Animals living about 500 million years ago spent time on mudflats that were periodically exposed to the air. The finding suggests that some of the earliest animals were able to survive outside of water, if only for a limited time – tens of millions of years before some animals started living permanently on land.
“They must have had mechanisms to cope with some of the stressors of this environment,” says at the . “There was already the genetic toolkit, the physiological toolkit, to make these brief excursions into very landward environments.”
Mussini and his colleagues re-examined sedimentary rocks from the Pika Formation in Jasper National Park, Canada. They were originally collected in 1999 but then went largely unexamined for two decades. The rocks are estimated to be about 498 million years old, placing them in the middle of the Cambrian Period (), when many of the major animal groups originated.
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The team found thin layers of shale with telltale cracks – indicating that the rocks formed from mud that had dried in the sun. Combined with other evidence, this suggested that the mudflats were sometimes exposed to the air.
Mussini emphasises that we can’t be sure whether the mudflats were regularly exposed, as in a modern tidal zone, or only intermittently. “All we can tell from the evidence I’ve seen, at least, is that sometimes this was exposed,” he says. “Some of the time, if not most of the time, I would say it was underwater.”
Despite this exposed setting, the team found abundant animal fossils. There were annelid worms, which are related to modern earthworms, and marine worms called priapulids. They also found fragments of animals similar to Wiwaxia, which Mussini calls a “spiny, slug-like creature with blades covering its body”.
These types of animals are known from other Cambrian fossil beds, including the Burgess Shale in British Columbia, also in Canada, which famously preserved many soft-bodied creatures. However, most such deposits formed in deep waters. This suggested the animals couldn’t survive out of water. The Pika Formation finds indicate that some of them could, at least for a while.
“Animals, by this time, were not confined to these deeper water environments,” says Mussini. “Some of them already had physiological tolerances and environmental tolerances that allowed them to edge closer to exposed environments.”
“They expanded the ecological ranges for the Cambrian fauna,” says at the University of Exeter in the UK. that another set of Cambrian fossils, the Chengjiang Biota, probably came from a river delta. “Nevertheless, they were entirely submerged under the water,” she says.
Spending time out of water required tolerance for sharp temperature changes and for drying out, says Ma. Modern intertidal organisms, like molluscs on seashores, have similar tolerances. “They can endure quite harsh environments,” she says.
Plants and animals wouldn’t start until the Silurian and Devonian periods, tens of millions of years later, says Ma. However, she says the new study fits with growing evidence that some Cambrian animals were able to survive being exposed to the air. For instance, trace fossils suggest .
Palaeontologists used to think animals first evolved as a group during the Cambrian – in which case it would seem they started venturing into shallow water within a few million years of their origin. However, evidence has accumulated that peculiar organisms from the preceding Ediacaran Period (635-539 million years ago) were actually animals, meaning the group could be significantly older than previously thought.
Unfortunately, almost all Ediacaran fossils are from deep water, says Mussini. “We don’t really know what was going on close to shore,” he says. It may be that Ediacaran animals were confined to deep waters – or it may be that some lived in the shallows and even spent a bit of time out of water, but we don’t have the fossils. Mussini points to , an early cnidarian related to today’s jellyfish, from the Ediacaran. “It looks like it was transported from shallow water into a deeper marine deposit where it was preserved,” he says.
Palaeontology
