Discovery of sponge fossils 890 million years old
Several strange sponge fossils, related to the large toilet sponge pictured above, may be the oldest fossilized animals ever discovered.
Mesh-like in appearance, these fossils are believed to be 300 million years older than the oldest animal life known to date. Their discovery is already sparking a lively debate within the scientific community.
The ancestors of sea sponges filtered the waters of our planet for hundreds of millions of years, long before the first plants appeared on dry land. Because of their simplicity, scientists have suggested that sponges were the first form of animal life to appear on Earth. But the exact period of this appearance is still debated.
A study published in the journal Nature now suggests that structures discovered in an ancient reef would be sponges 890 million years old . If confirmed, sponge fossils found in limestone rock at 'Little Dal' in northwestern Canada would be 300 million years older than the oldest fossilized animal life form known to exist. this day.
Most discoveries of very ancient life forms are controversial, and the Little Dal fossils are no exception. For what ? Because the ancient creatures that populated the seas were undoubtedly very different from those that evolve today in the oceans. Furthermore, scientists are also divided regarding the type and abundance of evidence to distinguish animals from other life forms, or even from geological structures.
“Here we are faced with a kind of Rorschach test, with squiggles in the rock,” reveals Jonathan Antcliffe , paleontologist specializing in the first forms of life at the University of Lausanne, Switzerland.
During a Zoom interview, Elizabeth Turner , author of the study, made a point of showing a mustard yellow natural toilet sponge; this modern species is related to the fossilized sponges she studied. According to the scientist, the network of flexible tubes that give the sponge its spongy character had a structure “identical” to that of recently analyzed fossils and several less ancient fossils recently identified by other scientists.
“There's almost no question about it,” says Elizabeth Turner, a field geologist at Laurentian University in Ontario, Canada. She acknowledges, however, that the suggested identity of the animal will be controversial. "The time had come to publish the study to discuss and question it with the scientific community".
WEIRD FOSSILS
The newly described fossils have been discovered in nooks and crannies of the huge Little Dal Reef; it was formed when warm seas covered a large area of land in what is now North America. Over time and the work of tectonic plates, inland seas dried up and reefs turned to rock. Unlike most modern reefs erected by corals, the architectures of this ancient structure were cyanobacteria. These microbes, which grow in layers of slime, form layered mounds over time as their sticky surface traps sand and dissolved minerals in the water turn into strong fibers.
These fossilized layered microbial structures are known as stromatolites . Some date from 3.5 billion years ago and are the oldest traces of life on Earth.
Elizabeth Turner was a student at Queen's University in Ontario, Canada, when she began studying Little Dal decades ago. The researcher was interested at the time in the construction of the reef by cyanobacteria when a series of strange samples showing complex structures caught her attention.
“It was suspicious,” recalls the geologist. While Little Dal Reef is characterized primarily by "a streaky array of strata", some samples of these structures exhibited tube-like shapes that branched off before joining to form a three-dimensional polygonal network. What leave the researcher perplexed as to the identity of these strange forms.
“I put that in the back of my mind and forgot about it,” she says.
But in recent years, clues to their potential identity have begun to accumulate. In particular, researchers have discovered remarkably similar tortuous networks in rocks much younger than the Little Dal reef, in several places. According to them, the mesh formations with branches could be fossil sponges of the Kerato group.
The skeleton of most sponges is made up of small, rigid structures called spicules. The latter, made up of calcium carbonate and silica, have the shape of ossicles. In fossils, this rigid structure is found in early sponges, but is absent in Keratosa sponges. It's networks of spongin, a protein with a soft, spring-like texture that's great for grooming use, that give them their spongy structure.
But by examining very thin sections of rock under a microscope, Elizabeth Turner observed similarities between the shapes and tubular structures seen in the Little Dal samples and fossils previously identified as Keratosa sponges, as well as modern sponges.
Decades after first spotting these strange shapes, the geologist was finally ready to publish her discovery. "It's an ode to the soft sciences," she says.
EVIDENCE CONSIDERED INSUFFICIENT
This new study fuels the long debate about the appearance of the first animals and the evidence needed to confirm that a fossil is indeed that of an animal. As Keyron Hickman-Lewis , a geobiologist specializing in ancient microbes at the Natural History Museum in London, explains, the use of geochemical tracers for the identification of early life forms, or biomarkers, has become widespread in recent years. Fossilized remains of several types of lipids are commonly used as biomarkers, for example.
Since then, a large part of this so-called evidence of early life forms has turned out to be false, says the geobiologist. Some biomarkers resulted from contamination, while other chemical residues were not enough to prove that the fossil was indeed that of an animal. Scientists have recently discovered that algae, coupled with geological alterations, can produce the same compounds previously identified as belonging to ancient sponges extracted from 635 million year old sediments in Oman.
Premature enthusiasm gave way to a certain “distrust as to the origins of the first forms of animal life”, emphasizes Keyron Hickman-Lewis.
The study of the mesh networks discovered at Little Dal promises to revive the debate. "The evidence is strong," said Robert Riding , reviewer of the study and researcher at the University of Tennessee in Knoxville, United States. He recently published a study of similar fossils associated with a roughly 485 million year old stromatolite discovered in New York State.
For Elizabeth Turner, the association of these sponges with microbial reefs would be logical. The Earth's atmosphere has not always been rich in oxygen and the first sponges would have lived before this life-sustaining gas became more abundant in the sea . Nevertheless, "oases of oxygen" would have existed around the reefs of cyanobacteria, where the photosynthetic microbes would have spewed out the oxygen absorbed by the sponges.
“The fact that these two events are linked strengthens the argument in their favour,” says Keyron Hickman-Lewis.
Other, more skeptical scholars note that the sponge-like network is not unique to this group of sponges, as Elizabeth Turner and other researchers suggest. “Every form of life, whether bacteria, algae, fungi, plants and animals, can form structures resembling this one,” says Jonathan Antcliffe.
In 2014, while examining evidence for the presence of the first sponges , the scientist and his colleagues discovered that the oldest and most convincing animal fossils were spicules of sponges observed in Iran and dating from around 535 million years old. As Jonathan Antcliffe points out, no recent study has changed his mind on this.
Numerous analyzes have identified what he says are "suspicions" of older sponge structures. But none of them presents the indisputable characteristics, namely spicules or pores. The latter proved essential in confirming the identity of the controversial archaeocyath sponges. This group, which does not have spicules, has been identified in rocks 523 million years old.
For Drew Muscente , a paleobiologist at Cornell College in Iowa, USA, part of the challenge stems from the difficulty in identifying ancient sponges from other animals. Dinosaurs, for example, have a host of highly recognizable bony features, such as eye sockets and skull sutures, that help scientists distinguish their fossils from non-living things. “You don't have those little details when you're dealing with a sponge or something similar,” he says.
Abiotic, or non-living, chemical processes can also form structures remarkably similar to those of life forms, adds Rachel Wood , a carbonate geologist at the University of Edinburgh. “[Elizabeth Turner] may be right. But I think we have to examine and refute all the other possibilities before making such comments,” she said. For now, "I don't think his evidence is irrefutable," she adds.
A complementary analysis could close the debate. As Rachel Wood points out, designing three-dimensional models of the tube network could provide more detailed insight into the structures. Robert Riding meanwhile hopes the new study will spark scientists' interest in other stromatolites to discover new mesh structures.
“That's not the end of the story,” admits Robert Riding. "This is just the beginning of a very interesting phase."
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