ICHNODIVERSITY DOES NOT EQUAL BIODIVERSITY: BIVALVE TRACE FOSSILS FROM THE UPPER MISSISSIPPIAN PENNINGTON FORMATION OF GEORGIA

Improved recognition of compound trace fossils shows that a diverse suite of interconnected or intergradational trace fossils may be attributed to a single species of tracemaker. Identification of one tracemaker for an ichnoassemblage is also supported in cases where the ichnotaxa share a bioprint diagnostic of tracemaker anatomy and dimensions. In this study, we apply these principles to a morphologically diverse ichnoassemblage in the Upper Mississippian Pennington Formation of Dougherty Gap in northwest Georgia. We propose that one species of bivalve made nearly all of the trace fossils at this locality. Other, less common trace fossils include topotypical Cymataulus undulatus and indistinct epichnia.

Paleoenvironments of the Pennington at this locality are interpreted as an interdistributary bay succeeded by an interdistributary channel, both nearly freshwater but with some tidal influence. The substrate inhabited by the tracemakers consisted of stiff clay layers alternating with thixotropic sand, a situation that led to finely detailed preservation of the traces. Most of the trace fossils are preserved as endichnia to positive-relief hypichnia of “Hillichnus”: the first record of a trace fossil resembling this complex ichnogenus reported in Paleozoic strata. Type Hillichnus is interpreted as a bivalve resting, locomotion, and feeding trace fossil.

The Dougherty Gap specimens are similar but are associated with Lockeia, Protovirgularia, and donut- to anchor-shaped traces: 14 ichnotaxa were named in one previous study. The bioprint of the resting and locomotion traces shows that the maker was an inflated, elongate bivalve nearly equal in width and height, with small umbo and pronounced costae: perhaps Wilkingia. Thus, it is possible to interpret the tracemaker's range of behaviors as well as its probable identity. We urge other researchers to look beyond a morphologically split nomenclature to illuminate biological and behavioral connections between trace fossils in a holistic manner.