Mongolian microfossils undercut the case that animals first appeared in the Ediacaran

Kheseen is a positive control, and a neat one. The bed formed about 550 to 526 million years ago ^[12], some 24 to 48 million years after the oldest fossils accepted as animals, the roughly 574-million-year-old Ediacaran Biota ^[10][20]. Animals were around. If Weng'an-style preservation reliably recorded them, Kheseen should show some. It did not. The scanning electron microscope turned up new species instead, including acritarchs, spherical forms with spines ^[14].

Taken as evidence of absence, Weng'an had confined animal origins to the roughly 16 million years between its 590-million-year age and the Ediacaran fossils ^[19]. "The Kheseen Biota breaks the argument that the exceptional microfossils of Weng'an mean we would have seen animal fossils in the assemblage had they existed at the time," said Ross Anderson, the senior author and an associate professor at Oxford ^[15][16]. The study was published in Science Advances ^[5].

The logic runs one way. A deposit that misses animals when they were present shows the method can fail; it does not place animals at 590 million years. The researchers make the matching point themselves: Weng'an cannot prove animals were absent when it formed, just as Kheseen does not prove they were absent 40 million years later ^[17]. "Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago," said first author Orin Lole Durbin, who did the work as an Oxford undergraduate and is now a PhD student at Virginia Tech ^[7][9]. "However, our new fossil evidence from Mongolia undermines one of the main arguments for restricting animal origins to the Ediacaran interval," Lole Durbin said ^[8].

Chemical traces linked to ancient sponges point to animals by 650 million years ago, about 60 million years before the first animal body fossils, and Discover reports that the findings may help explain that mismatch ^[6].

The older limit comes from the second half of the paper, and Discover's description of it is brief. The three 800-million-year deposits are exceptionally preserved and hold no soft animal tissue. The team takes that to mean animals arose after they formed ^[2]. "Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend," Lole Durbin said ^[4].

The thing this doesn't tell you is why three empty deposits make a firmer maximum age than Weng'an turned out to be. Discover's account does not address it. In my view the 800-million-year limit rests on the same kind of absence that Kheseen has just shown can mislead ^[2][17].

The Snowball Earth framing needs its own check. Earth may have been covered in ice during the Cryogenian ^[3][1]. Of the roughly 100-million-year window the team proposes, only the youngest 20 million years fall inside that period; the other 80 million predate it ^[18]. A Cryogenian origin fits these numbers, though most of the window lies before the ice. The 650-million-year sponge biomarkers do sit inside the Cryogenian ^[21].