FOSSILIZATION AS A PRESERVATIONAL PROCESS: NEW INSIGHTS ON BIOAPATITE IN THE FOSSIL RECORD

The mineral matrix of calcium phosphate, the primary structural component of bioapatite, is resistant to complete replacement and dissolution. As a result, a rich fossil record exists, particularly for vertebrates whose skeletons are comprised of compositionally and structurally complex apatite. Chemical elements acquired through food and drinking water are incorporated into the mineral and organic matrix of carbonate hydroxyapatite in bones and teeth. The incorporation of authigenic minerals in the burial environment during fossilization results in compositional heterogeneities and concentration gradients in bones and teeth, which in turn are subject to subsequent chemical changes. Nonetheless, both enamel and bone retain exceptionally well-preserved histological and structural details from the micron to the nano-scale. This preservational complexity is revealed by the recovery of secondarily mineralized osteocytes and semi-transparent, pliable vessels in the Cretaceous dinosaurs Tyrannosaurus rex and Brachylophosaurus canadensis. These are not pseudomorphs, but structures with an endogenous origin that share identical location, texture, morphology and molecular characteristics with their extant counterparts. Iron and oxygen chemistry is hypothesized to effect this extraordinary preservation based on the close association between goethite particles and soft tissues recovered from demineralized dinosaur bone. High-resolution X-ray computed tomography and hard tissue histology of dinosaur skulls reveals internal sutural details, tissue complexity and growth patterns critical to life history and growth stage determinations. Keratin apatitic structure similar to modern baleen is also confirmed in an exceptionally well-preserved example of fossil baleen from the upper Miocene of northern California. The original hydroxyapatite is altered to francolite. Iron-rich rings in the baleen tubules and FeOOH infilling point to the influence of Fe in bioapatite preservation. While all fossils are altered to some degree and diagenetic effects of burial cannot be ignored, the transition from unaltered bone to a fossil is better understood as a preservational process that modifies bioapatite, and differentially affects and preserves organic biomolecules across geological time.