The stem and crown group concept. The two pink groups represent a pair of crown groups, the last common node of which is the basal node. Terminology is from Craske, A. J. and Jefferies, R. P. S. (1989) A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion. Palaeontology 32, 69–99 and Budd, G. E. (2001) Tardigrades as ’stem-group’ arthropods: the evidence from the Cambrian fauna. Zoologischer Anzeiger 240, 265–279.

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