Fig. 4.

Archosaur osteoderms. (A–C) Edmontonia rugosidens (Ankylosauria, Late Cretaceous). (A) Reconstruction on display at the Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, demonstrating various plate-like and spine-shaped osteoderms. (B) Skull (American Museum of Natural History, New York, 5381) in right rostrodorsal view. (C) Computed tomography reconstruction of the skull in (B) with the rostrum truncated to indicate the in situ position of the cheek region osteoderms, and a series of small, granular osteoderms across the throat (small arrows). The asterisks (*) in both (A) and (B) identify the presence of an osteoderm embedded in the cheek region, lateral to the tooth rows. (D) Paleosuchus palpebrosus skull (Crocodylia, Extant: Royal Ontario Museum, Toronto, Paleobiology collection R6692) demonstrating a well-developed osteoderm (= palpebral) within the eyelid (large arrowhead). (E) Alligator mississippiensis (Crocodylia, Extant). Cervical osteoderms demonstrating a common pattern of ornamentation among archosaurs: superficial pitting. (F) Caiman c. crocodilus (Crocodylia, Extant: Royal Ontario Museum, Toronto, Paleobiology collection R7719). Transverse section through a cervical osteoderm. The structural organization includes an outer and an inner cortex of compact bone (com) surrounding a cancellous core (can). (G,I–K) Longitudinal sections (dorsal towards the top). (G) A. mississippiensis adult, cervical osteoderm, Masson's trichrome staining. Note the resorption of woven-fibred bone and newly deposited lamellar bone matrix. (H) A. mississippiensis subadult, cervical osteoderms prepared as whole-mounts using Alizarin red (single-stained). The initial site of mineralization (red staining) is within the keel of the largest presumptive element. (I) Same specimen as (H), sectioned and stained with a modified Masson's trichrome (Cole & Hall, 2004). Mineralization (red staining) is initiated without the formation of a cell condensation. This mode of ossification is consistent with bone metaplasia. (J) A. mississippiensis subadult [slightly older than (I)], cervical osteoderm, Mallory's trichrome. Numerous extrinsic collagen fibres are becoming incorporated into the osteoderm matrix. Note the absence of a clear osteoblastic front. (K) Sequence of osteoderm development beginning as a weakly defined primordium of dense irregular connective tissue (top panel), followed by mineralization within the centre of the keel (middle panel), and finally expansion of the osteoderm into the majority of the keel and the deposition of bone (bottom panel). Scale bars: B–C = 50 mm, D = 100 mm, E = 30 mm, F,K = 1 mm, G,I,J = 40 µm, H = 5 mm. en (external naris), or (orbit). Micrograph (F) courtesy of M. Burns, University of Alberta, Edmonton, Canada.