Earliest “land animals” were actually victims of a prehistoric Pompeii, say researchers

That’s the view of some researchers of the Ordovician trackways from 455 million years ago. They eyed the tracks skeptically because all other trackways on land dated from about 420 mya (Silurian era), and they found something interesting. From ScienceDaily:

What they discovered is that the trackways occur within volcanic ash that settled under water, and not within freshwater lake and sub-aerial sands (as previously thought). This means that the site is not the oldest evidence for animal communities on land, but instead “is actually a remarkable example of a ‘prehistoric Pompeii’,” says Shillito — a suite of rocks that preserve trails made by distressed and dying millipede-like arthropods as they were overcome by ash from volcanic events.

Further investigation proved that this was the case. In the course of their study, they found 121 new millipede trackways, all within volcanic ash with evidence for underwater or shoreline deposition.

Volcanic ash is known to cause mass death in some modern arthropod communities, particularly in water, because ash is so tiny it can get inside arthropod exoskeletons and stick to their breathing and digestive apparatus. Shilllito and Davies noticed that most of the trails were extremely tightly looping — a feature which is commonly associated with “death dances” in modern and ancient arthropods.

This study, published in Geology, overturns what is known about the earliest life on land and casts new light onto one of the key evolutionary events in the history of life on Earth. Shillito notes, “It reveals how even surprising events can be preserved in the ancient rock record, but — by removing the ‘earliest’ outlier of evidence — suggests that the invasion of the continents happened globally at the same time.” Paper. (paywall) – Anthony P. Shillito, Neil S. Davies. Death near the shoreline, not life on land: Ordovician arthropod trackways in the Borrowdale Volcanic Group, UK. Geology, 2018; DOI: 10.1130/G45663.1 More.

Now it gets even more interesting. Why would it have happened globally at the same time?

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See also: First transitional land fossils never walked on their legs?

Transition to land remake: Now starring … the trilobite

and

Early tetrapod (“fishapod”) sheds light on transition to land—maybe