The ‘Alien Goldfish’ Finds a Home

The early fossil record is littered with bizarre creatures that do not resemble anything living today. And few of those evolutionary enigmas are as perplexing as Typhloesus, an ancient sea animal so strange that paleontologists have referred to it as an alien goldfish.

The bloblike animal has defied taxonomic placement for nearly 50 years. Scientists weren’t sure whether the animal, which had a substantial tail fin and a gut often packed with the remains of early fish species, was more closely related to a worm, a jawless fish or something else entirely.

However, the discovery of a tooth-covered tongue in several Typhloesus fossils may bring these seemingly extraterrestrial animals down to earth. “It helps us find the branch of the tree of life that Typhloesus belongs to,” said Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum in Toronto. “It’s no longer a problematic orphan.”

Dr. Caron and Simon Conway Morris, a paleontologist at the University of Cambridge, made the discovery while examining several Typhloesus specimens that had recently been added to the Royal Ontario Museum collection. Those fossils, which are only a couple of centimeters long, were dug up from the Bear Gulch Limestone in Montana, a 330 million-year-old fossil deposit.

When those fossilized creatures were living, this area was blanketed by a balmy bay and was home to sharks that sported swordlike spines, coelacanths and the oldest known ancestor to vampire squids. Local monsoons washed nutrients into the bay, sparking algal blooms that sapped oxygen from the water and kept scavengers at bay. Those conditions allowed myriad soft-bodied invertebrates to be preserved in incredible detail.

Because many of these ancient sea creatures are delicately imprinted onto the limestone, most of their identities are easy to deduce. However, Typhloesus has perplexed scientists since it was described in 1973. The vaguely fishlike critter was once believed to be a conodont, a jawless, eel-like vertebrate. But a closer inspection revealed that the conodont remains were inside an animal’s digestive tract. That led scientists to conclude that Typhloesus had snacked on conodonts.

When Dr. Caron stuck several of the newly gained specimens under a high-powered scanning microscope, he spotted a ribbonlike structure studded with recurved teeth on both sides, like the business end of a chain saw. Because the toothy apparatus is lodged within the animal’s gut, past analyses had mistaken these rows of tiny teeth for muscle tissue.

In a study published Wednesday in the journal Biology Letters, the researchers describe the new structure as a radula, a tonguelike structure covered in teeth that snails and other mollusks use to scrape food into their mouths. The researchers hypothesize that the tooth-studded structure in Typhloesus was most likely attached to a retractable trunk. When Typhloesus approached an undulating conodont, its tooth-covered tongue would emerge to scarf down its meal.

The existence of Typhloesus’s toothy radula led the scientists to deduce that the alien goldfish was in fact a mollusk.

“It is a very exciting find to have a radula, because that is definitive,” said Christopher Whalen, a paleontologist at the American Museum of Natural History who studies cephalopods from the Bear Gulch and was not involved in the new study. “Just like how all vertebrates have a backbone, all mollusks have a radula.”

However, it is difficult to pin down what type of mollusk Typhloesus was. Dr. Caron proposes that the creature was similar to modern sea elephants. These gelatinous slugs swim through the water column and stick their radula through a trunklike proboscis to snag prey, a hunting style similar to what the new study proposes for Typhloesus. Although Typhloesus lacked eyes, its flexible body and large tail fin suggest it was an active swimmer that propelled itself through the water column as opposed to inching along the seafloor.

But Typhloesus fossils predate the rest of the swimming snail fossil record by over 100 million years. According to Dr. Whalen, that may be because these seagoing slugs lacked easily fossilized features like shells, which made them more maneuverable in the water. As a result, they are scarce in most fossil deposits.

Having a better grasp on Typhloesus’ identity can help paleontologists learn more about the evolution of mollusks, the second largest group of invertebrates on the planet today. According to Dr. Caron, the strangest creatures often have the most important tales to tell.

“The twists that life can give us are presented by these strange fossils,” he said. “They are enigmatic, but they reveal a lot of important evolutionary information.”