Heterochrony in the Cambrian trilobite Hsuaspis

Abstract

Comparison of ontogenetics suggest that the Middle Cambrian Xystridura evolved from the Lower Cambrian species Hsuaspis bilobata by changes in the rate or timing of development of characters during ontogeny. Extension of the juvenile growth period delayed maturation, or hypermorphosis resulted in exaggeration of normally slow, early holaspid growth patterns within the rapid, late meraspid growth stage, producing a marked progradation of the glabella in Xystridura. Forms referable to H. cerastes from the upper Early Cambrian Cymbric Vale Formation of New South Wales appear to show the beginnings of this hypermophic trend.