Part 2, Conodont biostratigraphy and paleoecology of the Ellis Bay formation, Anticosti Island, Quebec, with special reference to late ordovician-early silurian chronostratigraphy and the systematic boundary
Authors: McCracken A D , Barnes C R 
Published: August 2015

The Ellis Bay Formation on Anticosti Island, Quebec, is subdivided into six members and is composed of calcareous shale and argillaceous limestone and varies in thickness from 53 m in the west to about 96 m in the east. Member 6 accounts for about half the total
thickness of the formation and near its base small bioherms are developed widely across the island. Lateral variation in lithology represents a gradual transition from a nearshore facies (east) to an offshore, shallow subtidal facies (west) . The members may reflect eustatic changes produced by the
Ashgillian glaciation in North Africa.
Conodont samples were collected from three composite stratigraphic sections, at Ellis Bay, and the Vaureal and Salmon Rivers, over a distance of about 140 km. Thirty-five multielement species representing 18 multielement genera are recogni zed in the fauna of 14 665 conodont elements from 183
samples. New species are Oulodus? nathani , Panderodus clinatus andStaufferel/a inaligera. Eighteen form species , ten of which are new and unnamed, are also present, as is one new, unnamed form genus. Panderodus is the major component of the Ordovician fauna and is represented by seven multielement
and four form species. Two species of Gamachignathus comprise about 45 per cent of the Ordovician conodont fauna of the Ellis Bay section. An almost complete faunal replacement occurs in the Silurian with only three species of Panderodus and one species each of Decoriconus and Pseudooneotodus
continuing across the systemic boundary. Other than these taxa, the Silurian fauna contains 5 multielement genera and 8 multielement species. Panderodus graci/is (Branson and Mehl) dominates the Silurian fauna; Ozarkodina and Oulodus? are the most abundant ramiform conodont genera.
The Ordovician fauna, although including many taxa from the underlying Richmondian Vaureal Formation, is characterized by Gamachignathus. The strata containing this fauna are regarded as representing a latest Ordovician time interval bearing a fauna referred to as Fauna 13 and for which the
re-introduction of the Gamachian Stage is advocated. An Ordovician-Silurian transition zone, about 1.5 m thick, is present with species from both periods co-occurring. The Silurian fauna is similar to that of the Distomodus kentuckyensis Zone, the oldest conodont zone of the North American
Llandoverian. However, the new species appear to represent an earlier, new, Llandoverian zone, the Ou/odus? nathani Zone. The Ordovician-Silurian boundary is herein drawn at the first appearance of species of Ozarkodina and this occurs about 2 - 3 m above the base of Member 6 of the Ellis Bay
Formation.
The distribution of the Ordovician conodonts indicates the presence of three laterally segregated communities corresponding to the offshore subtidal, intermediate and nearshore environments. The offshore community (Community A) is dominated by Gamachignathus and Panderodus. The intermediate
community (Community B) includes several species of Panderodus , Oulodus, Amorphognathus and Aphelognathus. Community C, the nearshore community, is characterized by large and robust elements belonging to Aphe/ognathus and Panderodus. In Silurian strata, Community D is characterized by the presence
of Ozarkodina oldhamensis (Rexroad) and Oulodus? nathani n.sp. and is developed in an intermediate subtidal facies. Ozarkodina hassi (Pollock, Rex.road and Nicoll) and Ou/odus? kentuckyensis (Branson and Branson) dominate within Community E and reflect a shallow subtidal
environment.