DIACHRONOUS APPEARANCES OF THE PENNSYLVANIAN FUSULINID PROFUSULINELLA IN EURASIA AND NORTH AMERICA

Journal of Paleontology, Mar 2007 by Groves, John R, Kulagina, Elena I, Villa, Elisa

ABSTRACT-

The Pennsylvanian fusulinid genus Profusulinella appeared in sub-Arctic North America in Medial Atokan (=Early Moscovian) time, roughly 4-5 My later than its oldest known occurrence in the Eurasian-Arctic province. The genus originated in the latter area in late Early Bashkirian time and then underwent significant diversification, so that by Early Moscovian time a range of shell morphologies existed. The first sub-Arctic North American species in the genus are interpreted as immigrants from Eurasia, with their migration through the Franklinian corridor having been facilitated by generally east-to-west currents during a glacio-eustatic flooding event. Previous work suggested that North American Profusulinella spp. may have derived from a local ancestor such as Eoschubertella. This possibility seems unlikely given that early North American species in Profusulinella are very similar to age-equivalent Eurasian forms, that they differ from North American Eoschubertella in a number of morphologic features, and that there are no known North American intermediates between Eoschubertella and Profusulinella. Fusulinoidean faunas apparently migrated from Eurasia to North America on multiple occasions during Pennsylvanian time. These migrations were an important source of North American diversity, and their recurrence is a dominant theme in fusulinoidean biogeography.

INTRODUCTION

THE GENUS Profusulinella Rauser-Chernousova and Belyaev in Rauser-Chernousova et al., 1936 was first described from the northern Urals and it includes the earliest fusiform fusulinid foraminifers. Its widespread geographic distribution and short stratigraphic range led Thompson (1945, 1948) to erect the Zone of Profusulinella within his fusulinoidean subdivision of the North American Pennsylvanian System (now Subsystem). The Zone of Profusulinella was proposed originally as a taxon-range-zone (sensu Hedberg, 1976), although later work showed that in North America the upper range of the genus overlaps the lower range of the genus Fusulinella von M�ller, 1877, the next younger zonal index (e.g., Ross and Sabins, 1965; Douglass, 1987). During the years 1945-1970, most North American stratigraphers regarded the Zone of Profusulinella as the lower part of the Atokan ( = Derryan) Stage (Thompson, 1960, 1966). Subsequently, Mamet and Skipp (1970) and Lane et al. (1972) identified an interval of clearly post-Morrowan but pre-Profusulinella-beanng strata at numerous localities throughout the midcontinental and southwestern United States. This interval, characterized by occurrences of the smaller fusulinids Eoschubertella Thompson, 1937 and Pseudostaff ella Thompson, 1942, now is considered as the lowest part of the Atokan Stage. Following this revision, the Zone of Profusulinella was equated with the middle part of the Atokan Stage, and the Zone of Fusulinella with the upper part (Sutherland and Manger, 1983; Groves, 1986; Clopine, 1992). Profusulinella was equally important in the historical development of biostratigraphic and chronostratigraphic schemes in Europe, Asia, and North Africa. For example, in the former Soviet Union, the appearance of the genus was used to characterize the Askynbashsky Horizon of the Lower Bashkirian Substage in the southern Urals (Teodorovich et al., 1956, 1959; Semikhatova et al., 1979; Sinitsyna and Sinitsyn, 1987; Kulagina et al., 2001), the Prikamsky Horizon of the Moscow Basin and East European Platform (Rauser-Chernousova et al., 1951), and the C^sub 2^^sup 1^ (F) Suite of the Dneipr-Donets Basin (Donbass) (Brazhnikova et al., 1967). The appearance of the genus was used to correlate marine sections in Western Europe and North Africa to equivalents in the former Soviet Union (van Ginkel, 1965, 1986, 1987; Villa, 1995), and the Zone of Profusulinella defined the Atetsuan Stage of Japan (Toriyama, 1967) and the DaIa Substage of the Weiningian Stage of China (Zhang, 1987).

As late as the 1960s, the earliest occurrences of Profusulinella in various parts of the world were thought to be essentially isochronous, so that biostratigraphic and chronostratigraphic units defined on the appearance of the genus were equated from region to region (Thompson in Loeblich and Tappan, 1964; Toriyama, 1967). These interpretations were largely axiomatic, however, as independent evidence for the age of Profusulinella-beanng strata was rarely considered. With heightened interest in integrated, multidisciplinary, international chronostratigraphy in the 1970s, it became apparent that the stratigraphically lowest Profusulinella spp. in Eurasia and the Arctic are significantly older than their counterparts in the sub-Arctic Western Hemisphere. Ross (1979, p. A267, fig. 3) was among the first to recognize that "A nearly independent evolutionary history of the Eurasian-Arctic and Midcontinent-Andean faunal provinces is indicated by the different stratigraphie ranges of Profusulinella, Fusulinella, Beedeina, and Fusulina in the two provinces." There are two possible explanations for the diachroneity. One is to assume that Profusulinella originated in the Eurasian-Arctic province and then arrived later in the Midcontinent-Andean province as a consequence of timetransgressive dispersal. Alternatively, as first suggested by Groves and Sanderson (1990, p. 117), forms assigned to the genus may have originated independently in the two provinces.