‘Changing of the guard’ amongst holasteroid echinoids in the upper Maastrichtian of the south-east Netherlands: Exit Echinocorys, enter Hemipneustes

Abstract

The search, over several decades, for overlap (if any) in the stratigraphical ranges of two common holasteroid sea urchin genera in the type area of the Maastrichtian Stage (south-east Netherlands, north-east Belgium), has come to an end. Here, the first examples of representatives of Echinocorys and Hemipneustes are recorded in situ from the middle portion (between flint levels 10 and 12/12a) of the Lanaye Member (Gulpen Formation) at the former ENCI-HeidelbergCement Group quarry (Maastricht, the Netherlands). At this level, the typically Tethyan genus Hemipneustes is accompanied by other echinoid ‘newcomers’ such as Nucleopygus scrobiculatus, N. coravium, Oolopygus pyriformis, Catopygus fenestratus, Bolbaster prunella, B. koninckanus and Leymeriaster eluvialis. Most of these genera can be linked to shallower settings, and two at least (Nucleopygus, Catopygus) connected with shallower, coarser-grained and more permeable facies. The ‘last’ Echinocorys and the ‘first’ Hemipneustes in the Lanaye Member are separated by a single flint/chalk couplet, indicative of the c. 21-kyr precession cycle. At flint level 10 of the Lanaye Member, a negative carbon isotope excursion, suggesting a regressive phase, has recently been determined and tentatively equated with M3-(b) in the Stevns-1 borehole (Denmark), situated within calcareous nannoplankton zone UC20a and close to the last appearance datum (LAD) of the dinoflagellate marker species Triblastula utinensis. The exchange in echinoid faunas at this level may thus have been controlled by bathymetry (and concomitant differences in temperature), as well as by increased nutrient levels on the seafloor.