Life-span and growth cycles in Praeradiolites ciryi and Hippurites quintanalomensis (Hippuritacea, Bivalvia, Upper Cretaceous of Gredilla de Sedano, Northern Burgos, Spain)

Several specimens of Praeradiolites ciryi FLOQUET (1991) and Hippurites quintanalomensis FLOQUET (1991), taken from the Quintanaloma Formation sediments (Upper Campanian-Lower Maastrichtian) at Gredilla de Sedano section (Burgos, Northern Spain) have been studied for determination of their growth velocity, life-span and different types of cyclicity. H. quintanalomensis has a mean growth velocity of 0.30-0.45 cm/year, with a maximum of 0.79 cm/year and having a life-span of 20 years. On the contrary, P. ciryi shows more variable growth velocities, usually between 0.2 and 0.8 cm/year, with a maximum of 1.52 cm/year, varying the life-span from 9 to 13 years. Daily, tidal, monthly and annual cycles have been detected into sclerochronological profiles of these rudist shells. Fortnightly tidal cycles currently show an incomplete number of daily cycles (8-9 for H. quintanalomensis; 10-75 for R ciryi) represented by an alternance of clear/obscure laminae couples. Monthly cycles enclose a pair of tidal «doublets» and annual cycles (megarhythms) may include up to 25 «doublets» limited by densely-condensed obscure growth-lines. Annual cycles are more easily observable when P. ciryi develops «honeycomb» microstructures. This cellular microstructure tends to disappear coinciding with the wintry limit between year