Tubular structures that may be prehistoric horny sponges and found in ancient reefs in northern Canada could be the oldest known fossils of animal life ever discovered.
Researchers found the unusual features in vast reefs that were built by bacteria 890m years ago and then pushed up by geological processes to form part of the Mackenzie Mountains in north-western Canada.
Examined under a microscope, a small number of rock samples revealed tubules about half the width of a human hair that branch and reconnect to form 3D structures that are strikingly similar to those seen in fossils of bath sponges.
“Initially, when you look at these features they look like a bunch of wiggles, but when you try to follow each of the strands, you realise that even in thin sections they form complicated 3D meshworks,” said Prof Elizabeth Turner, at Laurentian University in Sudbury, Ontario.
The discovery was made in northern Baffin Island, Nunavut.
“Thanks to the wonderful work of others, younger examples of the same microstructures have been found in sponge body fossils and these structures have been compared to the skeletons of a variety of keratose, or horny, sponges,” she added.
While modern reefs are built by corals and algae, in Earth’s distant past communities of photosynthetic cyanobacteria created enormous carbonate reefs measuring many kilometres wide and hundreds of metres thick.
Prof Turner first spotted the weird tubules in a handful of thin sections of rock she gathered as a PhD student during field work at the site. Two decades on, she has built up the collection and discovered more examples of the features in the rocks.
Writing in the journal Nature, Turner describes how the fossils may have formed when putative sponges, measuring a few millimetres to a centimetre across, became mineralised. The soft tissue is first to fossilise, encasing the 3D network of collagen-like fibres that form the sponge’s skeleton. Over time, these skeletal fibres decay, leaving hollow tubules that fill up with calcite crystals.
If the structures are confirmed as early sponge fossils, they would predate the next-oldest undisputed sponge fossils by about 350m years.