On the edge: early land plants and marine algae from the Lower Silurian Kalana Lagerstätte, Estonia

The skeletal apatite is known to have a very variable nature, with biochemical, environmental and diagenetic signatures merged in the shell composition. Several investigations have demonstrated the existence of two discrete apatite phases in the lingulate shells that are thought to be related with different types of lamellae in the shell structure. We show for the first time using combined infrared (ATR FT-IR) and energy dispersive spectroscopic (EDS) mapping of the cross sections of Ungula ingrica shells the compositional variation of apatite that follows the lamination of brachiopod shell. The compact laminae are rich in PO43-, Na, Mg and poor in F and Ca, the less compact (baculate) laminae are rich in carbonate, F and Ca, but contain relatively less Na and Mg. The differences in chemical composition of the apatite in different types of laminae suggest different origin of the apatite phases that can be interpreted as the original skeletal apatite and secondary apatite, respectively. It is likely that the compact laminae have the highest preservation potential, and are composed of original apatite secreted by the organism, whilst the less compact laminae are mostly composed of secondary apatite.