Paleogeographic impact of a newly described 'cruciform' sub-group of Tetracyclus (Bacillariophyta, Diatomaceae)

摘要

Since the description of Tetracyclus lacustris Ralfs (=Tetracyclus glans (Ehrenb.) F.W. Mills), many new varieties and forms of it have been described. Examination of specimens, using scanning electron microscopy, has demonstrated that some should be recognised as species; many have restricted (palaeo-) distributions. This paper describes three species from the T. glans complex, two of which are new: Tetracyclus parvula (Forti) D. M. Williams stat. nov. and Tetracyclus quadriformis D. M. Williams sp. nov. As relationships within Tetracyclus are still under investigation, a 'cruciform' sub-group (within the group Tetracyclus) is discussed, which includes the three species described herein. It has not yet been demonstrated to be monophyletic. The significance of the species described herein is not just their stratigraphic position but the fact that they have distinct geographical distributions, primarily (but not exclusively) from Circum-Pacific areas (China, Russia, Central America, Africa and, possibly, specimens supposedly still living and in the tropics (New Caledonia).