Phosphatized organic nanostructures in the Cambrian linguloid brachiopod Ungula inornata (Mickwitz)

Scanning electron microscopy of untreated and uncoated fracture sections of a secondary shell of a Cambrian (Furongian) linguloid brachiopod Ungula inornata (Mickwitz) in concurrent backscattered electron and secondary electron imaging regimes revealed phosphatized organic fibril-like nanostructures, less than 200 nm in diameter. By analogy with published data on a living lingulate genus Discinisca, the nanofibrils are interpreted as parts of the organic biopolymer matrix, which are composed of axial protein strands of bacula connected by chitin and participate in the formation of baculate sets. The nanofibrils are relevant structural units in the hierarchical structure of lingulate brachiopods with baculate shell structure. It is concluded that these structures are preserved by instant post-mortem precipitation of apatite. The shell structure of U. inornata is most similar to the other species of the genus Ungula Pander and the species of the genus Obolus Eichwald.