A segmented worm left tracks on the ocean floor when most animals were sedentary.
Ars Technica said:The Cambrian is rightly famous for being the period when animal life first exploded into a dizzying diversity of forms, including some body plans that remain with us today. But the first animals in the fossil record predate the Cambrian by tens of millions of years. Entire ecosystems of creatures appeared over about 50 million years near the end of the Ediacaran Period and then vanished at the start of the Cambrian.
But the Ediacaran animals were rather strange, with body plans that don't even have the same starting materials as the more familiar Cambrian forms do. And most of them weren't mobile, instead simply attaching themselves to surfaces and staying there. There were, however, limited indications that the ancestors of Cambrian animals had already evolved in the Ediacaran. Traces left in sediments indicated that something was moving through them, and one odd disk-like creature appeared to have had a bilateral body plan. But the tracks weren't left by anything disk shaped, raising questions about what else might have been moving around in the Ediacaran.
Those questions have been at least partially resolved with today's announcement of Yilingia spiciformis, an Ediacaran worm that wouldn't look entirely out of place in today's oceans. Yilingia was segmented, it created tracks, and it even appears to have been able to burrow into sediments. And it managed to do all that without any prominent structures at its head.
Yilingia was found in Ediacaran deposits in the Hubei Province of China. The researchers involved in the study say they have 33 samples in their lab, many of them partial. A 34th was left in place where it was discovered. The animal is a long, thin worm, less than 3 centimeters (1 inch) at its widest, but up to 27 centimeters (nearly a foot) long. Its body is composed of a series of segments, with each segment having three parts: a central piece flanked by two lobes that extend toward its tail. Like a millipede, the segments don't appear to be specialized; they look to differ only in size, with those near the head and tail narrowing slightly compared to those in the trunk.
There are some ambiguous signs that there may be arthropod-like appendages attached to some segments, but the evidence isn't clear, and the authors are cautious about their interpretation here.
What is clear is that the apparent lack of specialization among the segments included the head and tail, in that there really weren't any. The authors describe the evidence for a specialized head as "weak, if not totally absent." Which should leave you scratching your (obviously specialized) head, as they also argued that the segments pointed toward the tail. How did the researchers know which was the tail?
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