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.

Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs

“…Nevertheless, recent work on the systematic value of ankylosaur osteoderms(Burns 2008;Burns & Currie 2014) has resulted in several new characters based on osteoderm gross morphology, surface texture, internal histology and arrangement on the body. These characters are supplemented by several new osteoderm characters identified in this study.…”

Bone histology sheds light on the nature of the “dermal armor” of the enigmatic sauropod dinosaur Agustinia ligabuei Bonaparte, 1999

“…Regardless of their microanatomical features, the histology of the primary bone is common in all titanosaur osteoderms. Similar to that observed in ankylosaurs (e.g., Scheyer and Sander 2004;Burns and Currie 2014), the matrix of the primary bone is formed by coarse mineralized collagen fibers which are organized in thick bundles. These bundles exhibit different orientations, giving an interwoven appearance to the tissue under low magnifications (Fig.…”

First thyreophoran dinosaur from the Middle Jurassic (Bajocian) of Luxembourg

“…Among ankylosaurs, nodosaurid and 'polacanthid' osteoderms have a thickened cortex relative to the thinwalled found in ankylosaurids; comparison indicates that the osteoderm histology of ankylosaurs, mainly nodosaurids and ankylosaurids, is highly derived from the basal thyreophoran type (Scheyer & Sander, 2004). Unlike the basal thyreophorans and stegosaurs, the ankylosaur osteoderms possess extensive structural fibres (Scheyer & Sander, 2004;Hayashi et al, 2010;Burns & Currie, 2014). A destructive (histological) analysis is not envisaged for the Luxembourg osteoderm, so this pattern cannot be tested.…”

“…12;Kirkland et al, 2013, figs 28-30, type D-E armour). However, the external surface of ankylosaurian osteoderms is generally characterized by its rugosity, whereas the smooth, sparse pitting ornamentation observed in the Luxembourg osteoderm looks more like the one of basal thyreophorans (Burns & Currie, 2014). According to these authors, the relatively smooth osteoderms of basal thyreophorans is the primitive surficial condition (also retained in primitive ankylosaurs).…”