BIOSTRATIGRAPHIC SIGNIFICANCE OF ORNAMENTED GNATHODUS SPECIES (CONODONTA) IN THE FIVE DEPOSITIONAL CYCLES OF THE WINDSOR AND CODROY GROUPS (MISSISSIPPIAN; LATE VISÉAN- E. NAMURIAN) OF EASTERN CANADA

Environmental controls on conodont distribution in evaporite-rich, shallow-water environments of the Windsor and Codroy groups (late Viséan-early Namurian) of eastern Canada, resulted in Gnathodus species being rare in the Clydagnathus-Cavusgnathus dominated faunas. This rarity, plus the environmental effects on conodont growth and maturity, complicate assessment of the biostratigraphic value of ornamented Gnathodus species.

Rare small gnathodids, associated in basal rocks of depositional cycle 1 of western Newfoundland with controversial Mississippian brachiopod-rich fissure fillings, were for many years identified as immature G. girtyi. These have been re-identified as Gnathodus pseudosemiglaber Thompson & Fellows, relying in particular on a mature individual from basal cycle 1 in New Brunswick. This species, first described from Kinderhookian and Osagean rocks in North America, has been reported worldwide, and was used to recognize a local-range subzone in China. In the British Isles, the region with Mississippian rocks most directly and commonly correlated with those of Atlantic Canada, Gnathodus pseudosemiglaber ranges from late Courceyan to late Chadian, whereas in Poland it extends from the late Courceyan into the early Asbian. These ranges agree approximately with the late Chadian to early Asbian palynomorph-based age determination for cycle 1 in eastern Canada; however, the recent range extension of the species in Spain into the late Asbian, and possibly the early Brigantian, suggests caution in using the species as an age determinator at this time.

Basing taxonomy on immature individuals can lead to taxonomic problems. Thus, Gnathodus girtyi of von Bitter and Plint-Geberl (1982) from cycles 3-5 of Newfoundland, was identified by Belka (1985) as Gnathodus austini. This is an immature, small specimen of Gnathodus girtyi and the existence of an austini Zone in Atlantic Canada seems unlikely. In a parallel, but dissimilar situation, Gnathodus bilineatus of von Bitter and Plint-Geberl (1982), from cycle 3-5 of Nova Scotia and Newfoundland, identified by Belka (1985) as G. praebilineatus, are with their strong posterior nodes, mature specimens of G. bilineatus bilineatus. G. praebilineatus is restricted to cycle 2 of the Windsor & Codroy groups, as will be the eponymous zone, if it is identified in eastern Canada.