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Ankylosaur osteoderm structure
A paper in the new issue of the Journal of Vertebrate Paleontology:
Michael E. Burns & Philip J. Currie (2014)
External and internal structure of ankylosaur (Dinosauria,
Ornithischia) osteoderms and their systematic relevance.
Journal of Vertebrate Paleontology 34(4): 835-851
Ankylosaurian systematics can be assessed using morphological,
textural, and histological characters of osteoderms. Archosaur
osteoderms have cortices surrounding cancellous cores. Ankylosaurs are
united by an external cortex distinguishable from the core and by the
presence of mineralized structural fibers. Nodosaurid osteoderms lack
a well-developed basal cortex and have dense external cortical fibers.
Ankylosaurid osteoderms are thinner than those of other ankylosaurs.
Polacanthine osteoderms have a cancellous core, but share this feature
with other derived and primitive taxa. Cortical thickness overlaps
among groups, so a thick cortex is not diagnostic for polacanthines.
Specialized elements diverge histologically from the primitive
condition to suit their specific functions. Some shapes and external
textures are diagnostic for specific taxa, such as Ankylosaurus and
Glyptodontopelta. Parsimony analyses suggest osteodermal support for a
monophyletic Polacanthinae (excluding Mymoorapelta) and Shamosaurinae.