The fossil of a recently discovered animal that trod both above and possibly underneath the Earth has received the name of a sea serpent that, according to Norse mythology, will invoke the world’s end and consequential rebirth.
It’s a fitting title given that Joermungandr bolti arrived at the cusp of the reptile’s evolution. “If you saw this thing in real life, you’d say it’s a reptile,” says Arjan Mann, a paleontologist and postdoctoral fellow at the Smithsonian National Museum of Natural History. Mann led the study published this July in Royal Society Open Science that analyzed the tiny ancient animal.
The fossil was discovered at the Mazon Creek fossil beds in Illinois by a collector who later donated it to the Chicago Field Museum of Natural History. While working on his doctorate at Carleton University in Ottawa, Canada, Mann encountered the fossil as he examined the Field Museum’s collections. Now, he and his colleagues from Carleton University are the first to etch it into the evolutionary record.
The Mazon Creek fossil bed formed between roughly 307 and 309 million years ago during the Carboniferous period, when some of the earliest known terrestrial animals first appeared in the fossil record. While amphibians were previously semiterrestrial, they still needed to return to water to lay their eggs. The group of animals still reproduces similarly.
And while people have found fossils in Mazon Creek for over a century, most of these traces come from insects or fish — tetrapods like J. bolti are much rarer. When these proto-reptiles died out, the Mazon Creek area was likely a mangrove forest, Mann says.
Joermungandr bolti measured about 2 to 3 inches long with small legs and about 40 vertebrae in its spine. It may have appeared similar to today’s snakes and longer lizards like skinks. J. bolti had scales and, while it’s unclear whether they’re composed of keratin like all modern reptiles, they appear to be smooth and thin. The scales were probably similar to those of reptiles that use them to shed dirt because they spend at least part of their lives underground.
The head shape also appears similar to snakes and lizards that spend some part of their lives down there and dig around with their heads. “These groups of animals might be early reptiles adapted to burrowing,” Mann says.
The newly described creature may have belonged to a group called microsaurs, which were some of the most abundant creatures during the Carboniferous and subsequent Early Permian. J. Bolti now complicates the debate over whether microsaurs were amniotes, an ancestral clade of reptiles that split from amphibians, or a type of proto-reptile just outside the amniote group. “If they are reptiles, that really increases early fossil reptile diversity,” he says.
While the name bolti comes from the late Field Museum paleontologist John R. Bolt, Mann also dubbed the new critter after the infamous Jörmungandr. This mythological sea serpent unleashes the world’s demise when it chomps on the end of its own tail. Eventually, a new world emerges after the old one is submerged in water. Mann says he chose the name because J. bolti was among a wave of life that evolved to diversify the various niches on land.
At that time, the rainforests had begun to break apart and certain sections grew more arid — conditions favoring animals that didn’t depend on water, like the amphibians and fish that had once dominated the world. “These ecosystems were becoming unstable,” Mann says. “It’s really this time period which allowed amniotes to become the most dominant colonizing things on land.”
