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HETEROCHRONY IN THE CAMBRIAN TRILOBITE HSUASPIS

CHRISTOPHER NEDIN and RICHARD. J. F. JENKINS

Department of Geology and Geophysics, University of Adelaide, South Australia 5005

Nedin, C. & Jenkins, R.J.F. 1999. Heterochrony in the Cambrian trilobite Hsuaspis. Alcheringa, 23:1-7.

ABSTRACT

Comparison of ontogentics suggest that Middle Cambrian Xystridura evolved from Lower Cambrian Hsuaspis bilobata by changes in the rate or timing of development of characters during ontogeny. Extention 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.