Dead, fossil or alive: Bioapatite diagenesis and fossilization

Calcium carbonate, silica and calcium phosphate have been selectively used by organisms in the production of mineralized hard parts throughout the Phanerozoic. Among these materials, bioapatite has enabled fundamental acquisitions in the evolution of life. Despite the remarkable biological success, the crystallography of bioapatite and the eventual modification of its lattice parameters over geological time have in contrast been scarcely investigated. In our study, we analyzed living, dead and fossil remains of both vertebrate and invertebrate organisms that biomineralized apatite, ranging from the Cambrian to the Recent, a time interval spanning over 500 million years. We detected in this way the bioapatite crystal features of the major phosphatic phyla (Brachiopoda, Arthropoda, Bryozoa, and Chordata: the latter including conodonts, cartilaginous and bony fishes, amphibians, reptiles, birds and mammals). Groups were investigated using either fossil or recent material (dead and/or alive, the former indicating organisms who died in recent times, the latter referring to material extracted from living organisms). The experimental results revealed that phosphatic materials from living, dead, and fossilized organisms have a distinct crystallographic signature. In fact, bioapatite in fossils is characterized by lower values of the crystal lattice cell parameter a (9.320–9.439 Å compared to 9.355–9.466 Å in dead and alive organisms), whereas the cell parameter c is less variable (6.857–6.911 Å for fossils and 6.861–6.902 Å for recent bioapatite); Student t-tests, applied to the means of these ranges of values (a¯=9.369 Å, c¯=6.887 Å, volume¯=523.6 Å3 for fossil values, a¯=9.415 Å, c¯=6.878 Å, volume¯=528.0 Å3 for recent values), highlighted significant differences between fossils and recent samples at a level p < 0.01 for the three cell parameters. These changes, which begin at the death of the organism and only stabilize in the ultimate stages of fossilization, mirror the isomorphic chemical substitutions within the crystal lattice and drive to a general decrease of the cell volume (i.e., the volume of the bioapatite hexagonal crystalline cell frame) over time, with an average reduction of 4.4 Å3 (0.8%) from alive to fossil organisms.