Shuhai Xiao of Harvard University and colleagues, writing in the journal Nature in 1998, were among the first to propose that such fossils are the remains of multicellular animal embryos. Since then, numerous “embryo-like” fossils have been unearthed at the 600-million-year-old Doushantuo Formation of South China.
The Mongolian fossils, discovered at that country’s Khesen Formation to the west of Lake Khuvsgul, are younger. Anderson and his team dated them to 540 million years ago.
The scientists decided to excavate the site in northern Mongolia because it consists of numerous phosphorites, which are sedimentary rocks that contain a high proportion of calcium phosphate. The rocks at Doushantuo are of a very similar composition.
“In terms of the fossils present (at Khesen) there are a lot of similarities” with those at Doushantuo, Anderson said. “The fossils are also preserved in the same manner as those found in the Doushantuo Formation, yielding the exceptional cellular level of preservation.”
He and his colleagues suspect that many of the microfossils preserve different stages of embryonic development whereby cells divide and multiply.
“For example,” he said, “we have specimens with only a single internal cell, but others with up to 100.”
As for how single-celled organisms evolved into multicellular ones, there are many possibilities.
Anderson explained that an increase in the availability of oxygen; genomic evolution, which may provide new biological equipment; changes in nutrient cycling; continental reconfiguration, which opens up new habitats; and greater predation could have been factors.
He added that evaluating these hypotheses requires a good temporally calibrated fossil record to test correlations to environmental change. Since the Mongolian microfossil assemblage extends both the location and timing of Megasphaera, it moves the researchers one step closer to that goal.