Occipital spine of Orthacanthus (Xenacanthidae, Elasmobranchii): Structureand growth

The morphology of 16 occipital spines of the xenacanthid Orthacanthus from Upper Carboniferous deposits of Robinson (Kansas, USA), Nyran (Czech Republ ic) and Puertollano (Spain) is described. The nonreplaced spines reveal the growth pattern of the shark. Moreover, the relationship between growth and paleoenvironmental conditions can be used to determine paleoecological con ditions. Both external and internal morphology indicate that the spine was superficially inserted in the skin. During growth, the spine moved from a d eep position in the dermis, in which trabecular dentine is formed, to a mor e superficial location in which centrifugally growing lamellar dentine was formed. Centripetally growing lamellar dentine was deposited more slowly th an the centrifugally growing dentine; it obliterated the pulp cavity. The d enticles are independent dermal elements formed by a dermal papilla and sec ondarily attached by dentine to the spine proper. The number of denticles p er annual cycle and the density of denticulation vary with the growth rate. Moreover, the ratio of length of denticulated region to total length of th e spine changes throughout ontogeny. In consequence, those features cannot be used for systematic purposes without a careful analysis of the variabili ty. Centrifugally growing lamellar dentine in spines from Robinson shows a regular alternation of layers, suggesting tidal conditions in the environme nt in which the sharks lived. Monthly and seasonal cycles also occur. Tidal (lunar) cyclicity is also observed in the denticles: size and distance bet ween denticles increase and decrease gradually, forming waves that are cons idered seasonal and yearly cycles. The observed regularity could be related to the variation in calcium phosphate deposition following the cyclical ch anges in water temperature produced in the tidal zone. Monthly and seasonal cycles are the result of the interaction of the solar and tidal (lunar) cy cles. The cyclical pattern of growth is used to determine the age and growt h rates. Orthacanthus was a fast-growing shark like the Recent sharks Isuru s, Mustelus, and Negaprion. (C) 1999 Wiley-liss,Inc.