Ancient armored fish survived a massive impact before Earth’s largest mass extinction

Near the end of the Permian, an asteroid hit central Brazil and folded the rock layers around the crater it left. Farther out, one of those layers held a small armored fish fossil, and there the rock was never disturbed at all.

The fish came out nearly complete, down to its skull bones, jaws and roughly 50 rows of ridged scales. Researchers have now named it Leolepis matogrossensis, the only whole fish known from its part of a huge lake.

That lake was drying up in the last stretch of the Permian, just before the largest mass extinction in Earth’s history.

An asteroid struck nearby

Geologists call the crater the Araguainha Dome, and it’s the largest impact crater in South America. It measures about 25 miles (40 km) across, and the asteroid that made it was roughly a mile or two (2 to 3 km) wide.

Shaking from the strike left marks in rock as far as 620 miles (1,000 km) away. Around the rim, the same formation the fish came from is folded. The fish itself was buried about 30 miles (50 km) out.

Valéria Gallo of Rio de Janeiro State University and her co-authors think that layer was already deep underground at the time of the strike. That would explain why the fossil in it came through unbroken.

“Most fishes of this age in Brazil are generally known from scattered teeth, scales and isolated bone fragments,” said Martha Richter, a scientific associate at the Natural History Museum in London and a co-author of the study.

“Leolepis, on the other hand, is remarkably complete, especially considering a major meteorite strike warped the nearby rocks!”

The fossil surfaces in a ravine

Léo Adriano de Oliveira, a geologist who worked on the Permian rocks of the region, found the fossil in 2005. It came out of a ravine north of the MT-100 highway near Alto Garças, in Mato Grosso state.

He deposited it at the Federal University of Mato Grosso in Cuiabá, where it is cataloged as UFMT-353.

The rocks in that ravine formed on the floor of the lake, and geologists working the same beds have found freshwater clams, tiny shelled crustaceans and scraps of other fish.

A 2008 paper pictured the fossil, but no one described it formally until now. Gallo’s team built Leolepis out of Oliveira’s first name and lepis, Ancient Greek for scale. The species name comes from the state where he found it.

The animal was about 6 inches (150 mm) long, roughly the length of a dollar bill. This one fossil is all anyone has. It’s preserved in two matching slabs of reddish siltstone.

Armored scales covered the body

Leolepis was narrow at both ends and deepest through the middle. Roughly 50 rows of scales ran from head to tail, and at least 15 rows were counted around the body. Each scale carried a coating of a hard material called ganoine.

Every scale is diamond-shaped and ridged. A single one can bear 8 to 20 parallel ridges, depending on where it was on the animal, and the rear edge of each flank scale is serrated.

Toward the tail, the ridges get shallower and smoother, and on some scales they wander or fan out.

The head takes up about a quarter of the total length. Small cone-shaped teeth line the edge of the upper jaw, and the lower jaw appears to carry them too.

An unexpected European link

Leolepis belongs to the ray-finned fishes, the group whose fins are held open by thin bony rods. They account for 96% of living fish species, more than 30,000 of them.

Fewer than 20 early members have ever been described from rocks this old in South America.

Gallo’s team compared 37 species across 92 anatomical features, most of them details of skull bones, and ran the comparison through a program that builds family trees.

Leolepis came out closest to Paramblypterus, a genus known from older lake beds in Europe.

Until now, that family had only been found there. Leolepis is its first member from Gondwana, the southern supercontinent, and it lived much later than the European ones.

The family tree isn’t settled, though. Gallo’s team also scored how strongly each branch is supported, and most of the scores were under 50%.

The tail and fins are missing

Its paired fins and pelvic bones are gone, along with most of the tail. The back fin is down to ten loose rays. The skull was flattened top to bottom before it fossilized, and the trunk is twisted.

That twist helped in one way, because both sides of the head are visible. The snout came through poorly.

Dating these rocks is hard, too. Geologists date beds like these from crystals of a mineral called zircon. Grains from beds at the top of the same formation have been dated at about 254 million years by one team and 265 million by another.

The authors give the crater’s own age as 251.5 million years in their summary, then cite two later studies closer to 254.

More fossils may surface

Those ridged scales are distinctive enough to be worth hunting for. Gallo and her colleagues suggested the same species could be recognized elsewhere in the Paraná Basin from its scales and a few bones.

Geologists could then match up rock layers between sites far apart.

“The discovery of Leolepis also shows that there could be many incredible new fossils still to come not just from Brazil, but South America as a whole,” Richter said.

Whether a strike this size did lasting damage to life across southern Gondwana is still unsettled. Two enormous volcanic episodes, one in Siberia and one in China, are the main suspects for the extinction.

That die-off, about 252 million years ago, killed more than 70% of land species and 94% of the species in the sea.

To test whether impacts the size of Araguainha made anything worse, researchers need more fossils and more dated rock from the Southern Hemisphere.

They also need a firmer date for the crater than zircon crystals have given anyone so far.

The full study was published in the journal Journal of South American Earth Sciences.

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