INTERNAL STRUCTURE OF CAMBRIAN CONODONTS

Based on material of the Upper Cambrian of Sweden the internal structu re of proto-, para-and euconodonts has been investigated. The protocon odontids Gapparodus and Gumella show some special development of the m edian unit of the skeletal tissue. In Gapparodus, long, parallel-runni ng lamellae suggest a continuous growth, even of previously secreted l amellae, until their eventual outcropping. In Gumella the fibrous medi an layer decreases considerably during growth until its complete reduc tion, which coincides with the disappearance of the posterior wall. Pa raconodonts also produced several specialities in their mode of growth . Elements with lateral processes such as Proacodus and Serratocambria developed by allometric growth. Tne lamellae are much more widely spa ced 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 p araconodontid growth with lamellar remains in the tip of the median de nticle, which are isolated from the growth lines in the lateral dentic les. Based on this observation a hypothetical growth model explains th e lamellar development as a result of stress and strain. The Ordovicia n euconodontid Chosonodina clearly shows white matter and is thus unre lated, but homeomorphic to Westergaardodina. Cambropustula from the lo wer Upper Cambrian is the oldest euconodont yet but it lacks white mat ter. The latter was an evolutionary novelty, which progressively devel oped in the euconodont line. The systematic position of conodonts is b riefly discussed; the studied material cannot contribute to the hypoth esis of neural crest derived skeletal tissue.