Correlation between vertebral Hox code and vertebral morphology in archosaurs

The evolution of the vertebral column is marked by profound
morphological changes that have a strong impact on organismal
biology. The vital functions of the axial skeleton range from
protecting the neural structures through sustaining the body
posture to physiological aspects such as breathing. Archosaurs
(crocodiles, birds and dinosaurs), as a group, display a striking
variety of body plans and vertebral morphologies. This dissertation
aims to contribute to the understanding of the pattern and the
genetic basis for the evolution of the vertebral column in
archosaurs. The transdisciplinary project comprises five chapters.
Framed by a general introduction (chapter 1) and the conclusion
(chapter 5), the second chapter considers, from a morphofunctional
point of view, the question of (1) why differences in the vertebral
column evolved. The present thesis revealed a strong link between
the digitally simulated flexion pattern of the presacral vertebral
column and the axial movements of modern archosaurs during related
activities such as feeding and locomotion: this correlation allowed
the inference of the feeding range and locomotor options in the
extinct archosaur Plateosaurus. This long-necked dinosaur was
primarily adapted as mid-level browser, obtaining food that was at
or above the horizontal level of its head. There is currently no
evidence to unambiguously interpret the locomotion style of
Plateosaurus. The morphofunctional analysis supported both a
quadrupedal and a bipedal posture. The third chapter addresses,
from a molecular biology point of view, the question (2) of how
modern taxa develop their vertebral columns. It provides insights
into the genetic basis for the embryonic development of the
vertebral column in modern archosaurs, which includes the highly
conserved Hox genes. The Hox gene expression pattern was detected
in the Nile crocodile (Crocodylus niloticus) via whole-mount in
situ hybridisation experiments. Hox paralog genes 4 and 5 are
expressed in the cervical region of the crocodile. The anterior
expression limit of HoxC-6 marks the cervicothoracic transition.
The expression of Hox paralog genes 7 and 8 is restricted to the
dorsal series. The same Hox genes are expressed along the
anteroposterior body axis of crocodiles, chickens and mice, but the
pattern of expression is different. The comparative analysis
revealed two general processes that are accompanied by evolutionary
differences in the axial skeleton: 1) expansion and condensation as
well as 2) a shift of genetic activity corresponding to different
vertebral counts. The strong association between the anterior
limits of the expression of specific Hox genes and the borders
between morphological regions of the vertebral axis in a variety of
vertebrate species stimulated the work presented in the fourth
chapter. It considers the question (3) of whether we can infer that
the development of the vertebral column took place in extinct
animals. The direct correlation between vertebral Hox code and
quantifiable vertebral morphology shows that the genetic code is
deducible from vertebral morphology in modern crocodiles, chickens
and mice. Applying these findings to the fossil relative
Plateosaurus revealed that the hypothetical Hox code for the
dinosaur would be generally similar to the crocodilian Hox gene
expression pattern, but with the variation that the anterior region
is expanded, as in birds. The integrative analysis (morphology,
genes and fossils) of the vertebrae greatly enhanced our knowledge
of evolutionary processes and provided valuable information about
the possible reasons, genetic basis and pattern for evolutionary
changes of the vertebral column in extant and extinct archosaurs.