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Researcher image of field work being conducted in the Walcott Quarry located in the Burgess Shale of British Columbia. (Photo: Orla Bath Enright)

Fossil secret may shed light on the diversity of Earth’s first animals

A large group of iconic fossils widely believed to shed light on the early evolution of many of Earth’s animals and their communities may be hiding a secret.

A group of scientists, led by Dr. Nic Minter and Dr. Orla Bath Enright from the University of Portsmouth’s School of the Environment, Geography and Geosciences, U.K., are the first to model how exceptionally well-preserved fossilized animals could have been moved by mudflows. These fossils record the largest and most intense burst of evolution ever seen.

University of Saskatchewan researchers Dr. Gabriela Mángano (PhD, McLeod Enhancement Research Chair in Geology) and Dr. Luis Buatois (PhD, Professor at the Department of Geological Sciences), were involved in the study’s field work and collaborated in the writing of the study’s results, exploring the implications of this research. Both researchers received NSERC Discovery grants to support their work. Dr. Esther Sumner from the University of Southampton was involved in both fieldwork and experiments.

The finding, published in Communications Earth & Environment, offers a cautionary note on how palaeontologists build a picture from the remains of the creatures they study.

Until now, it has been widely accepted that extraordinary fossils buried in mudflows in the Burgess Shale in Canada, that show the result of the Cambrian explosion 505 million years ago, had all lived together, but the results of this study may suggest otherwise.

The Cambrian explosion was responsible for kick-starting the huge diversity of animal life now seen on the planet.

Now, the authors of the study have found that some of the animals which became fossils could have remained well preserved even after being carried large distances, throwing doubt on the idea the creatures all lived together.

“This finding might surprise scientists or lead to them striking a more cautionary tone in how they interpret early marine ecosystems from half a billion years ago,” Minter said. “It has been assumed that because the Burgess Shale fossils are so well preserved, they couldn't have been transported over large distances. However, this new research shows that the general type of flow responsible for the deposits in which they were buried does not cause further damage to deceased animals. This means the fossils found in individual layers of sediment, and assumed to represent animal communities, could actually have been living far apart in distance.”

The team of scientists studied a classic Burgess Shale succession in British Columbia, both in the field via a detailed sedimentologic analysis and with laboratory experiments.

The site is an area rich in fossils entombed in the deposits of mudflows and is one of the world’s most important fossil sites, with more than 65,000 specimens already collected and, so far, more than 120 species counted.

The Burgess Shale has been fundamental to scientists in understanding the early history of animal groups and the communities they lived among and has been closely studied multiple times.

The researchers used fieldwork to identify how the mudflows would have behaved, and then used flume tank laboratory tests to mimic the mudflows and are confident that the bodies of certain creatures could have been moved over tens of kilometres without damage, creating the illusion of animal communities.

“In the case of the Walcott Quarry, the classic locality of the Burgess Shale in the Canadian Rockies where the study was focused, the absence of biogenic structures left by the activities of animals provides significant evidence of environmental stress, suggesting that locally at the sediment-water interface and within the sediment, anoxic conditions were prevalent at times,” said Mángano.

The Burgess Shale was discovered in the early 1900s and led to the idea of the 'Cambrian explosion' of life, with the appearance of animals representing almost all the modern phyla, and inspiring copious research and discoveries.

 “Many would argue that it is fundamental, even ground zero for scientists in understanding the diversity of life,” Bath Enright said.

It’s not known precisely what caused the mudflows which buried and transported the animals which became fossilized, but the area was subject to multiple flows, causing well-preserved fossils to be found at many different levels in the shale.

“We don't know over what kind of overall time frame these many flows happened, but we know each one produced an ‘event bed’ that we see today stacked up on top of one another. These flows could pick up animals from multiple places as they moved across the seafloor and then dropped them all together in one place,” said Bath Enright.

“When we see multiple species accumulated together it can give the illusion we are seeing a single community. But we argue that an individual ‘event bed’ could be the product of several communities of animals being picked up from multiple places by a mudflow and then deposited together to give what looks like a much more complicated single community of animals.

“Palaeontologists need to appreciate the nature of the sediments that fossils are preserved within and what the implications of that are. We could be overestimating the complexity of early marine animal communities.”

The researchers hope to do further study to investigate whether differences in the species that are present in other exceptional fossil sites are due to evolutionary changes through time, different environments of deposition, or the nature of the flows and the effects of transport and preservation of the fossils.

 

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