It’s called Yilingia spiciformis:
More than half a billion years ago, a strange, worm-like creature died as it crawled across the muddy sea floor. Both the organism and the trail it left lay undisturbed for so long that they fossilized. Now, they are helping to revise our understanding of when and how animals evolved.
The fossil, which formed some time between 551 million and 539 million years ago, in the Ediacaran period, joins a growing body of evidence that challenges the idea that animal life on Earth burst onto the scene in an event known as the Cambrian explosion, which began about 539 million years ago …
“It’s such a bizarre-looking organism,” says Darroch. The creature, which has been named Yilingia spiciformis and was up to 27 centimetres long, seems to be a biologically complex animal with a distinct front and rear end. “We don’t really have many of those from the Ediacaran,” he says.Colin Barras, “Ancient worm fossil rolls back origins of animal life” at Nature
The claim that the worm challenges the Cambrian explosion which followed this Ediacaran period is weird because we knew there were worms in the Ediacaran on account of the tracks (and comb jellies too) but the explosion of multi-faceted life in the Cambrian is a unique event in any case.
(The vid is in French but the pictures tell the story pretty much.)
The late Precambrian- era sediments around the world have yielded four main types of fossils, all of which are dated between about 570 and 543 million years ago. … The third group includes what are called trace fossils, the possible remains of animal activity such as tracks, burrows, and fecal pellets. Some paleontologists have attributed these trace fossils to ancient worms.
This absence of clear affinities has led an increasing number of palaeontologists to reject ancestor-descendant relationships between all but (at most) a few of the Ediacaran and Cambrian fauna. Nevertheless, some have suggested that trace fossils may establish a link. In an authoritative 2011 paper in the journal Science, Douglas Erwin and colleagues described the discovery of Ediacaran trace fossils consisting of surface tracks, burrows, fecal pellets, and feeding trails, which, they argue, though small, could only have been made by animals such as worms with a relatively high degree of complexity. On the basis of these findings, Erwin and other paleontologists have argued that these trace fossils suggest the existence of organisms with a head and tail, nervous systems, a muscular body wall allowing creeping or burrowing, and a gut with mouth and anus. Other paleontologists suggest that these characteristics may indicate the presence of a Precambrian mollusk or a worm phylum.
Graham Budd, a British paleontologist who works at Uppsala University in Sweden, and others, have disputed these associations. Budd and geologist colleague Sören Jensen argue that many alleged trace fossils actually show evidence of inorganic origin: “There are numerous reports of older trace fossils, but most can be immediately shown to represent either inorganic sedimentary structures or metaphytes [land plants], or alternatively they have been misdated.” Still others have suggested that surface tracks and trails could have been left by mobile single-celled organisms, including a known form of a giant deep-sea protist that leaves bilaterianlike impressions. As one paper explains, “Some such traces date back to 1.5 billion to 1.8 billion years ago, which outdates even the boldest claims of the time of origin of animal multi-cellularity and forces researchers to contemplate the possibility of an inorganic or bacterial origin.”
Even the most favorable interpretations of these trace fossils suggest that they indicate the presence of no more than two animal body plans (of largely unknown characteristics). Thus, the Ediacaran record falls far short of establishing the existence of the wide variety of transitional intermediates that a Darwinian view of life’s history requires. The Cambrian explosion attests to the first appearance of organisms representing at least twenty phyla and many more subphyla and classes, each manifesting distinctive body plans. In a best case, the Ediacaran forms represent possible ancestors for, at most, four distinct Cambrian body plans, even counting those documented only by trace fossils. This leaves the vast majority of the Cambrian phyla with no apparent ancestors in the Precambrian rocks (i.e., at least nineteen of the twenty- three phyla present in the Cambrian have no representative in Precambrian strata).
Third, even if representatives of four animal phyla were present in the Ediacaran period, it does not follow that these forms were necessarily transitional or intermediate to the Cambrian animals. The Precambrian sponges (phylum Porifera), for example, were quite similar to their Cambrian brethren, thus demonstrating, not a gradual transformation from a simpler precursor or the presence of an ancestor common to many forms, but quite possibly only an earlier first appearance of a known Cambrian form. The same may be true of whatever kind of worm may be attested by Precambrian tracks and burrows.
Stephen Meyer, Darwin’s Doubt, pp. 81, 85-85 (HarperOne, 2013) (citations omitted).
See also: Earlier than thought: Ancient and well-travelled worm
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