External and internal structure of ankylosaur (Dinosauria, Ornithischia) osteoderms and their systematic relevance

ABSTRACT

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.

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

ACKNOWLEDGMENTS

We thank M. Vickaryous for expert advice on osteoderm histology and different tissue types. Thanks also to M. Caldwell and D. Begg for advice and assistance. V. Arbour read an early version of the manuscript. E. Koppelhus, R. Sissons, and all members of the UALVP have also provided support. I. Cerda and T. Scheyer provided thorough reviews that greatly improved the quality of the manuscript. We also benefited greatly from discussions with K. Carpenter, T. Ford, D. Fowler, D. Martill, P. Penkalski, T. Scheyer, R. Sullivan, and D. Tanke. Thanks are also due to the many people that have provided access and assistance at their respective institutions, especially those that have provided material for destructive sampling. They include M. Borsuk-Białynicka (ZPAL), K. Carpenter (DMNH), D. Evans (ROM), M. Getty (UMNH), B. Iwama (ROM), L. Ivy (DMNH), C. Mehling (AMNH), M. Norell (AMNH), K. Seymour (ROM), K. Shepherd (CMN), B. Striliski (TMP), and R. Sullivan (SMP). The use of thin sectioning equipment was possible courtesy of R. Stockey (University of Alberta). I. Jakab and the Earth and Atmospheric Sciences Digital Imaging Facility (University of Alberta) provided a slide scanner equipped with PPL and XPL. Funding was provided by the Department of Biological Sciences, Dinosaur Research Institute, Faculty of Graduate Studies and Research, Jurassic Foundation, and University of Alberta Graduate Students’ Association.

Handling editor: Randall Irmis