Dinosaur osteoderms from the Adamantina Formation, upper Cretaceous of São Paulo State, Brazil

Abstract

In the austral spring of 2001, the Laboratório de Macrofósseis/IGEO/UFRJ conducted an expedition to some Cretaceous fossil localities in the state of São Paulo. The expedition produced several fossils, including icnofossils, molluscs, fishes, turtles, crocodilomorphs, and dinosaurs. We describe herein two dinosaur osteoderms from the Fazenda Furnas (Adamantina Formation, Upper Cretaceous, Bauru Basin), Municipality of Jales, São Paulo State. Reddish clays and sands of fluvial-lacustrine origin, deposited during times ofwarm and humid climates, dominate the Adamantina Formation. Based primarily on its vertebrate fossil content, the Adamantina deposits were formed between the Santonian and Maastrichtian. The osteoderms are subquadrangular in shape, presenting the diagnostic ornamentation (coarse and fibrous texture, grooves, pits and tiny projections) observed in titanosaurid osteoderms already described. The function of all of these features is unclear - foramina were probably passages for blood vessels and rugosities for anchoring the osteoderms. However, both present some particularities: the largest one has a prominent ridge that separates the dorsal surface into two portions. Foramina are sparsely distributed along both surfaces. The rugose texture is found around the borders, except in a more straight one which presents a pattern of dorsoventral laminae. The ventral surface is broken superficially, and differs from dorsal surface by having a marked depression and a comparatively smoother fibrous texture. The other specimen is smaller and thinner. Some dorsal fractures permit to observe its cancelous bone internal constitution. Stereoscopic microscopic analysis of the straight border shows no bone fractures, so we interpreted it as an articular surface. This indicates that the dinosaur that had it may bear dermic plates composed by two or more articulating osteoderms, or these dermal bones belonged to a different dinosaur to which osteoderms have not been described. As this feature has never been reported in a Titanosauridae, and the osteoderm, though similar, is distinct to the one previously described to Bauru Basin, we found no strong basis to assign it to a titanosaurid. These findings add a new dinosaur locality for the Bauru Basin, and comprise the first record of dinosaur osteoderms in the Cretaceous of the São Paulo State.