- The Paleontological Society
The classification of fossils has long been controversial. Should traditional taxonomic concepts be expanded to encompass plesiomorphic extinct relatives that exhibit subsets of essential defining characters in so many shades of gray? Should rank taxa, established on extant taxa alone, and in a pre-Darwinian, pre-Hennigian framework, remain restricted to their living constituents and integral extinct relatives? Or should these taxonomic concepts be restricted to a membership exhibiting a defining suite of essential characters, regardless of whether they are extant or extinct?
Hennig (1981) argued that rank taxa should be defined on the basis of extant organisms because the latter are often better known than fossil taxa. However, he further argued that traditional taxonomic concepts should be expanded to include all extinct taxa more closely related to the living members than to any other extant clade. The extant clade he denoted the *group, later renamed the crown group (Jefferies 1979), and its paraphyletic complement of extinct taxa, the stem group; Jefferies (1979) later coined the term “total group” to describe the monophyletic sum of the stem and crown group, equivalent to Hennig's (1981) more inclusive version of the rank taxon (see Fig. 1 for a diagrammatic representation of these concepts).
There are, however, inherent problems associated with the implementation of the stem group. This is because fossil organisms may fail to exhibit the full inventory of characters deemed diagnostic of a crown group, not because of genuine plesiomorphy but because of incomplete anatomical preservation. Although this holding position was considered “unsatisfactory” even in its original formulation, Hennig (1981) argued that prolonged study of fossils would reveal the structure of stem groups and that, ultimately, this could be reflected in the hierarchy of classification.
Hennig's vision of reconstructed stem groups has begun to …