Fluctuations in the body size of marine invertebrates through the Pliensbachian–Toarcian extinction event

Abstract

The sizes of a selection of common fossil marine invertebrates, including belemnites, ammonites and bivalves, from the Cleveland basin of northern England were measured, in order to assess whether there were any statistically significant changes in body size through the Pliensbachian–Toarcian extinction event. Bed-by-bed fluctuations in body size of pre-extinction bivalves from the Upper Pliensbachian may be influenced by taphonomic effects, whereas other size changes recorded probably reflect real biological change. One taxon, the belemnite species Passaloteuthis bisulcata, records a significant size increase at the start of the extinction interval (hawskerense through to tenuicostatum subzones). The taxa Dactylioceras and Pseudomytiloides dubius both survive the extinction interval and record their smallest sizes in the immediate aftermath of the event (the early survival interval): a phenomenon known as the Lilliput effect. This is the first documented account of the Lilliput effect in the aftermath of the Pliensbachian–Toarcian extinction event. Apart from the belemnite Acrocoelites subtenuis, all taxa present in the aftermath of the event increase in body size through the later survival interval and subsequent recovery interval. In the Early Toarcian, size decrease may have been caused by low oxygen conditions and, possibly, changes in water depth and food supply. Growth line data indicate that a bedding plane assemblage of Pseudomytiloides dubius from the semicelatum Subzone had slower rates of growth (more growth lines per millimetre) than a larger-sized assemblage from an adjacent horizon. Changes in variance also occur during the Pliensbachian–Toarcian event. Species show greatest variability in the survival interval, which may be the result of reduced interspecific competition experienced by those few taxa fortunate enough to survive the extinction crisis.