Function and evolution of ankylosaur dermal armor

[bh480@scn.org]

From: Ben Creisler bh480@scn.org

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
http://www.app.pan.pl/issue.html?issue=forthcoming


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.