Heliolitine corals of the topmost Douro and overlying formations (Upper Silurian), Canadian Artic Islands

Journal of Paleontology, Nov 1998 by Dixon, Owen A

In the studies of these Arctic heliolitines, assemblages acquired through exhaustive field search and collection are used as the principal basis for taxonomy, while isolated specimens, or small so-called "representative" collections acquired during stratigraphic or other field work, are used mostly in a supplementary role. The combined use of numerical and descriptive morphological data from the larger assemblages at each of three stratigraphic levels (Faunas 1 to 3) has proven effective in delimiting groups having normal patterns of continuous morphological variation. Thereby, they provide a basis for recognizing and defining meaningful morphotaxonomic or "operational" species within the few genera present. Although species composition of the faunas changes with stratigraphic level, no intraspecific variation that could be attributed to evolution was discriminated, perhaps because of the short time range represented. Paleoenvironmental aspects of variation were addressed by comparing assemblages from different localities and different stratigraphic units that span the range of depositional settings known for these corals in the region. At each level, examples of cooccurrence further helped to confirm the distinctness of most of the species recognized. The geographic extent of this study was insufficient for an appraisal of geographic variation, for example, in terms of paleolatitude or isolation.

The substantial variation portrayed by these corals favors a taxonomic philosophy of "lumping" rather than "splitting," and is certainly analogous to the population variation known to typify many living species. For example, from extensive study of modern scleractinians, Veron (1995, p. 142) considered that "The amount of variation found in most extant coral species is often more than the amount of variation frequently described in several genera of fossils." Veron (1995) suggested that in effect there may be no clear boundaries between many zooxanthellate scleractinian species, and that distinctions between species break down with geographic distance and through evolutionary time. He attributed this breakdown to repeated isolation and rejoining of species populations and their genetic pools through time, due to fluctuating dispersal patterns of planktonic planulae with changing patterns of ocean surface circulation. Although this mechanism may have been less relevant for heliolitines, if they were azooxanthellate as their generally deeper water occurrence in this study may imply, their degree of variation remains an important and challenging taxonomic consideration.

SYSTEMATIC PALEONTOLOGY

Repository.-Type and figured specimens (numerals with GSC prefixes) are deposited in the National Type Fossil Collection at the Geological Survey of Canada in Ottawa.

Class ANTHOZOA Ehrenberg, 1834

Order HELIOLITIDA Frech, 1897

Suborder HELIOLITINA Frech, 1897

Family HELIOLITIDAE Lindstrom, 1876

Genus HELIOLITES Dana, 1846

Type species.-Astraea porosa Goldfuss (1826, p. 64, pl. 21, fig. 7a-g) from the Middle Devonian of the Eifel District, Germany (OD).