ON THE PHYLOGENETICS OF THE TRILOBITE FAMILY PHACOPIDAE WITH SYSTEMATIC IMPLICATIONS FOR ELDREDGEOPS (GREEN, 1832)

Trilobites belonging to the family Phacopidae are of particular interest partly due to their unique schizochroal eyes. Work by Eldredge documenting cephalic and eye morphology within the Middle Devonian Phacops rana (=Eldredgeops) group helped shape early arguments on punctuated equilibrium. However, morphological similarity between taxa, genera, and tribes among members of Phacopidae result in polytomies, homoplasy, and paraphyly. Further, extensive taxonomic splitting in the last several decades has left phacopid systematics in a state of mild disarray. Work by Strüve (1990, 1992) has brought the identity of the Phacops rana-group into question. The process of erecting the genus Eldredgeops from Phacops rana has led to the following uncertainties: (1) which of the former Phacops rana subspecies are to be considered members of the genus of Eldredgeops; (2) what are the relationships among these members; (3) if Eldredgeops is to be considered a valid genus, what is the relationship of Eldredgeops to other members of the family Phacopidae?

To answer these questions a suite of characters comprising all sclerites of the trilobite were coded for 41 species, representing 16 genera across the trilobite family Phacopidae, including all ‘subspecies’ formerly assigned to Phacops rana (=Eldredgeops). Parsimony analysis run on a subset of the data matrix reveal the following systematic implications for Eldredgeops: (1) the two African ‘subspecies’ described by Burton and Eldredge (1974) are nested within Eldredgeops and should be systematically considered Eldredgeops; (2) all 'subspecies' of Phacops rana are united in a monophyletic clade suggesting that Eldredgeops is a valid monophyletic genus; (3) Eldredgeops is a sister genus to Geesops (G. schlotheimi) as hypothesized by Eldredge (1972); (4) Together Geesops-Eldredgeops are sister to Phacops s.s.

With a clearer picture of the relationship among members of Eldredgeops and of Eldredgeops to the rest of Phacopidae, we may better understand the morphological evolution of an enigmatic clade. Future work uniting morphology and phylogenetics could have important macroevolutionary consequences.