Abstract

A phylogenetic analysis was used to determine evolutionary relationships within the Early Cambrian superfamily Olenelloidea Walcott, 1890. Phylogenetic patterns within the suborder Olenellina Walcott, 1890, which contains the Olenelloidea and the Fallotaspidoidea Hupe, 1953, are also discussed. The Olenelloidea are monophyletic, and synapomorphies uniting them include the condition of the ocular lobes where they intersect the frontal lobe of the glabella, and the condition of the lateral margins of the glabellar lobes. In contrast, taxa formerly assigned to the Fallotaspidoidea are shown to represent a paraphyletic grade of several genera, some more closely related to the Olenelloidea, and some more closely related to the Redlichiina Richter, 1933. Seventy-nine exoskeletal characters were coded for 26 taxa within the Olenellina. These included 22 ingroup Olenelloidea and four outgroup taxa that have traditionally been assigned to the Fallotaspidoidea. When subjected to parsimony analysis these character data yielded a single most parsimonious cladogram that provides an hypothesis of relationship for the generic clades within the superfamily. Two new genera are recognized herein, Fritzolenellus and Lochmanolenellus. It has been argued that genetic flexibility was so great and trilobite morphology was so plastic in the Early Cambrian that suprageneric classification of Early Cambrian trilobites is precluded. Although levels of intraspecific variability may have been slightly higher in the Early Cambrian relative to the mid Paleozoic, based on the extent of polymorphic character codings, it was not so high as to obviate attempts at recovering phylogenetic structure in a major clade of Early Cambrian taxa. In addition, the consistency index recovered by this analysis is not unduly low for a phylogenetic database of this size. The phylogenetic analysis also has bearing on patterns of allometric heterochrony, which have often been held to be significant in Early Cambrian trilobites. The paedomorphic retention of advanced genal spines into the adult probably evolved at least four times. Three of the episodes can be best described as neoteny, the fourth, as progenesis. Finally, based on the phylogeny, it is likely that rates of speciation in trilobites may have been two to three times higher in the Early Cambrian than in the mid Paleozoic.

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