Eight radiolitid shells (Hippuritacea, Bivalvia) collected from the Upper Campanian-Lower Maastrichtian sediments of the Quintanaloma Formation at Gredilla de Sedaño section (Northern Burgos, Spain) have been studied with regard to their arrangement of growth lines for a first approximation to their life span. The walls that separate internal skeletal cells («honeycomb») as well as the compact layers showing the alternance of obscure and clear lines (microrhythms) have allowed to us to make a first account of laminae sets. The very thin microrhythms are grouped in sets separated by growth lines. The more vigorous growth of the rudist, the thicker development of laminae within the walls, even overlying the growth line. The calculations suggest that Biradiolites could have had a more continuous growth in comparison to Radiolites, and could live in normal environmental conditions as much as 18-19 years in age. On the other hand, the individuals of Radiolites could live between 15 and 24 years in age. Some specimens of Biradiolites could grow up to 1.5 cm. in length during the better youth year, in contrast with Radiolites, which could not reach up to 1 cm each year lacking growth lines in practically all the annual cycles. Nevertheless, some slight Interruptions into the laminae series of Biradiolites suggest reproductive periods, or perhaps a response to variations In sedimentation rate. Radiolites shows non-erosive annual truncations in their sclerochronological profiles suggesting "biological" rather than physical gaps. The absence of evaporitic manifestations in the sediments leads us to think in possible annual changes in water salinity owed to seasonal factors (rain periods)