Contrasting behavioral and feeding strategies recorded by tidal-flat bivalve trace fossils from the Upper Carboniferous of eastern Kansas

Abstract

Upper Carboniferous tidal-flat deposits near Waverly, eastern Kansas (Stull Shale Member, Kanwaka Shale Formation), host abundant and very well-preserved trace fossils attributed to the activity of burrowing bivalves. Thin shell lenses with an abundant bivalve fauna are associated with the ichnofossil-bearing beds and afford an unusual opportunity to relate trace fossils to their makers. Two distinctive life and feeding strategies can be reconstructed on the basis of trace fossil analysis and functional morphology. Lockeia siliquaria hyporeliefs commonly are connected with vertical to inclined, truncated endichnial shafts in the absence of horizontal locomotion traces. These structures record vertical and oblique displacement through the sediment, and suggest relatively stable domiciles rather than temporary resting traces as typically considered. Crowded bedding surfaces displaying cross-cutting relationships between specimens of L. siliquaria and differential preservation at the top (concave versus convex epireliefs) record a complex history of successive events of colonization, erosion, deposition, and recolonization (time-averaged assemblages). Irregular contours of some large hypichnia indicate the cast of the foot, while other outlines closely match the anterior area of Wilkingia, its suggested tracemaker. Relatively stable, vertical to inclined life positions and dominant vertical mobility suggest a filter-feeding strategy. Moreover, the elongate shell and pallial sinus of Wilkingia provide a strong independent line of evidence for an opisthosiphonate, moderately deep-tier inhabitant. Wilkingia may represent a pioneer attempt at siphon-feeding in the late Paleozoic, preceding the outcome of the Mesozoic infaunal radiation. A second strategy is represented by Lockeia ornata and associated locomotion and locomotion/feeding structures. Lockeia ornata is commonly connected with chevron locomotion traces that record the bifurcated foot of a proto-branch bivalve. Lockeia ornata exhibits distinctive, fine, parallel lines that mimic the ornamentation of Phestia, a nuculanid protobranch bivalve. Rosary and radial structures give evidence of a patterned search for food. Lockeia ornata and associated Protovirgularia record dominant horizontal locomotion and suggest the activity of deposit-feeding bivalves. Morphologic variability of Protovirgularia was controlled by substrate fluidity, which was dependent on sediment heterogeneity and tidal-cycle dynamics. This study demonstrates that detailed analysis of bivalve traces provides valuable information on bivalve ethology and paleoecology, evolutionary innovations, environmental dynamics, and substrate fluidity.