Ecological continuity between Lower and Upper Bed II, Olduvai Gorge, Tanzania

The sediments of Bed II at Olduvai Gorge, Tanzania, have received less scientific scrutiny than Bed I strata, likely due to the lower density of fossil hominins in the younger layers. Nevertheless, Bed II provides important contextual information about changes over time in early hominin environments. Mammal fossils from Bed II are separated into older and younger faunas by a regional disconformity under which lies an aeolian tuff, the Lemuta Member, dated to ~1.74Ma (millions of years ago). Differences between the faunal communities above and below the Lemuta Member have been previously noted and interpreted as evidence for distinct environmental change. Here, we investigate the palaeoecological conditions that characterised the transition between older and younger Bed II sites via analysis of their mammalian community ecological niche exploitation profiles, comparing these to known modern habitats. We present a comprehensive species list for both Bed II faunas. An initial correspondence analysis points to palaeoecological differences in pre- and post-Lemuta Member assemblages, although neither is representative of a forest habitat. When taphonomic differences in body mass profiles are taken into account, however, ecological differences appear far less important. Based on a resampling procedure to generate modern locality data with body mass profiles similar to the Bed II faunas, discriminant correspondence analyses liken both fossil assemblages to modern woodland habitats, with Lower Bed II having some affinity to floodplain and marshy woodlands, likely related to the presence of the Olduvai palaeolake in this stratigraphic interval. A comparison of the Bed II faunas to each other suggests that their differences could reasonably be sampled from within a single modern habitat. Although additional evidence points to an increase in aridity subsequent to the Lemuta Member, the importance of woodland habitats throughout Bed II is clear.