THE PHYLOGENY AND EVOLUTION OF SOME PHACOPID TRILOBITES

Abstract

Abstract A data matrix of 32 characters, with a total of 132 character states and 48 terminal taxa of phacopid trilobites, was analysed using parsimony. The 108 most parsimonious trees have a length of 301 steps and a consistency index of 0.29. Two major clades were found, one including species referred to Acernaspis and one with species referred to Ananaspis, Paciphacops, Viaphacops, and Phacops. Most homoplasy occurs between the major clades of the trees, rather than within them. The major clades were also analysed separately, resulting in trees with significantly higher consistency indices. We redefine the following supraspecific taxa to make them monophyletic: Acernaspis, Ananaspis, and Paciphacops. In addition, Kainops, new genus, is described for a clade consisting of six taxa previously assigned to Ananaspis and Paciphacops. The evolutionary pattern resulting from our cladogram is compared with the approaches of Campbell and Eldredge, and it is argued that some of the assumptions made by these authors regarding the phylogeny of the group are not valid. The biogeography of the group was also analysed. It is found that phacopids from the Ludlow and older strata are distributed globally, with an emphasis, perhaps due to a large number of suitable localities, on Northern Europe. However, the lineage of phacopines leading up to Phacops is, with some slight exceptions, basically confined to the United States, being entirely absent in Northern Europe. The main phacopine lineage does not re‐establish itself geographically until the middle Devonian. Finally, we analysed the evidence for polymorphism in phacopid trilobites, and reject this hypothesis on the basis of stratigraphy and the present cladogram. Copyright © 1991, Wiley Blackwell. All rights reserved