THE SILURIAN MULTIELEMENT CONODONT GENUS ALDRIDGEODUS JEPPSSON IN NORTH AMERICA

The genus Aldridgeodus Jeppsson 2008 was based on a multielement reconstruction of six diminutive element types from the latest Sheinwoodian through early Ludfordian of Gotland, Sweden. Only a single species, A. minimus Jeppsson 2008, was recognized, and the generic, and by default, the specific diagnosis was based on an unusual M element with radiating denticles. On Gotland, the species ranges from the Kockelella ortus ortus Zone through the Polygnathoides siluricus Zone.

We recovered Aldridgeodus from the Silurian Eramosa Formation in two cores at Guelph, Ontario, Canada. There, an M element confirms the presence of A. minimus, and other data suggest the range of A. minimus be extended to at least the Lr. Kockelella walliseri Zone (middle Sheinwoodian). Although the diagnostic M element has not yet been identified from Hepworth, Ontario, the genus is also present there, as natural assemblages and discrete elements in the Eramosa Lagerstätte (von Bitter et al., 2007), possibly as a second, younger species. Aldridgeodus dominates just west of Hepworth, and appears to define a distinct conodont biofacies with associated Ctenognathodus, Panderodus, and Pseudooneotodus.

Aldridgeodus is abundant in Gotland, in “an atypical middle Silurian shale" (Calner et al., 2008). These authors concluded the shale most likely represents part of the maximum flooding interval (MFI) associated with a late Wenlock transgression across the Laurussian craton. The position of this shale in the interior of a carbonate platform was explained by a sea-level high-stand, and the restricted fauna present was regarded as a result of low salinity, possibly in a sediment-starved lagoon, or analogous environment.

Whether the occurrence just west of Hepworth is "atypical", whether the lithologies of the Eramosa at this locality reflect the same MFI associated with the late Wenlock transgression recorded on Gotland, or whether they can be related to a sea-level high-stand, remains to be determined.