EARLY SILURIAN TRILOBITES FROM NORTHEASTERN MISSOURI

Abundant trilobite sclerites occur as molds in weathered blocks of the early Llandovery (Rhuddanian) Bowling Green Dolomite (Edgewood Formation) in a section near Clarksville, Missouri. The trilobite fauna is entirely undescribed, and includes at least ten species belonging to seven families, including Encrinuridae (49% of the total 233 specimens), Calymenidae (25%), Phacopidae (11%), Odontopleuridae (5%), Proetidae (4%), Aulacopleuridae (3%), and Illaenidae (3%). The complete absence of other common Silurian families, such as Cheiruridae and Lichidae, is unusual. The shelly fauna is dominated by articulate brachiopods, and the unit was likely deposited in a deep, subtidal, outer shelf environment, below storm wave base.

The trilobite fauna is important due to the extremely limited global sampling of Rhuddanian assemblages. The recorded diversity corroborates recent work indicating that the end-Ordovician mass extinction, which it immediately post-dates, had little or no effect on trilobite alpha (within-habitat) diversity. In addition, although many Silurian trilobite faunas from the eastern United States have been recorded, few have received modern systematic treatment. The Bowling Green fauna can be used to test hypotheses of lineage survivorship across the end-Ordovician extinction. The Transcontinental Arch exerted considerable influence on Early Silurian Laurentian trilobites, dividing the continent into northern and southern geographic realms. The Bowling Green fauna indicates that southern genus-level endemism was entrenched very early during the Silurian.