UNDERSTANDING THE SILICIFICATION OF DINOSAUR BONES IN THE CRETACEOUS OF URUGUAY

Much of the research into silicification of terrestrial fossils to date has focused on petrified wood. There has been little investigation into the processes leading to the silicification of dinosaur bones. The Late Cretaceous, terrestrial Mercedes and Guichon formations, exposed on the western border of Uruguay, hold clues to some of the pathways of silica permineralization and recrystallization of large vertebrates. These formations preserve a silicified assemblage of isolated titanosaur sauropod elements and occasional exquisitely preserved dinosaur eggs. Here we focus on the processes that led to bone silicification, although similar processes likely influence the preservation of the eggs.

Samples of titanosaur bone from each formation were examined in thin-section both optically and under SEM, and were analyzed using EDS and XRD. Despite the stratigraphic proximity of the two formations, the style of silicification differs between the Mercedes and Guichon formations. The Guichon Formation bone has been completely replaced by silica with minor amounts of barite. The Guichon bone retains its internal structure, but shows at least three stages of diagenetic modification. In contrast, the Mercedes Formation shows only a single stage of modification. The preserved bone was permineralized with silica without the loss of the original fluorapatite of the bone.

These analyses suggest that at least some of the modification of the Uruguayan bones occurred relatively early in their history, otherwise both sets of bones would show the same pattern of modification. This is supported by the presence of hollow silicified dinosaur eggs in the Mercedes Formation. We hypothesize that one source of the silica was from the intense weathering of the uppermost Asencio Formation that produced the striking laterites present there.