By-the-wind-sailors from a Late Devonian foreshore environment in western Montana

Abstract

Three remarkable fossil medusoid hydrozoans (Plectodiscus latinautilus n. sp.) with disc, topsail, and pendant tentacular appendages were recovered from the latest Late Devonian beds of the Sappington Member of the Three Forks Formation in the Gallatin Range, southwest Montana. This is the first velellid reported from a Paleozoic beach paleoenvironment. Top and underside patterns of these chondrophorine velellids display well-preserved casts and mold imprints in fine siltstone. Outlines of the circular disc and the profile of the wide sail and tentacular structures embedded in soft tissue can be recognized.These floating colonial animals were moved along the southeast shores of the Sappington Basin during a time of eustatic sea level lowering and regression in a tropical setting. Colonies were washed up on the upper beach where they were stranded in the bubbling swash left behind by the surf. The foreshore was constructed of tabular, planar cross-bedded, seaward-dipping, foreset accretionary beds which contain parallel laminations and lime-coated grains. Bedding is inclined 17° to 24° seaward.