Western Science Center 16505 consists of small fragments of a dorsal rib, six complete or partial identifiable osteoderms (WSC 16505.1–WSC 16505.6), and fragments of additional osteoderms. Although this specimen includes fewer osteoderms than the two referred specimens, UMNH VP 28350 and UMNH VP 28351 (Fig. 2), it was selected as the holotype because its osteoderms are the best-preserved among the three specimens and most clearly exhibit two of the three features in the unique combination of characters that distinguishes I. zephyri.

Cervical/pectoral osteoderms

Western Science Center 16505.1 is identified as a possible right medial cranial/pectoral osteoderm. The medial cervical/pectoral osteoderms of Edmontonia rugosidens (AMNH 5665) (Plates 5, 8 in Gilmore (1930)), Panoplosaurus mirus (Plates 5, 6 in Lambe (1919)), and G. mimus (Figs. 2 and 5B in Burns (2008)) have straight medial margins, suggesting that the straight preserved margin on WSC 16505.1 is the medial margin (Figs. 3A–3D). WSC 16505.1 preserves the caudal and medial margins, but is broken along the cranial and lateral margins (Figs. 3A–3F). The caudal margin is straight immediately medial to the caudal end of the keel, but then curves craniomedially into the straight medial margin, suggesting a rectangular or subrectangular shape reminiscent of the medial cervical/pectoral osteoderms of Edmontonia rugosidens (AMNH 5665) (Gilmore, 1930) and G. mimus (Morphotype F of Burns (2008)). Lateral to the keel, the caudal margin curves craniolaterally. The caudal margin is overall convex, with the caudal-most point on the osteoderm being the caudal end of the keel, as on the medial cervical/pectoral osteoderms of E. rugosidens, P. mirus, and G. mimus. The preserved margins are rugose, with an intricate morphology of furrows, small projecting bumps, and, at the caudal end of the keel, abundant neurovascular pits (Figs. 3A–3F). The osteoderm thins considerably toward its preserved margins. At the thickest preserved point on the keel, the osteoderm is 1.8 cm thick, while it is only 0.6 cm thick along its medial margin. This thinning occurs gradually, such that the medial flank of the keel exhibits a very gentle gradient and the keel is poorly defined compared to the rest of the osteoderm’s external surface. The keel itself is low, rounded, and thins cranially so that it appears not to have reached the cranial margin (Figs. 3A–3F).

Figure 3: Cervical/pectoral osteoderms of WSC 16505, holotype of I. zephyri. WSC 16505.1, right medial cervical/pectoral osteoderm in (A) external, (B) basal, (C) medial, (D) lateral, (E) cranial, and (F) caudal views. WSC 16505.2, left medial cervical/pectoral osteoderm in (G) external, (H) basal, (I) medial, (J) lateral, (K) cranial, and (L) caudal views. WSC 16505.3, left distal osteoderm from the second cervical half-ring in (M) external, (N) basal, (O) medial, (P) lateral, (Q) cranial, and (R) caudal views. Study site: bng, bifurcated neurovascular groove. Cranial is toward the top of the figure in A, B, G, H, M, and N. Scale bars equal five cm.

Western Science Center 16505.1 is the only osteoderm in WSC 16505 that exhibits any degree of projecting rugosity on its external surface. The entire external surface is covered in small subcircular neurovascular pits of more or less uniform size, though a few larger pits occur adjacent to the caudal margin and on the caudal end of the keel (Figs. 3A and 3F). On the keel and near the caudal margin of the osteoderm, the pits are interspersed with small patches of projecting rugosity. This rugosity diminishes toward the cranial margin. Toward the cranial margin, the pitted texture continues, but the surfaces among the pits are smooth, as is the case on the entire external surfaces of all other osteoderms in WSC 16505. The transition from pitted and rugose to pitted and smooth texture is abrupt. There are no neurovascular grooves present on the preserved portion of the osteoderm.

The basal surface of this medial cervical/pectoral osteoderm is similar to the external in exhibiting a large number of small pits of uniform size distributed randomly over the entire surface (Fig. 3B). Projecting bumps are present on the basal surface as well, though compared to the external surface these bumps are more prominent, fewer in number, more widely spaced, and extend the entire preserved craniocaudal length of the osteoderm. The basal surface has a woven appearance, with visible structural fibers. Adjacent to the medial margin of the osteoderm, the basal surface is flat. However, lateral to this flat expanse, the basal surface becomes arched, with a broad, shallow concavity extending craniocaudally and corresponding to the keel on the external surface (Fig. 3B).

