ENVIRONMENTAL GRADIENT IS REFLECTED IN FAUNAL COMPOSITION AND DISTRIBUTION OF EXCEPTIONALLY PRESERVED CONODONTS AND OTHER VERTEBRATES FROM THE SILURIAN ERAMOSA LAGERSTÄTTE, BRUCE PENINSULA, ONTARIO, CANADA

The distribution of early jawless vertebrates in the Eramosa Lagerstätte of the Eramosa Member of the Guelph Formation (Silurian, Wenlock) of southern Bruce Peninsula, Ontario, may be explained by the existence of an environmental gradient in shallow Silurian seas.

Just N. of Park Head, our most southerly locality, the vertebrate fauna consists solely of fluorescescent corvaspid and tolypelepid heterostracan fish, associated with the pterygotid sea-scorpion Erettopterus, as well as ostracodes and articulated polychaete worm jaws.

At Hepworth, 3.5 km N. of Park Head, abundant and well-preserved skeletons of the conodont Ozarkodina excavata (Branson and Mehl) dominate, both as articulated assemblages and fused clusters; associated, but rare, articulated conodont skeletons are those of a new taxon, as well as those of Ctenognathodus cf. murchisoni, and Panderodus and Pseudooneotodus species.

Approximately 3 km W. of Hepworth, the faunal composition of vertebrates is almost exactly reversed. There, articulated skeletons of Ctenognathodus cf. murchisoni, associated with rare articulated skeletons of the new taxon (above) and those of Panderodus sp. dominate, to the apparent exclusion of Ozarkodina excavata (Branson and Mehl) and other vertebrates.

The Park Head biota, lacking conodonts but containing polychaetes and ostracodes, most likely lived under brackish-water, near-shore, marine conditions. The abundance of Ctenognathodus cf. murchisoni (and the apparent absence of Ozarkodina excavata) west of Hepworth, suggests that this biota lived under shallow-water, lagoonal conditions. The abundance of Ozarkodina excavata (Branson and Mehl) at Hepworth suggests that the biota, of which this conodont species was a part, lived under open-marine conditions.

Based on the preceeding, we postulate the existence of an environmental gradient in the Silurian shallow-water marine environments of the southern Bruce Peninsula. This gradient is expressed by marginal marine, brackish-water conditions to the S., shallow-water lagoonal conditions to the NW., and open marine conditions to the N. Although host lithologies mirror these hypothesized palaeoenvironmental trends only poorly, the taxonomic composition and distribution of the vertebrates that lived there, closely corresponds to the hypothesized palaeoenvironmental trends and changes.