THE ONSET OF NON-PLANKTOTROPHY IN SCHIZASTERID SPATANGOID SEA URCHINS

In a broad study of developmental mode in all sea urchin groups Jeffery (1997, Geology 25: 991-994) found that non-planktotrophic (non-feeding) larvae first evolved immediately prior to the Cretaceous-Tertiary boundary and that non-planktotrophy was adopted almost synchronously in five different orders at this time. However, the smaller scale patterns within orders and at lower taxonomic levels remain poorly known. Important questions include: Are there more switches to non-planktotrophy than previously thought? Are switches scattered through time and space or concentrated in particular stratigraphic horizons or geographical areas? Which factors drive switches to non-planktotrophy?

This study addresses these questions by means of an investigation of the Cretaceous representitives of the family Schizasteridae (Echinoidea: Spatangoida). Developmental modes were determined from the adult test by using either morphological or crystallographic criteria and were then mapped onto a new phylogeny of the group. This method allowed the number, direction and timing of the switches in larval mode to be determined. The resulting data were then used to determine the temporal and geographic distribution of switches, and to assess which factors drove these switches.