Bromleyia magnifica n. igen., n. isp.: a feeding trace of a protobranch bivalve

Abstract

The new ichnotaxon Bromleyia magnifica n. igen., n. isp., attributed to the feeding activity of bivalves, is proposed. This ichnotaxon consists of clusters of closely spaced curved ridges that form a fan-shaped structure oppositely distributed on both sides of a longitudinal axis or, more rarely, being present only on one side. Intergradation between Protovirgularia, Lockeia, and Bromleyia forms a compound trace fossil that records the activity of a cleft-foot protobranch bivalve while burrowing, moving through the sediment, and stopping to deposit feed. A specimen from the Carboniferous of Arkansas, previously regarded as Lophoctenium isp., is here included in Bromleyia magnifica. The spreite in Lophoctenium reflects complex behavioural patterns of horizontal strip-mining deposit feeders, including bundles of tubes bending to one side in a pectinate way or arranged on both sides in a highly systematic fashion, which contrasts with the coarse, horizontal fan-shaped, curved bundle of ridges from the Arkansas specimen. The trace fossil Hillichnus lobosensis records the activities of tellinacean deposit feeders and displays feather-like spreite structures reminiscent of Bromleyia. However, the spreite in Hillichnus is more organized, recording repetitive probing through the sediment in horizontal, oblique, and vertical directions of the tubular inhalant siphon. The resulting complex feeding structure, recorded in multiple preservational tiers, displays an alternate arrangement of ridges on either side of an axial basal structure. This configuration differs from the mostly horizontal, less-organized, coarse bundle of ridges with broadly opposite distribution to the sides of an axis present in Bromleyia. Moreover, contrary to Hillichnus, Bromleyia is commonly associated with Lockeia, indicating significant differences in burrowing strategy and mode of construction between these two ichnotaxa.