Mongolian Fossil Bed Challenges Timeline for Animal Origins

A fossil site in Mongolia, the Kheseen Biota, preserves microscopic organisms in remarkable detail but contains no fossils that can be confidently called animals, even though animals are known from other regions at that time. This undermines the assumption that an exceptionally preserved deposit would necessarily capture any animals alive then, a conclusion from a University of Oxford-led study in Science Advances. The finding suggests animals may have originated as much as 200 million years earlier than their first unambiguous fossils.

The Kheseen Biota in Mongolia contains microfossils preserved with cellular detail, including embryo-like forms, yet lacks fossils confidently classed as animals. It is younger than China’s Weng’an deposit by over 40 million years and overlaps in age with known animal fossils elsewhere.

Researchers surveyed over 140 samples with scanning electron microscopy, finding new acritarch species alongside previously undocumented localities. Because early animals were small and soft-bodied, their preservation required particular environmental and chemical conditions. Their absence at Kheseen may reflect different habitats or unsuitable preservation chemistry, weakening arguments based on missing animal fossils at Weng’an.