The role of heterochrony in the evolution of spatangoid echinoids

Abstract

Despite many imperfections the fossil record still providesthe only direct evidence of patterns of morphological change through time. By analysing the ontogenetic development of successive species of a lineage in a stratigraphic succession, it is possible to evaluate the nature of any heterochronic change and assess which particular paedomorphic or peramorphic processes might have been involved in morphological evolution. Spantangoids are useful in such studies because morphological characters which are used to differentiate living species are generally all preserved in fossil species. Furthermore, the autochthonous nature of many spatangoid assemblages and the intimate relationship between test morphology and enclosing sediment type allow the impact of environmental factors on the direction of morphological evolution to be assessed. Four lineages of Cainozoic spatangoids from Australia are discussed. The Paraster-Schizaster lineage shows peramorphic evolution, by a combination of hypermorphosis and acceleration of specific characters. Fossil species in a Hemiaster lineage provide an example of a lineage in which morphological evolution progressed by dissociated heterochronic events: some characters followed peramorphoclines, others paedomorphoclines. The Protenaster lineage is characterised by its great morphological conservation in most characters, except for the pore pairs of the phyllode: these form a peramorphocline produced by acceleration. The three species of the Lovenia lineage also show dissociated heterochronic events. The pronounced intra-specific variability in species of this lineage also occured by the action of heterochronic processes. In all these heterochronic lineages, and in the classic Micraster lineage in the European Cretaceous, which itself was probably driven by heterochrony, selection pressure favoured those species which were better adapted to burrowing in progressively finer-grained sediments.