Big oyster, robust echinoid: an unusual association from the Maastrichtian type area (province of Limburg, southern Netherlands)

Abstract

Large, denuded tests of holasteroid echinoids were robust benthic islands in the Late Cretaceous seas of northwest Europe. A test of Hemipneustes striatoradiatus (Leske) from the Nekum Member (Maastricht Formation; upper Maastrichtian) of southern Limburg, the Netherlands, is encrusted by a large oyster, Pycnodonte (Phygraea) vesiculare (Lamarck). This specimen is a palaeoecological conundrum, at least in part. No other members of the same oyster spatfall attached to this test and survived. Indeed, only two other, much smaller bivalve shells, assignable to the same species, attached either then or somewhat later. The oyster, although large, could have grown to this size in a single season. The larval oyster cemented high on the test and this would have been advantageous initially, the young shell being elevated above sediment-laden bottom waters. However, as the oyster grew, the incurrent margin of the commissure would have grown closer to the sediment surface. Thus, the quality of the incurrent water probably deteriorated with time.