THE PHANEROZOIC DIVERSITY OF AGGLUTINATED FORAMINIFERA: ORIGINATION AND EXTINCTION RATES OF A “CONSERVATIVE” GROUP OF ORGANISMS

The agglutinated foraminifera (AF) represent a large group of marine protozoans (over 750 genera), possessing the longest and most complete fossil record of any foraminiferal group, extending back to the Pre-Cambrian. AF are thought to be “conservative” in terms of evolution, and genera that evolved in the Cambrian are still present in the modern ocean. It is, therefore, instructive to quantify what is meant by “conservative” in terms of origination and extinction rates. New diversity curves for AF genera are presented based on the stratigraphic ranges of 764 genera distributed over the 96 Phanerozoic subdivisions in the 2008 ICS timescale. The data are based on stratigraphic ranges published by Loeblich & Tappan (1987), 218 of which were modified based upon newer studies. Additionally, 136 genera were newly described or reinstated since 1987. The mean standing diversity was compiled using the number of boundary crossers rather than the number of genera in each stage. This diversity curve displays a general upward trend throughout the Phanerozoic, punctuated by peaks and troughs. The curve shows a radiation from the Early Cambrian to the Early Silurian, followed by a plateau to the Late Permian. The Permian/Triassic and the Triassic/Jurassic boundaries show small dips in the diversity record. The Jurassic begins with an exponential rise in diversity that continues to the Cenomanian. The Cenomanian to Holocene diversity record shows peaks and troughs that are roughly in line with cycles of global climate, with reductions at the end-Cenomanian, end-Cretaceous, and end-Miocene. Excluding modern values, the Phanerozoic maximum number of genera is observed in the Cenomanian, whereas the maximum mean standing diversity is observed in the mid-Miocene. The highest per-capita origination rates are observed in the Hettangian, Dapingian, Pleistocene, and Sheinwoodian. Linear regression of the origination rates shows a decrease towards the Holocene, in agreement with findings of Raup & Sepkoski (1982). The highest per-capita extinction rates are observed in the Messinian, Late Silurian (Gorstian), Hirnantian, and Maastrichtian. The background extinction rate shows an increasing trend towards the Recent, in disagreement with Raup & Sepkoski.