Unusual ichnofossils in homo erectusbearing beds of the Pleistocene lake deposits in central-eastern Eritrea, East Africa

摘要

The continental Pleistocene Alat Formation in Eritrea hosts one-million-year-old Homo erectus remains and includes lacustrine calcareous beds with exceptionally numerous and well-developed ichnofossils. They are referred to two different types with distinctive morphologies based on the shape of their casts on the bed soles. Ichnofossils of the first type, termed donuts, have a pronounced ring shape with an average diameter of ～13 cm. Those of the second type have a rosette shape with a diameter of up to 35 cm and more than 20 short fingers arranged around a flat, circular area. We interpret the donut-shaped structures as casts imprinted by the conical pedal disk of an unknown organism on the soft floor sediment. A similar organism with a more complex pedal disk, characterized by protrusions, may have made the rosette-shaped ichnofossils. Both ichnofossils are interpreted as resting traces of soft-bodied animals anchored in a vertical position within the sediment. After their deaths, the traces were preserved as casts on the lake floor and subsequently filled by calcareous sand. The peculiar plug-shaped resting traces, the lacustrine habitat of the tracemakers, and the lack of similar traces in the literature suggest that these fossils represent two new ichnotaxa left by invertebrate organisms of unknown affinity.