In the late 1990s, word began to spread of something paleontologists had long searched for: a fossil bed more tantalizingly close to the beginning of animal life than any yet seen.
Unearthed by phosphate miners in southern China, the microfossils of the Doushantuo formation just predate -- or are the earliest known members of -- the Ediacaran biota, the first bizarre animal life. The Ediacaran itself predates the 550 million-year-old Cambrian Explosion when most modern animal groups emerged.
In this bed were what appeared to be animal embryos.
20 years and reams of papers later, we still don’t really know what the heck they were. A 2017 review in the Journal of the Geological Society concluded that although they have many animal-like characteristics, none of these features are exclusive to animals. Debate continues.
Other possible Doushantuo finds reviewed by the paper include the first sponge and possible red algae. But nothing except their beauty and marine origin are definitive.
Here are a few:
These fossils bear an unmistakable resemblance to animal embryos. Although the authors of the review agree it is likely the embryo-like fossils represent a single group of organisms, and their characteristics – an ornate envelope, Y-shaped cell junctions, and a particular form of cell division called palintomic -- don’t rule out an animal origin, they also don’t definitively support it.
Some have suggested these stacks of coin-like objects are now-extinct tabulate coral. But corals abandon old chambers, and all of the cells in these forms appear occupied by some sort of biological structures. The authors conclude there’s no good evidence that they were animals of any kind.
G, H, and I all remain mysterious. Some have suggested I is the embryo of a cnidarian, a group that includes coral and jellyfish.
J is a suspected red alga, but the authors feel its non-flattened shape seems at odds with that idea (most algae have flattened blades like leaves to maximize the illumination of their cells). K is an acritarch, the frequently spiny biological equivalent of a UFO. L is a suspected sponge. But other than looking like one, scientists have been unable to find any characteristics in the fossil that are definitively sponge-worthy.
The sculpted, exquisitely precise forms of all these fossils belie their frustrating taxonomic ambiguity. After 600 million years in the ground patiently waiting to be found, they are still waiting to be conclusively identified.
Cunningham, John A., Kelly Vargas, Zongjun Yin, Stefan Bengtson, and Philip CJ Donoghue. "The Weng'an Biota (Doushantuo Formation): an Ediacaran window on soft-bodied and multicellular microorganisms." Journal of the Geological Society 174, no. 5 (2017): 793-802.