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Function and evolution of ankylosaur dermal armor
From: Ben Creisler email@example.com
In case this paper has not been mentioned yet:
Shoji Hayashi, Kenneth Carpenter, Torsten M. Scheyer,
Mahito Watabe, and Daisuke Suzuki.
Function and evolution of ankylosaur dermal armor.
Acta Palaeontologica Polonica in press
available online 19 Jan 2010
Ankylosaurs have spike-, plate-, and club-shaped
osteoderms probably used as defensive and/or offensive
weapons. Previous studies have proposed the evolution and
function of small ankylosaur osteoderms, but histological
variations in their defensive weapons are little known.
Here, we provide comparisons of the internal structures
in defensive weapons of ankylosaurs, which shed light on
understanding their evolutionary history and function.
Histological features of spikes, plates, and clubs are
similar to those of small osteoderms in having thin
compact bone, thick cancellous bone with large vascular
canals, and abundant collagen fibers. A previous study
demonstrated that each of the three groups of ankylosaurs
(the Polacanthidae, Nodosauridae, and Ankylosauridae)
have distinct arrangements of collagen fibers in small
osteoderms. This study shows that spikes and clubs of
ankylosaurs maintain the same characteristic features for
each group despite the differences in shapes and sizes.
These histological similarities suggest that various
types of osteoderms in ankylosaurs retained the thin
compact bone and abundant fiber structures of the small
osteoderms during their evolution. Polacanthid spikes
show thin compact bone, with less collagen fibers than in
spikes of nodosaurids and spikes and clubs of
ankylosaurids. Also, ankylosaurid plates with hollow
bases are very thin in morphology and show thin compact
bone. These results imply that the bone strengths of
polacanthid spikes and ankylosaurid plates are lower than
spikes and clubs of other ankylosaurs, indicating that
they may be used more probably as display and/or
thermoregulation rather than as weapons. It is thus
probable that ankylosaur armor in general played more
than just a defensive role.
Key words: Ankylosauria, Thyreophora, dermal armor, bone
histology, evolution, growth, function.