Before life explodes in the Cambrian, there were worms

The Cambrian is rightly famous for being the time when animal life exploded for the first time in a dizzying variety of shapes, including some body plans that remain to us today. But the first animals in the fossil record date back tens of millions of years. Entire ecosystems of creatures appeared in the last 50 million years towards the end of the Ediacaran period, then disappeared in the early Cambrian.

But the Ediacaran animals were rather strange, with bodily plans that do not even have the same starting materials as the more familiar Cambrian forms. And most of them were not mobile, they simply attached to the surfaces and stayed there. However, there were few indications that the ancestors of the Cambrian animals had already evolved in Ediacaran. Traces left in the sediments indicated that something was passing through them and a strange disk-like creature appeared to have had a bilateral body plan. But the tracks are not left by a disc-shaped disc, raising questions about what could have moved in Ediacaran.

These issues have been at least partially resolved with today's announcement of Yilingia spiciformis, an Ediacaran worm that would not seem totally out of place in today's oceans. Yilingia has been segmented, it has created tracks and it even seems that it has been able to bury itself in the sediments. And he managed to do all this without any major structure at his head.

A box of worms

Yilingia was found in Ediacaran deposits in China 's Hubei Province. Researchers participating in the study report having 33 samples in their laboratory, many of which are partial. A 34th was left at the place where it was discovered. The animal is a long thin worm, less than 3 centimeters (1 inch) wide, but up to 27 centimeters long. His body is composed of a series of segments, each segment being composed of three parts: a central piece flanked by two lobes that extend towards the tail. Like a centipede, the segments do not seem to be specialized; they appear to differ only in their size, those near the head and tail narrowing slightly with respect to those of the trunk.

There are ambiguous signs that there may be arthropod-like appendages attached to certain segments, but the evidence is unclear and the authors are cautious about their interpretation.

What is clear is that the apparent lack of specialization among the segments included the head and the tail, in that there were not really any. The authors describe the evidence of a specialized head as "weak, if not totally absent". Which should leave you scratching your head (obviously specialized) because they also argued that the segments were pointing to the tail. How did the researchers know what the tail was?

Follow the path

It turns out that there were a dozen traces in the sediments, which corresponds to the trace left by these worms as they crawled on the sediment. A 13th trace ended on the body of a Yilingia. (The technical term used to refer to such discoveries is "mortichnia", which essentially means a track that ends at the body of the animal that made it.) In some cases, the tracks are are completed at what appeared to be burrows, indicating that Yilingia was able to dig into the sediments as well as to cross their surfaces. So, there is no doubt that it was a moving animal.

The key question is what type of animal it is. An obvious mission would be to Yilingia in Annelida, a group that includes many segmented worms. But if the limbs turn out to be real, then it would probably be grouped with arthropods. Arthropods, however, are defined as having compound eyes, a brain, and other elaborate features that are clearly absent from the newly discovered creature. Perhaps he would be closer to being an arthropod ancestor if the problem of the members ended up favoring this interpretation.

Although some of the evidence is ambiguous, the main aspects of the discovery are perfectly clear. Bilaterally symmetric animals are pre-Cambrian and have previously involved segmentation and mobility. (The authors argue that segmentation is perhaps the key to bilateral mobility, but many Ediacarean creatures are also segmented but never displaced.) And, critically, all these innovations were present well before the Cambrian, implemented place the basic ingredients for the explosion of forms that could possibly occur.

Nature, 2019. DOI: 10.1038 / s41586-019-1522-7 (About DOIs).