A transitional fauna shows that the “Cambrian explosion” was happening before the Cambrian

The Cambrian Period, beginning at 538.8 Ma (million years ago) and lasting about 52 million years, is famous for marking the transition from simple and largely unicellular animals to, beginning at the period’s inception, representatives of modern groups.  This apparently rapid onset of modern forms of multicellular animals constitutes the famous “Cambrian Explosion.”

The Cambrian was preceded by the 96-million-year-long Ediacaran period, extending from 635 million years ago to the beginning of the Cambrian. The Ediacaran fauna, consisting of some multicellular animals of unknown affinity and things looking like members of some modern groups like cnidarians (represented today by jellyfish, corals and anemone). But most of the Ediacaran groups appeared to have died out at the end of the Ediacaran, and for unknown reasons.

The boundary between the Ediacran and the Cambrian thus marks a major transition in animal life.   Many of the “modern” groups that first arose during the Cambrian don’t have apparent ancestors in the Ediacaran, and so those modern groups were thought to have evolved almost instantaneously (in geological time!). But surely modern groups had ancestors during the Ediacaran: unless you’re a Biblical fundamentalist, you realize that ancestors of modern groups had to have existed well before the Cambrian explosion.

Now a paper in Science, based on a fossil group called the Jiangchuan Biota that spans the period from 559-534 million years ago, shows that representatives of “modern” groups seen in the Cambrian explosion were indeed present in the late Ediacaran, pushing back the time of origin of modern phyla 4-5 million years.  This conclusion was possible because of the remarkable preservation of the animals (and some algae), all present as carbonaceous films on rocks—the same kind of films (presumably due to rapid burial) that enabled us to see the remarkable Burgess Shale fauna of the middle Cambrian. The new find was in the province of Yunnan in Southwestern China.

You can see the paper by clicking the screenshot below, reading the pdf here, or reading the shorter blurb at an Oxford University sit. at the bottom. All photos below are taken from the paper.

I won’t go into all the terminology involved in identifying the groups but will show a few fossils from the paper strongly suggesting that some “modern” groups arose in the late Ediacaran.

First, an anomalous animal that appears to be some kind of worm, but one with a “holdfast” disc on its butt. We don’t know what this one is, but it has oral projections or tentacles. The disc is very clear:

Another wormlike animal (note that these are small: a few millimeters) having a clear oral region. Again, we’re not sure what this is, but the preservation as a carbon film is remarkable:

A deuterostome (animals where the first opening in the embryo becomes the anus rather than the mouth), a group thought to have appeared in the Cambrian but here seen in the Ediacaran: this one resembles  Herpetogaster, known from the early Cambrian which, according to Wikipedia, “possessed a pair of branching tentacles and a tough but flexible body that curved helically to the right like a ram’s horn and was divided into at least 13 segments”. This one, like Herpetogaster, has tentacles (at leat four) and a stalk.  It’s interpreted as a relative of acorn worms, relatives of modern echinoderms which are hemichordates, the closest living group to modern chordates (animals with notochords and a dorsal nerve chord, which include all vertebrates).

The one below,described in the paper as “Margaretia-like animal now known as a dwelling tube for an enteropneust hemichordate worm”. It’s also described as having “regular, oval-shaped holes running along its length”. Again, we see what is likely an early hemichordate, showing that the relatives of modern chordates seem to have been present several million years before the Cambrian explosion began.

The one below is identified as a ctenophore, or comb jelly, a phylum of early animals previously known only from the mid-Cambrian. “OS” stands for “oral skirt”, described as “a specialized, often scalloped, muscular, or rigid structure surrounding the mouth, primarily found in Cambrian-era fossil comb jellies such as Ctenorhabdotus and Thalassostaphylos. Unlike modern ctenophores, these ancient species used the skirt for feeding, potentially to engulf large prey.”

Finally, this animal is thought to be an early cnidarian with tentacles and a holdfast (HF). Although one form identified as a cnidarian had already been recognized from the Ediacaran, here we have another that’s different, showing a radiation of cnidarians before the Cambrian.

These fossil data support already-existing molecular data suggesting that animal groups had already evolved and diversified before the Cambrian, though until now no fossils, or only a few suggestive fossils, were known.

The authors’ summary below, though written in scient-ese, basically says that a major radiation of animal phyla had already begun before the Ediacran/Cambrian boundary, but we did not know about it because the conditions for forming this kind of trace fossil, requiring rapid burial in marine sediment (and subsequent finding by investigators!) were infrequent:

The new Jiangchuan animal fossils, dominated by bilaterians of apparently diverse affinities, with rarer fossils more typical of late Ediacaran deposits, could be described as a “Cambrian-type” assemblage from the late Ediacaran. A dominantly bilaterian assemblage from the late Ediacaran may not have been discovered until now as a result of the paucity of carbonaceous compressions from this time, hinting at a broader taphonomic bias (51).

If you want a short, readable summary of the importance of this fine, click below to read a shorter summary from Oxford University.