Asociación Paleontológica Argentina. Publicación Especial 7 ISSN 0328-347X VII International Symposium on Mesozoic Terrestrial Ecosystems: 171-174. Buenos Aires, 30-6-2001 Histology of dermal ossifications in an ankylosaurian dinosaur from the Late Cretaceous of Antarctica 1 Armand de RICQLES , Xabier PEREDASUBERBIOLN,Zulma GASPARINPand Eduardo OLIVEReY Abstract. Ankylosaurian remains from the Late Cretaceous (Campanian) Santa Marta Formation of the James Ross Island in the Antarctic Peninsula include several types of armour, the most abundant being tiny, button-like ossicles (less than 5 mm in diameter). An histological study of these small ossicles evi- dences an original tissue structure. We notice the very small amount of vascularisation and of bone re- modeling. Some structural aspects strongly suggest a direct (metaplastic) mineralization of the preexisting siratum compacium of the dermis. However, some contradictory evidences support instead the hypothesis of the structures originating de nava at the limit of the stratum compacium and stratum spongiosum of the der- mis and experiencing further grawth via neoplasy. Keywords. Ankylosauria. Late Cretaeeous. Antaretiea. Dermal armour. Histology. Introduction Material and methods In 1986, ankylosaurian remains were discovered The dermal armour is represented by five differ- in the vicinity of the Santa Marta Cove on James Ross ent kinds of elements: keeled, hollow-based scutes, Island, at the northern tip of the Antarctic Peninsula. massive bulging plates; co-ossified flat scutes, over- This was the first dinosaur to be found in Antarctica lapping each other and enclosed by small polygonal (Gasparini et al., 1987).The material, a partial skele- ossicles; oval, low-keeled scutes; and tiny, button- ton of a small single individual, was recovered from like ossicles (Gasparini et al., 1987, 1996). The latter shallow marine Late Cretaceous (Campanian) de- are the most abundant dermal elements and were posits of the lower Gamma Member of the Santa found scattered around the skeletal remains. As pre- Marta Formation, in the lowermost part of the served in the rock matrix, the dermal ossicles look at Marambio Group (Olivero et al., 1991).It consists of a first sight as mineralized scutes (figure 1.A). These lower jaw, teeth, cervical, dorsal, sacral? and caudal ossifications are oval to subrectangular in outline; vertebrae, ribs, parts of the scapula and ilium, au- some ossicles are nearly semicircular, others are flato tapodial bones, and dermal elements (Museo de La The internal side is convex and show faint concentric Plata 86-X-28-1). Preliminary descriptions of the ridges and crossing fibres (figures 1.B-D),while the ankylosaurian material were given by Gasparini et al. external side is flat or concave and very irregular. (1987)and Olivero et al. (1991).Gasparini et al. (1996) This small armour (diameter less than 5 mm) proba- discussed the affinities of the Antarctic ankylosaur bly floated in the skin and formed a continuous and referred to it as Nodosauridae gen. et sp. indet. pavement between the large dermal elements per- The aim of this paper is to describe the most abun- mitting supple movements of the body and limbs dant type of dermal armour found in association (see Carpenter, 1997). with the Antarctic ankylosaur and to examine the The preparation of the fossil remains was carried bone histology of these ossifications. out in the laboratory of the Museo de La Plata using mechanical techniques. Some osteoderms were freed IUMR 8570 CNRS, Université París 7, Laboratoire d' Anatomie from the matrix for photography and further studies. Comparée, 2 place [ussieu, 75251 Paris Cedex 05, and College de Photographs of figure 1 were made by D. Serrette France, 11 place Mareelin-Berthelot, 75005 París, Franee. (UMR 8569 CNRS). The histological work has been 'Universidad del País Vaseo/EHU, Departamento de Estratigrafía y Paleontología, Apartado 644, 48080 Bilbao, Spain; and Muséum completed in the Laboratoire d'Anatomie Compa- National d'Histoire Naturelle, Laboratoire de Paléontologie, 8 rue réee of the Université Paris VII (UMR 8570 CNRS). Buffon, 75005 Paris, Franee. Osteoderms were studied mainly on petrographic JMuseo de La Plata, Depar~amen~o de Palconlologia d~ thin sections. Vertical sections were selected from the Vertebrados, Paseo del Bosque, 1900 La Plata, Argentina. 