PALAEOBIOLOGY OF THE CLIMACTICHNITES TRACEMAKER

Abstract

Abstract: Trace fossils such as Climactichnites offer rare insights into the palaeobiology of Cambrian soft‐bodied animals, especially those that inhabited emergent sand flats and are not known from body fossils. Analysis of field and museum Climactichnites, together with experiments on the preservation of similar modern trails, indicates that the tracemaker was an elongate, bilaterally symmetrical, dorsoventrally flattened, soft‐bodied animal with a muscular foot. These characteristics are consistent with the tracemaker being a primitive mollusc or mollusc‐like animal. Unlike most Neoproterozoic and Cambrian molluscs, the tracemaker could reach considerable size; at up to c. 69 cm long, it was one of the largest Cambrian animals of its time. When moving on the sediment surface, locomotion resulted from muscular waves generated along the sole of its flexible foot; the foot was extended and then clamped onto the substrate. Contraction of pedal muscles then pulled the body forward. Sedimentary structures associated with Climactichnites wilsoni, such as polygonal desiccation cracks, raindrop impressions, adhesion structures and gas escape structures demonstrate that the animal inhabited intermittently subaerially exposed environments. The tracemaker’s method of locomotion is similar to that employed by modern intertidal gastropods, which make Climactichnites‐like trails on exposed sand flats. However, these modern trails are not preserved because of erosion by wind, waves, tides and subsequent bioturbation. Abundant microbial sedimentary structures are associated with C. wilsoni, and together with low levels of vertical bioturbation, intimate that microbial binding may have mediated the preservation of these early mollusc trails.