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Bone histology of archosauriform osteoderms (phytosaurs, aetosaurs, doswelliids)

From: Ben Creisler

A new online paper:

Torsten M. Scheyer, Julia B. Desojo & Ignacio A. Cerda (2013)
Bone Histology of Phytosaur, Aetosaur, and Other Archosauriform
Osteoderms (Eureptilia, Archosauromorpha).
Anatomical Record (advance online publication)
DOI: 10.1002/ar.22849

As in other archosauriforms, phytosaurs and aetosaurs are
characterized by the presence of well-developed osteoderms. Here we
provide a comparative study on the microstructure of phytosaur (five
taxa) and aetosaur (thirteen taxa) osteoderms. For outgroup
comparison, we sampled osteoderms of the sister taxon to Aetosauria,
Revueltosaurus callenderi, and the doswelliid Jaxtasuchus salomoni.
Phytosaur, aetosaur, and Jaxtasuchus osteoderms are composed of a
diploe structure, whereas the Revueltosaurus osteoderm microanatomy is
more compact. The external cortex of phytosaurs, Revueltosaurus and
Jaxtasuchus osteoderms is mainly composed of parallel-fibered bone. In
aetosaurs, the external cortex mainly consists of lamellar bone, with
lines of resorption within the primary bone indicating successive
cycles of bone erosion and deposition. The basal cortex in all the
specimens is composed of parallel-fibered bone, with the cancellous
internal core being more strongly developed in aetosaurs than in
phytosaurs. Woven or fibro-lamellar bone was recorded in both
phytosaurian and aetosaurian taxa, as well as in Jaxtasuchus.
Structural fibers, which at least partly suggest metaplastic origin,
were only recorded in the internal core of two phytosaurs and in the
basal cortex of one aetosaur. Osteoderm thickness and cancellous to
compact bone ratios appear to be subject to ontogenetic change.
Minimum growth mark counts in osteoderms sampled indicate that some
aetosaurs and phytosaurs lived for at least two decades. Bone
microstructures are more uniform in phytosaur osteoderms and show a
higher level of disparity among aetosaur osteoderms, and at least in
the latter, histological features are potentially apomorphic for
species/genus level.