Intricate tube structures discovered in ancient reefs could be the remains of prehistoric horny sponges and the world’s oldest known animal fossils.
The strange patterns were discovered in enormous reefs created by bacteria 890 million years ago and then pushed up by geological processes to form part of the Mackenzie Mountains in northwestern Canada.
A tiny number of rock samples examined under a microscope revealed tubules approximately half the thickness of a human hair that branch and reconnect to form 3D structures strikingly similar to those found in bath sponge fossils.
“At first glance, these features appear to be a bunch of wiggles, but when you try to follow each of the strands, you realize that they form sophisticated 3D meshworks even in thin areas,” said Prof Elizabeth Turner of Laurentian University in Sudbury, Ontario.
“Younger instances of the same nanostructures have indeed been found in sponge body fossils thanks to the amazing work of others, and these structures have been compared to the skeletons of a variety of keratose, or hairy, sponges,” she noted.
Corals and algae build modern reefs, but in the distant past, populations of photosynthetic bacteria constructed massive carbonate reefs that were hundreds of kilometers wide and hundreds of meters thick.
After two decades, she has expanded her collection and uncovered new examples of rock features.
Turner outlines how the fossils may have evolved when putative sponges measuring a few millimeters to a centimeter across became intake of calcium in the journal Nature.
The soft tissue is the first to fossilize, encasing the sponge’s skeleton, which is made up of a 3D network of collagen-like fibers. These skeletal fibers degrade over time, leaving empty tubules filled with calcite crystals.
If the structures are proved to be early sponge fossils, they would be 350 million years older than the next oldest undisputed sponge fossils.
Sponges, according to earlier research, are among the first organisms on Earth, having appeared between one billion and half a billion years ago during the early Neoproterozoic era.
However, the precise timing is unknown. According to another study, oxygen levels were too low for species to live until the Neoproterozoic Oxygenation Event occurred between 800 and 540 million years ago.
Early sponges may have found a way to thrive in the nooks and crannies of microbial reefs even on an oxygen-depleted Earth if the photosynthetic bacteria that suffocated the reefs released enough oxygen into the sea.
Prof Turner explained, “These probable sponges, or whatever they were, were living in a reef that was an oxygen factory, it was nirvana.” They could have eaten the polysaccharide slime that sloughed off the microbial mats and into the surrounding ocean as a source of sustenance.
“If my interpretation of these data is correct, the early animals may not have had the same oxygen requirements that we have thought up to this point.” “We probably had early sponges develop some time ago, and certainly by 890 million years ago, surviving in a world with limited oxygen,” she continued, “but the more intricate organisms had to wait a while.”
More research is needed to investigate if other rocks from the same epoch have indications of early animal life. Prof Turner stated, “We have to approach it with a much more open mind.”
“We need to consider what we can expect from early animals. Anything we consider to be an animal could be overly complicated. We’re looking for something that captures the essence of animals while being unmistakably familiar.”
