Ankylosaurus parascapular subsp. osteoderm

A parascapular osteoderm ?

Owen (1863: tab. III; see Fig. 4, Sc.ost) was the first to report the presence of an oval-based, ridged osteoderm located on the posterodistal external surface of the left scapular blade of the lectotype. An identically positioned oval-based and ridged osteoderm is also present on the blade of the left scapula in the referred specimen (Fig. 23). This coincident placement on the scapula reflects the fact that this osteoderm is just part of the mid-lateral row of medium-sized osteoderms and lay in the skin adjacent to the scapular blade before death, decay and compression pressed it on to the scapular blade (Figs 23, 24).

Comment: Among basal thyreophorans, such as Scutellosaurus and Emausaurus , there is as yet no skeletal evidence concerning the presence or absence of ‘scapular osteoderms’. Among more derived eurypodans, comparisons are more variable. In the nodosaurid ankylosaur Borealopelta a large, recurved and sharply pointed osteoderm, described as a ‘parascapular spine’ ( Brown et al., 2017: 19), is positioned adjacent to the scapular blade. Of the articulated ankylosaur skeletons known to date, this is the only one that preserves what might reasonably be described as a scapular osteoderm, although the dorsal osteoderm shield of the nodosaurid Edmontonia (see Fig. 41) has large, spine-shaped osteoderms in the vicinity of the scapula. After checking with Caleb Brown, the‘parascapular spine’ of Borealopelta is actually regarded as an osteoderm that lay in the dermis adjacent to the scapular blade (as in Scelidosaurus ) rather than being directly attached thereto (C. M. Brown pers. comm., May 2018). The term ‘parascapular spine’ was being used as a positional metaphor and was not intended to imply homology with the parascapular spines reported in stegosaurian eurypodans.

Parascapular spines have been reported in a number of stegosaurs, e.g. Chialingosaurus , Gigantspinosaurus , Huayangosaurus , Kentrosaurus , Lexovisaurus and Tuojiangosaurus (e.g. Galton & Upchurch, 2004). Morphologically, stegosaur parascapular spines are distinct from the ridged scapular osteoderm seen in Scelidosaurus . The stegosaur parascapular spine is prominent and has a bulbous ovoid base that is flattened medially so that it can adhere to the scapular blade. The distal portion of the parascapular osteoderm tapers to form a posterodorsally oriented elongate, conical spine. Parascapular spines have not been reported in stegosaurids viz. ‘Hesperosaurus’, Miragaia, Stegosaurus and Wuerhosaurus ; their absence/loss has been interpreted as a familial apomorphy (Maidment et al., 2008).