engleski

Intra-association development and paleobiology of Upper Cretaceous rudist Biradiolites angulosus

Undisturbed rudist associations can yield much information about original relations and interactions between individuals. Here, radiolitid paleobiology and paleoautecology are investigated from autochthonous specimens in Turonian peritidal limestones of Vis Island, Croatia. The outer shell layer of the radiolitid right valve is composed of stacked lamellae that are more or less inclined. In combination with other constructional elements, the lamellae form compact or cellular structures. Such a constructional mechanism provided a control on the vertical growth rate of the right valve. Biradiolites angulosus preferred to be attached to a firm host, and to maximize the vertical growth rate during ontogenesis. B. angulosus used its capacity for plastic growth also to reach a new host laterally in case the former was not present ahead. Besides, it used the capacity to develop ribs with the aim to improve stability in ambient sediment if growing alone. Predominance of antero-dorsal attachment is observed. The postero-ventral radial bands thus tended to be on the opposite side. In environments characterized by directed water currents, larvae that settled upstream of its attachment site probably had better chances of survival. Consequently, preferred life-orientations observed in some rudist in-situ associations are probably related to environments characterized by generally directed water-currents. Considering shell asymmetry related to a more or less accepted interpretation of the functional morphology of the shell, a refiltration risk for inappropriately oriented individuals is also suggested. Such a natural selection could have affected communities of both Late Cretaceous major groups of predominantly upright-growing rudists-radiolitids and hippuritids.

shallow-water organisms, original rudist associations, radiolitids, paleobiology, Late Cretaceous

SEPM = Society for Sedimentary Geology