Internal structure of Cambrian conodonts

Based on material of the Upper Cambrian of Sweden the internal structure of proto-, para- and euconodonts has been investigated. The protoconodontids Gapparodus and Gumella show some special development of the median unit of the skeletal tissue. In Gapparodus, long, parallel-running lamellae suggest a continuous growth, even of previously secreted lamellae, until their eventual outcropping. In Gumella the fibrous median layer decreases considerably during growth until its complete reduction, which coincides with the disappearance of the posterior wall. Paraconodonts also produced several specialities in their mode of growth. Elements with lateral processes such as Proacodus and Serratocambria developed by allometric growth. The lamellae are much more widely spaced in the process than in the main cusp to attain the required length of the former. In Serratocambria additional lamellae are inserted in the process. Thin sections of tricuspidate westergaardodinids reveal paraconodontid growth with lamellar remains in the tip of the median denticle, which are isolated from the growth lines in the lateral denticles. Based on this observation a hypothetical growth model explains the lamellar development as a result of stress and strain. The Ordovician euconodontid Chosonodina clearly shows white matter and is thus unrelated, but homeomorphic to Westergaardodina. Cambropustula from the lower Upper Cambrian is the oldest euconodont yet but it lacks white matter. The latter was an evolutionary novelty, which progressively developed in the euconodont line. The systematic position of conodonts is briefly discussed; the studied material cannot contribute to the hypothesis of neural crest derived skeletal tissue.