Dinosaur bones: Fossils or pseudomorphs? The pitfalls of physiology reconstruction from apatitic fossils

A prerequisite to any attempt to reconstruct thermophysiology of ancient animals through analysis of stable isotopes of oxygen is the assumption that apatitic fossils are isotopically unaltered parts of their original skeletons. We suggest that sufficient evidence is available to seriously question this assumption. Because living bones contain about 1/3 by weight of organic matrix, mass balance considerations show that at least half of an apatitic fossil must be new material added post mortem. Furthermore, living apatitic skeletons contain almost no rare earth elements and uranium, whereas apatitic fossils bear high concentrations of these elements. Additionally we found that while in living fish correlation between δ¹⁸O of oxygen in the phosphate and that in the carbonate of apatite is poor, these two values are linearly correlated in fossil fish. This suggests diagenetic recrystallization has taken place. Perfect preservation of micro-textures such as Haversian canals does not prove the pristine condition of a fossil; some of the most beautifully preserved fossils are highly silicified. Most apatitic fossils probably are pseudomorphs replacing original skeletal elements. Whereas distribution of stable isotopes of oxygen in fossils might provide information about their burial environment, it could be misleading in attempts to interpret the organisms' physiology. This cautionary note is supported by the similarity of δ¹⁸O(P) in bones of coexisting fossil fish, dinosaurs, and various other reptiles at different latitudes. The overlap of predicted δ¹⁸O(P) values in fish and mammals living in contact with the same environmental water also weakens the hope that ectotherms can be distinguished from endotherms by the isotopic composition of oxygen in their bones and teeth.