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

Abstract

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

Diagenetic fate of bioapatite in linguliform brachiopods: multiple apatite phases in shells of Cambrian lingulate brachiopod Ungula ingrica (Eichwald)

“…Indeed, previous studies on Cambrian (Furongian) linguloid brachiopods Obolus ruchini and Ungula inornata (Lang et al . ; Lang & Puura ) have shown that phosphatized fibrous networks are preserved with various fidelity in baculate laminae, depending possibly on the nature of early diagenetic processes.…”

“…The time scales and intensity of these changes are difficult to assess at this point, but since; 1, the overall post‐mortem compositional change of the apatite in compact laminae of linguliform brachiopods is rather limited; 2, baculate laminae show at least partial preservation of delicate mineralized organic fibrous frameworks (Lang & Puura ) and 3, shell laminae contain authigenic pyrite (Lécuyer et al . ) suggesting alteration mediated by sulphate‐reducing bacteria that commonly thrive in the uppermost few‐tens of centimetres thick sediment column at the sea bottom (Jørgensen ), then it is possibly a rapid early diagenetic phenomenon.…”

“…Therefore, it could be possible that some of the original fibrous network was preserved in the shells of U.ingrica and offered a template for the authigenic apatite to nucleate. Indeed, previous studies on Cambrian (Furongian) linguloid brachiopods Obolus ruchini and Ungula inornata (Lang et al 2011;Lang & Puura 2013) have shown that phosphatized fibrous networks are preserved with various fidelity in baculate laminae, depending possibly on the nature of early diagenetic processes.…”

“…Lang et al . () and Lang & Puura () described the originally organic but apatite‐replaced nanofibrils, representing the finest parts of the hierarchical construction of the shells of Cambrian lingulates Obolus ruchini Khazanovitch & Popov (in Khazanovitch et al . ) and Ungula inornata (Mickwitz ).…”

“…Comparing Carboniferous and modern Lingula, Cusack & Williams (1996) demonstrated that the structure and mineral parts of a shell may be preserved in nanometric detail. Lang et al (2011) and Lang & Puura (2013) described the originally organic but apatite-replaced nanofibrils, representing the finest parts of the hierarchical construction of the shells of Cambrian lingulates Obolus ruchini Khazanovitch & Popov 1984(in Khazanovitch et al 1984 and Ungula inornata (Mickwitz 1896). However, the preservation at ultrastructural level is not common and may vary even within a single shell (Lang et al 2011).…”