Macro-invertebrates and the Cretaceous-Tertiary boundary

Abstract

Most of Cretaceous macro-invertebrate groups such as ammonites, inoceramids, belemnites, and rudists whow a gradual decline towards the C/T boundary, and some of them disappear long before the boundary level itself. As in the terrestrial vertebrates, their disappearance is unrelated to an extraterrestrial impact as suggested by the widespread iridium anomaly occurring at the boundary. It is also unrelated to fluctuations in temperature, which have been recognized near the boundary level but which can probably be better correlated with such an event. The decline of ammonites is gradual and at the same time periodic in nature. Periodic events occurring through the Upper Cretaceous and the Phanerozoic as a whole are sea level changes. Indeed, the pattern of global transgressions and regressions shows a striking similarity with increasing and decreasing ammonite diversity.

In contrast, the turnover in calcareous plankton of oceanic surface waters as well as in angiosperms (Aquilapollenites Province) is a later and "instantaneous" event which can be related with observed fluctuations in temperature, the iridium anomaly, and presumed impact at the boundary level.

Unlike those groups of organisms affected by the terminal Cretaceous extinctions, a few others flourished at this time. From the boundary beds of Jylland, Denmark, bryozoans and specialized crinoids were reported as having their peak density at the time of mass extinction. Since both groups are very sensitive to changes in salinity or oxic conditions, botl factors can be ruled out as having caused the C/T boundary mass mortality. From available observations it can be concluded that the great faunal and floral break at the C/T boundary was not a monocausal phenomenon, but was the result of a complex scenario of both periodically fluctuating and instantaneous catastrophic factors.