A new type of shell structure in a phosphatic brachiopod from the Cambrian of Estonia

Shell structures of Ungula ingrica, U. inornata, and Ungula sp. 1 (Kallavere and Ülgase formations) from Cambrian sandstones of the Mäekalda section in Tallinn, Estonia, were studied. Specific alterations were recorded in the shell structures of U. ingrica and Ungula sp. 1. Several generations of precipitation of phosphatic matter inside the shells could be distinguished, marking (1) original biomineralization events, (2) phosphatization of soft tissues, and (3) later diagenetic alteration of mineral matter and/or structures inside the shells. The shells of U. inornata and U. ingrica were found to represent the baculate structure type typical of the genus. The shell structure of Ungula sp. 1 is different, consisting of alternating compact and primarily organic-rich laminae. No bacula were found in the latter laminae. Compact laminae are penetrated by tubuli, filled with phosphatized organic matter. This structure type is termed here as tubulate shell structure.