theropods, Tyrannosauroidea as well as
derived Oviraptoridae, Dromaeosauridae and Troodontidae was
probably influenced by peramorphosis. However, within Crocodylia
the short skull of Osteolaemus might result from a paedomorphic
event. This is also likely for the short-snouted basal theropods
Daemonosaurus and Limusaurus. The great similarity in the skull
shapes of the juvenile megalosaurid Sciurumimus and basal
coelurosaurs reveals that the skull shapes of the latter might be
also caused by paedomorphosis. Further paedomorphic trends are
suspected for the skull evolution of basal Maniraptora and Avialae.
The heterochronic events found seem to correlate with body size
evolution.