From ScienceDaily:
The fossils were discovered in sediment in the Corumbá region of western Brazil, near the border with Bolivia. The burrows measure from under 50 to 600 micrometres or microns (?m) in diameter, meaning the creatures that made them were similar in size to a human hair which can range from 40 to 300 microns in width. One micrometre is just one thousandth of a millimeter.
Dr Russell Garwood, from Manchester’s School of Earth and Environmental Sciences, said: ‘This is an especially exciting find due to the age of the rocks — these fossils are found in rock layers which actually pre-date the oldest fossils of complex animals — at least that is what all current fossil records would suggest.’
The fossils found date back to a geological and evolutionary period known as the Ediacaran-Cambrian transition. This was when the Ediacaran Period, which spanned 94 million years from the end of the Cryogenian Period, 635 million years ago, moved into the Cambrian Period around 541 million years ago. To put that into context, dinosaurs lived between 230 and 65 million years ago in the Mesozoic Era.
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‘The fossils that we describe were made by quite complex animals that we call bilaterians. These are all animals that are more closely related to humans, rather than to simple creatures like jellyfish. Most fossils of bilaterian animals are younger, first appearing in the Cambrian period.’ Paper. (paywall) – Luke A. Parry, Paulo C. Boggiani, Daniel J. Condon, Russell J. Garwood, Juliana de M. Leme, Duncan McIlroy, Martin D. Brasier, Ricardo Trindade, Ginaldo A. C. Campanha, Mírian L. A. F. Pacheco, Cleber Q. C. Diniz, Alexander G. Liu. Ichnological evidence for meiofaunal bilaterians from the terminal Ediacaran and earliest Cambrian of Brazil. Nature Ecology & Evolution, 2017; DOI: 10.1038/s41559-017-0301-9 More.
In short, even less time can be allowed for the development of complex animals.
From Colin Barras at New Scientist: “It’s also important that these animals were burrowing through sediment, says Nic Minter at the University of Portsmouth, UK. “It marks the point at which these animals became ecosystem engineers, modifying their environment in ways that affect sediment geochemistry and could impact upon other organisms.”
See also: Oldest known multicellulars are Ediacaran seaweed 555 mya