Evolutionary and ecologic controls on benthos distribution from an upper Cambrian incised estuarine valley: Implications for the early colonization of marginal-marine settings

Estuarine deposits of the Furongian Pico de Halcón Member of Argentina contain trace and body fossils that allow assessing the role of evolutionary and environmental factors on early colonization of marginal-marine environments. The outer region of the estuary was characterized by a subtidal sandbody flanked by tidal flats. Vertical burrows of suspension feeders and probably passive predators are abundant in the high-energy areas of the outer estuary. Horizontal trilobite and vermiform trace fossils record the activities of mobile deposit, detritus, and suspension feeders in more protected settings. Arthropod trackways, vertical burrows, and horizontal vermiform burrows have been recorded in middle estuarine deposits. The occurrence of trackways attests the ability of some late Cambrian marine arthropods to periodically migrate inland following tidal channels or via salt-water wedges. No trace and body fossils have been found in inner valley areas. Outer and middle estuarine deposits contain the trilobite Neoparabolina frequens argentina. Its record in brackish-water settings is envisaged as a result of its generalist nature and ability (physiologic or behavioral) to cope with a wide array of conditions, flexible resource requirements, and successful spreading.

Although trace-fossil distribution along the salinity gradient reflects the basics of the “brackish-water model”, the remarkably low ichnodiversity in the middle estuary and the lack of trace fossils in the inner estuary are interpreted as reflecting evolutionary controls. Teichichnus assemblages, forming monospecific occurrences or with Planolites, in fine-grained, heterolithic facies seem to be remarkably persistent through geologic time. The studied trace fossils yield insights on attempts of some clades to expand into stressful brackish-water settings early in their evolutionary histories. In addition, comparison of the environmental tolerance of the Furongian benthos with other occurrences in younger early Paleozoic estuarine settings allows to track early steps of the colonization of brackish-water environments as it developed during the Ordovician Radiation.