'CADIC (Centro Austral de Investigaciones Científicas), e.e. 92, samples in order to examine the histological features 9410 Ushuaia, Argentina. of the ossicles. Sections were studied by optical mi- ©Asociación Paleontológica Argentina 0328-347X/01$00.00+50 172 A. de Ricqles, X. Pereda Suberbiola, Z. Gasparini and E. Olivero B D Figure 1. Ankylosaur dermal ossicles from the Late Cretaceous (Campanian) of Antarctica, a11specimens MLP 86-X-28-1. A, pavement of ossicles as preserved in matrix; B-D, three ossicles in bottom view, after preparation using mechanical techniques. Scale bars, 1cm (A) and 1mm (B-D). croscopy under ordinary and polarized light. "valleys" are gently sloped and rounded (figure 2.B). Infography of the figures 1and 2 by H. Lavina (UMR The matrix is entirely formed by a dense packing of 8569 CNRS). mineralized (collagenous) fibers. Most fibers appear to run parallell to the free surface but there is anoth- Histological observations er, less developed fibrous system which runs perpen- dicular to the surface of the scute (figure 2.C). There Thanks to the availability of a few isolated scutes are no evidence that the superficial ornamentation is freed from the matrix, it has been possible to assess the result of either differential rates of deposition the shape of the scutes and their general orientation (with more active fibrillar deposition on the projec- relative to the skin surface. Accordingly, it proved tions and less active deposition in the valleys) or of possible to decipher the orientations of numerous differential resorption (in the valleys). Vascula- scutes on thin sections directly obtained from grind- risation of the matrix, if any, appears extremely ing of the matrix still containing the scutes. In this scarce. process, while most scutes are cut at random accord- ing to various oblique orientations, a few of them are The nan arnamented (basal) regian (figure 2.0) cut in almost perfect vertical orientations and allow Its free surface is regular and convexoThis region precise histological observations (figure 2.A). What contains a series of superimposed "growth lines" we interpret as the external (superficial) surface of a parallel to each other and to the free surface. It is en- scute is rather flat or irregular and shows a character- tirely formed by closely packed, highly ordered sys- istic ornamentation by a system of polygonal or irreg- tems of mineralized collagenous fiber bundles. Small ular pits lined by acute ridges. The internal (deep or cell spaces within or between fiber bundles are prob- basal) surface of the scutes is generally convex, even ably numerous but difficult to observe. Vascularity is hemispherical and is not ornamented. It clearly shows very low, accounting to a few simple primary vascu- evidences of a system of strong straight horizontal lar canals. The spatial organisation of the vessels is ill fibers which cross orthogonally. On a vertical section, defined but they would roughly diverge from the the material forming the external surface and about center of the scute, some of them opening on the con- the external quarter to the external fifth of a scute ap- vex (deep) surface. pears distinctly different from the rest of the scute (figure 2.A). It is much more translucent and less col- The central regian (figure 2.D) ored. For practical purposes one may distinguish this "ornamented región" from a "central región" and a It is formed by the same systems of collagenous "non ornamented (basal) region". Both the "central" bundles than the basal region but differs by its vas- and "basa!" regions have generally similar structures cularisation. Some vascular canals have a large diam- and optical reactions. Together, they form about the eter and a few erosion bays occur. A secondary 3/4 to the 4/5 of the total thickness of a scute. (Haversian) reconstruction has taken place around most erosion bays. The arnamented (superficial) regian Spatial arganizatian af the fibrillar systems On a vertical section, its free (external) surface is marked by projections and inward angles which Polarization microscopy allows to decipher the match the morphological ornamentations. Projec- complex and highly ordered spatial organization of tions may be acute at their tips while the intervening the fiber bundle s systems in the central and basal re- A.P.A. Publicación Especial 7, 2001 Dermal ossifications in an ankylosaur from Antarctica 173 gions of the scute. There are at least three orthogonal a cone pointing to the center of the scute. This is close systems of fiber bundles which criss cross each others to the "orthogonal plywoods" described in the basal from the center of the scute to its surface. One system plate of elasmoid scales in many advanced of fibrillar bundles is "vertical" (figure 2.E).Its fibers, Osteichthyans (Meunier, 1984;Sire, 1990). roughly parallel to each others (in fact fan shaped), reach perpendicularly the surface of the non orna- Discussion and conc1usions mented region. Thus, they cross at right angle the successive "growth lines" parallel to the scute sur- Nature af the ornamenied regían face. This fibrillar material, which actually forrns a cone in space (the summit of which beeing at the cen- The stricking optical difference between the su- ter of the scute) reminds of the system of attachment perficial (omamented) region and the central and fibers (or Sharpeys fibers) observed in the basal plate basal ones would not be caused by occurence of dis- of rhombic scales in early Osteichthyans (Meunier, tinct tissues but mostly by differences in the spatial 1984;Sire, 1990).Two other systems of fiber bundles, organization of the mineralized bundles systems.