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Bone ornamentation mechanisms in crocodylomorphs (Crurotarsi)

Ben Creisler

A new online paper:

V. de Buffrénil, F. Clarac, M. Fau, S. Martin, B. Martin, E. Pellé and
M. Laurin (2014)
Differentiation and growth of bone ornamentation in vertebrates: A
comparative histological study among the Crocodylomorpha.
Journal of Morphology (advance online publication)
DOI: 10.1002/jmor.20351

Bone ornamentation, that is, hollow (pits and grooves) or protruding
(ridges) repetitive reliefs on the surface of dermal bones, is a
frequent, though poorly studied and understood, feature in
vertebrates. One of the most typical examples of this characteristic
is given by the Crurotarsi, a taxon formed by the crocodilians and
their closest allies, which generally display deep ornamentation on
skull roof and osteoderms. However, the ontogenetic process
responsible for the differentiation and development of this character
remains controversial. This study was conducted to settle the question
on histological and microanatomical evidence in several crurotarsan
taxa. Observational and experimental data in extant and extinct
crocodyliforms show that bone ornamentation is initially created, and
later maintained during somatic growth (that is indefinite in
crocodilians), by a complex process of bone remodeling comprising
local resorption of superficial bone cortices, followed by partial
reconstruction. The superficial reliefs of crocodilian dermal bones
are thus permanently modified through pit enlargement, drift,
stretching, shrinking, or complete filling. Ridges are also remodeled
in corresponding ways. These processes allow accommodation of unitary
ornamental motifs to the overall dimensions of the bones during
growth. A parsimony optimization based on the results of this study,
but integrating also published data on bone histology in
non-crocodyliform crurotarsans and some non-crurotarsan taxa, suggests
that the peculiar mechanism described above for creating and
maintaining bone ornamentation is a general feature of the Crurotarsi
and is quite distinct from that attributed by previous authors to
other vertebrates.