LARGE RUSOPHYCID TRACES FROM THE LOWER TRIASSIC: AN EXAMPLE OF CONVERGENT EVOLUTION IN ARTHROPOD PREDATORY BEHAVIOR

Occurrences of Paleozoic Rusophycus have been interpreted as predatory traces produced by trilobite arthropods which went extinct during the end-Permian extinction. Traces with a rusophyciform morphology from the younger Lower Triassic Moenkopi Formation in Utah are also interpreted as predatory traces based on evidence for a change in digging strategy during trace production and their close association with other traces (Rhizocorallium). The traces range from 3 to 8 cm wide and 0.8 to 5 cm deep, penetrate the substrate at an angle, and are preserved in convex hyporelief on the base of siltstone beds originally deposited in a low-relief, tidally-influenced delta. They are characterized by an overall bilobate morphology with a median furrow separating bilaterally symmetrical, primary lobes sculpted by millimeter-scale bioglyphs. A distinct pair of secondary lobes representing the deepest portions of the trace are superimposed on the primary lobes. These may be smooth or sparsely sculpted by bioglyphs. Bioglyphs on the primary lobes are subparallel whereas those on the secondary lobes lack a shared orientation. This distinct primary and secondary lobe morphology and the bioglyph patterning of these lobes suggest that the tracemakers systematically changed their digging strategies during trace production. Rhizocorallium occur at the same bedding horizon with individual burrow tubes sometimes preserved protruding from the median furrow of the rusophycid traces. Close association with other traces and changes in digging strategy support a hunting interpretation where there was an active prey item and the rusophycid trace recorded the initial attack and subsequent adjustments made by the predator. Characteristics of the Moenkopi rusophycid assemblage include randomly oriented traces of varying size that do not overlap indicating a single, uncoordinated gathering of arthropod tracemakers for feeding purposes. Based on their age, their unusually large size and the appendage dexterity required to produce the trace and its bioglyphs, a decapod crustacean is considered the most likely tracemaker. These rusophycid traces are an example of behavioral convergence in the evolution of predatory feeding strategies and associated functional morphologies of Paleozoic trilobites and early Mesozoic arthropods.