Tracks, Trails and Trampling by Large Vertebrates in a Rift Valley Paleo-Wetland, Lowermost Bed II, Olduvai Gorge, Tanzania

The impact of large vertebrates on the sedimentary record in an East African groundwater-fed wetland in Ngorongoro Crater, Tanzania, is used as a modern analog to interpret an ancient (Plio-Pleistocene) wetland record in Olduvai Gorge, Tanzania. The 3 km2 wetland at Olduvai is characterized by massive silty claystones produced by intense vertebrate trampling, as well as trails by vertebrates frequenting the wetlands and isolated footprints that represent a “snapshot in time.” Based on modern analogs, a paleo hippo trail (1.2 m wide and 0.6 m deep) in the Plio-Pleistocene wetlands is interpreted as a frequently used corridor between hippo pools (days) and grazing meadows (nights). Groundwater-fed wetlands are low energy environments where the physical record appears to be dominated by plant and animal activity. The bioturbation record reflects a number of interacting factors such as substrate texture, moisture content, sedimentation rate, frequency of flooding, type of animals present, trampling rate, and post-depositional changes (compaction). Lithofacies in both the modern and ancient wetlands include muddy sandstone (drainage channels) and silty claystone (vegetated and nonvegetated mud flats). Organic-rich sediments eventually oxidize, eliminating most evidence of the habitat. Modern wetlands have organic-rich mud and peat, whereas the ancient analog has siliceous earthy claystones that contains plant remains, bone fragments, pollen, phytoliths, and localized beds of diatomite. Thus, the physical record of vertebrate bioturbation in conjunction with paleontological and lithological records provides crucial information on the ecology of ancient wetland environments.