Western Science Center 16505.2 is identified as a medial cervical/pectoral osteoderm based upon similarities with the medial cervical/pectoral osteoderms of Glyptodontopelta (Fig. 5B in Burns (2008)), including a straight, sharp keel that is dorsally convex toward its cranial end (Figs. 3G–3L). The preserved portion of the cranial margin of WSC 16505.2 is gently convex, similar to the craniolateral margins of the left medial cervical/pectoral osteoderms of Glyptodontopelta (Figs. 2 and 5B in Burns (2008)) and different from the square craniomedial margins of those same osteoderms. This suggests that WSC 16505.2 is a left medial cervical/pectoral osteoderm. WSC 16505.2 is broken medially and caudally, and is missing most of its lateral margin (Figs. 3G–3L). The texture of the cranial margin is similar to that of the medial and caudal margins of WSC 16505.1, though with less pronounced rugosity. As in WSC 16505.1, WSC16505.2 becomes much thinner along its margins and away from its keel. At its greatest preserved depth, the keel’s apicobasal thickness is 2.2 cm. At the cranial margin, the osteoderm thins to only 0.6 cm. However, in contrast to WSC 16505.1, the keel of WSC 16505.2 reaches the cranial margin and is well demarcated along its entire preserved length except at the cranial margin, where it diminishes and merges with the margin (Figs. 3G and 3I–3K). The keel is more sharply defined on WSC 16505.2 than on WSC 16505.1, with steeper lateral and medial flanks.

The entire preserved external surface of WSC 16505.2 is akin to that of the cranial portion of WSC 16505.1: smooth with no rugosity and numerous, randomly distributed, small pits (Fig. 3G). Most of the pits on WSC 16505.2 are miniscule, smaller than those on WSC 16505.1, although larger pits are present along the apex of the keel. WSC 16505.2 also differs from WSC 16505.1 in the presence of a small number of randomly distributed neurovascular grooves. Some of these grooves are simple, non-branching furrows, while others bifurcate, forming a Y-shaped groove with the opening of the “Y” directed laterally (Fig. 3G).

The basal surface of WSC 16505.2 lacks the projecting bumps present on WSC 16505.1. Otherwise, the basal surfaces of the two osteoderms are similar, with abundant small pits and a broad, shallow groove extending craniocaudally and corresponding to the keel on the external surface (Fig. 3H).

Western Science Center 16505.3 is identified as the distal osteoderm of the left side of the second cervical half-ring. It is nearly complete, missing only some small portions of the medial margin (Figs. 3M–3R). This osteoderm is more oval in shape than WSC 16505.1 and 16505.2, making it difficult to delineate the cranial and caudal margins from the medial margin. In contrast, the lateral margin is easily demarcated; the cranial-most margin of the osteoderm is incomplete, but the caudal-most margin is preserved and forms an abrupt, only slightly obtuse angle with the lateral margin. The preserved margins are similar to the rugose margins of WSC 16505.1 and 16505.2.

The keel of this distal cervical osteoderm is strongly laterally offset, as on the distal osteoderms of the second cervical half-ring of E. rugosidens (AMNH 5665). However, the distal cervical osteoderm of I. zephyri differs from those of AMNH 5665 in that the keel curves laterally, rather than medially, toward its cranial end (Fig. 3M). The keel is very pronounced, with a steep medial flank and precipitous, nearly vertical lateral flank. At its deepest point, the keel is 2.2 cm thick. At a point directly lateral to this along the lateral margin, the osteoderm is only 0.8 cm thick.

The external surface of this distal cervical osteoderm is very similar to that of WSC 16505.2: smooth with numerous miniscule pits and a smaller number of larger pits along the apex of the keel, and a small number of randomly distributed neurovascular grooves (Fig. 3M). The basal surface also is similar in texture to that of WSC 16505.2, with abundant small pits randomly distributed over the entire surface and a broad, shallow, longitudinal concavity corresponding to the position of the keel on the external surface. WSC 16505.3 measures 6.3 cm at its greatest mediolateral width and 8.4 cm at its greatest craniocaudal length.