Systematics and evolution of Phacops rana (Green, 1832) and Phacops iowensis Delo, 1935 (Trilobita) from the Middle Devonian of North America. Bulletin of the AMNH ; v. 147, article 2

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New York : [American Museum of Natural History]
"Two trilobite species, Phacops rana (Green, 1832) and Phacops iowensis Delo, 1935, from the Middle devonian of North America, are analyzed in detail from the point of view of geographic and stratigraphic variation. A purely morphological, as opposed to biostratigraphic, approach is used in analyzing relationships among subtaxa of the two species. Subsequent comparison of the relative sequence of inferred evolutionary events with documented biostratigraphic distributions allows an analysis of trends (biostratigraphic character gradients), character displacement between the two species, and mode of origin of the subtaxa. The schizochroal eye of these species is the most important anatomical complex in terms of both intrapopulational and interpopulational variation and species discrimination. Lens number per eye may be broken down into number of dorsoventral files (vertical columns of lenses) and number of lenses per dorsoventral file. The adult population number of dorsoventral files per eye in Phacops is reached early in ontogeny and is stabilized; the number of dorsoventral files is the most consistently reliable criterion for discrimination of the two species. Throughout its history, P. iowensis had 13 dorsoventral files in normal adults. This species belongs to a native North American phacopid lineage that can be traced back at least as far as the Gedinnian ('Phacops' logani Hall). Phacops rana, morphologically closest to P. schlotheimi (Bronn) from Europe and elsewhere, has from 15 to 18 dorsoventral files. Most of the history of species involved the reduction from 18 to 15 files; the reduction is an allopatric phenomenon, involving transitional populations acquiring a reduced number of dorsoventral files on the eastern margin of the craton (exogeosyncline), which subsequently invade the cratonal interior. Other evolutionary changes in the P. rana lineage appear to be phyletic trends. The two species are nearly mutually exclusive, though coeval and their geographic ranges overlap considerably. Although P. iowensis occurs from Iowa to New York, it was confined to the Michigan Basin for the larger part of its history. Phacops rana is found from New York south to Virginia and west as far as Iowa. Phacops iowensis was by far the more stable through time; it is invariably rare, and generally confined to purer limestones. The one case of sympatry between rana and iowensis occurs in the Hungry Hollow Formation of Ontario, resulting in morphological changes in the two species which are best explained as character displacement. The P. rana lineage as a whole converged on iowensis in number of dorsoventral files and in many ornamental features; the convergence was closest in the Taghanic. The distribution of, and interactions between, Phacops rana and Phacops iowensis are best explained as if the two taxa are considered true 'bio-species.' Although the little change that occurred within the iowensis lineage seems to have been phyletic in nature, the allopatric model is necessary to account for the more important evolutionary changes in P. rana. Five subspecies of Phacops rana, including P. rana paucituberculata, new subspecies, and three subspecies of P. iowensis, are recognized"--P. 49.
p. 47-113 : ill. ; 27 cm.
Includes bibliographical references (p. 111-113).