Respiration in a vagrant Ordovician cystoid, Amecystis

Abstract

Amecystis Ulrich and Kirk is a rhombless rhombiferan cystoid exhibiting adaptation to an active mode of life in the bilaterally symmetrical, dorso-ventrally compressed theca and to an active mode of respiration suggested by the lack of pore rhombs and greatly enlarged periproct. Like other members of the Pleurocystitidae, Amecystis probably wriggled along the bottom, but may have adopted a temporarily sessile habit by pushing the distal stele into the substrate or by grasping. Evolution of the genus began in the Middle Ordovician, probably as a development from Pleurocystites. General trends in the stratigraphic succession of recognized morphospecies: A. raymondi Parsley, A. woodi n.sp., and A. laevis (Raymond) show a reduction in angularity of thecal outline and obsolescence of surface features on the thecal plates. The use of the periproct as a pumping organ in respiration is regarded as an advanced trend that was probably utilized by many rhomb-bearing cystoids as an accessory respiratory mode; alone, it proved to be an unsuccessful adaptation in Amecystis.