The Middle Devonian (Emsian /Eifelian) Trout Valley Formation is exposed in the northeastern corner of Baxter State Park, Maine, and is noted for its abundant plant-fossil assemblages. However, to date no invertebrate macrofaunal assemblages have been reported in this fluvial-to-marine sequence; only isolated eurypterid parts have been reported. A previously undescribed outcrop of coarse- to medium-grained siltstone characterized by megaripples preserves a restricted, transported invertebrate assemblage. The macrofossils are randomly oriented and concentrated in the ripple crests. Similar sedimentological features in other parts of the stratigraphic section indicate an estuarine, tidally influenced depositional regime. The fossil assemblage is dominated by Phthonia sectifrons-an uncommon Devonian bivalve known previously from open-marine deposits. Few other fossil taxa occur. Typically associated with deep-shelf brachiopods, the presence of P. sectifrons in these estuarine deposits indicates that this taxon occupied a wider range of habitats, extending from the shallow silty shelf facies to more transitional settings. The facies associations in the Trout Valley Formation suggest that Devonian near-shore communities resembled modern bivalve-dominated nearshore communities.
