Paleohistology of the first ankylosaur (Dinosauria: Ornithischia) interstitial ossicles registered in South America (Cerro Fortaleza, Argentina), and the most austral continental record of the clade

IGNACIO A. CERDA; YOUNG-NAM LEE; ARIEL MENDEZ; ARIANA PAULINA CARABAJAL

virtual

Jornada; 1º Reunión Virtual de Comunicaciones de la Asociación Paleontológica Argentina; 2020

The most austral South American ankylosaur record correspond to the preliminary report of an isolate tooth recovered from a microsite (MPM-PV-18805, Museo Padre Molina, Río Gallegos) at Cerro Fortaleza (Cerro Fortaleza Formation, Campanian-Maastrichtian), in Southern Patagonia. Associate materials were recently identified as interstitial armor ossicles (6 mm diameter). They are heterogeneous in morphology from oblate spheroid-shaped (heptagonal contour) to strongly narrow and tall (rectangular outline). The ventral surface exhibits straight fibers crossing orthogonally, giving an interwoven texture. The dorsal surface is rugose and exhibits irregular depressions bounded by sharp ridges, a pattern also present in ossicles of Kunbarrasaurus and Antarctopelta (nodosaurs from Australia and Antarctica, respectively). The histological analysis shows that the ossicles are composed of compact primary bone tissue, with some vascular spaces located in medullary region. This bone tissue is composed of closely packed bundles of mineralized collagen fibers, which exhibits a complex and ordered spatial organization. There are three systems of fiber bundles: one vertical and two horizontals (arranged roughly perpendicular of each other). The vertical bundles are narrow in the deep portion of the cortex, becoming thicker towards the superficial cortex. Ill-defined growth marks are observed in the outer cortex. Although it is no possible to assign the specimens to a clade less inclusive than Ankylosauria, all these histological features are identical to those described for Antarctopelta ossicles. Despite its taxonomical uncertainties, this report not only increases the dinosaur diversity in south Patagonia, but also represents the first record of ankylosaur interstitial ossicles for South America.