100-million-year-old 12"-long fish in Cretaceous limestone from Lebanon: This is the first known proper fossil of a hagfish, Tethymyxine tapirostrum Tetsuto Miyashita / University

100-million-year-old Fossil Slime Shakes Tree of Vertebrate Evolution

Lebanese fossil of a hagfish and its slime shield from the Cretaceous supports contention that these eyeless, spineless sea creatures used to be real fish, then regressed



Believe it or not, “hagfish” is what people call these sea creatures when they’re being polite. Otherwise, the seabed scavengers are known as slime eels (though they aren’t eels). Slimy they are, and have been since dinosaurs were in diapers. The question that has been setting paleontologists and geneticists at each other’s theoretical throats was what came first: the first vertebrate or the first hagfish. They also never agreed on whether the hagfish is a fish or a living fossil of something pre-dating fish – a sort of ancestor to us all. Did we all arise from hagfishy things?

Maybe, but probably not. Dr. Tetsuto Miyashita of the University of Chicago and the team reported Monday in the Proceedings of the National Academy of Sciences that a 12-inch (30-centimeter) long fossil they found in Lebanon dating from 100 million years ago is definitely of a hagfish and its slime. Their discovery supports the contention that the vertebrates came first and the hagfish (and their relative – the equally uncharming lamprey) split off from the vertebrate tree, albeit over half a billion years ago, after which the hagfish degenerated, Miyashita explains.

Ergo, the ancestor to the presently boneless, jawless, eyeless, spineless hagfish may have had have a backbone 540 million years ago, and who knows, it may have had eyes too. But it regressed to a simpler, seemingly more primitive state.

It isn’t unthinkable for evolution to go “backward.” Regression happens. A one-celled parasite infecting fish farms turned out, to the shock of science, to be a parasitic jellyfish that had lost almost all its genome and reverted to a practically amoebic existence.

Tetsuto Miyashita / University of Chicago

Not exactly Cousin Vinny

The Lebanese fossil in question is identified as Tethymyxine tapirostrum (i.e., hagfish), lying in clouds of fossil slime that it almost certainly emitted itself, the whole lot embedded in a slab of Cretaceous sediment. For fossil slime, it had a lot to tell us, it seems.

“Certainly hagfish got a little closer to us than they used to be considered. But this is not just about the proximity between us and those alien-looking fish,” head author Miyashita told Haaretz. “The family tree of vertebrates has just got a lot deeper: Deeper in the sense that this big group includes something seemingly so distant from us like hagfish. Deeper in the sense of time: Our analysis suggests that the hagfish/lamprey branch and our lineage split from one another about 540 million years ago, just before we start seeing the diverse array of animals in the so-called Cambrian Explosion,” he says.

He does recoil from calling the hagfish a long-lost cousin. “We really have to look into the fossil record carefully to understand the evolutionary path of our ancestors,” Miyashita advises. “Try as we might, it is probably a futile effort to try seeing our ancestors in hagfish or lampreys or any other living animals.  They are mosaics of primitive and derived. The ancestors are somewhere in-between them, and we must utilize information from fossils to fill in these gaps.”

Living fossil, or bones and beyond?

It is easy to understand why science finds the hagfish so baffling. Morphologically they appear primordial, lying between vertebrates and invertebrates. It isn’t a huge stretch to think they’re a living fossil of a time before vertebrates evolved.

Hagfish and the lamprey are the only surviving members of the cyclostomes (“round of mouth”). Sci-fi flicks have been made in emulation of their round sucker mouths, but those terrifying teeth are really specialized skin. Hagfish scavenge on the seafloor while lampreys have specialized in vampirism.

Today’s hagfish have no scales, bones or proper vertebrae, but they do have whiskers, a la catfish, and rudimentary eyes beneath the skin on the head. They have skulls but, like the rest of their “skeleton,” it’s made of cartilage. And their nervous system is startlingly “backward.”

“For example, hagfish do not have taste buds or a fully functional lateral line – the vibration and electric sensors that most fishes have,” Miyashita explains. “Hagfish only have a single canal in their inner ear to assist with the sense of balance, although most living vertebrates have three. These differences used to be considered evidence that hagfish are not fully vertebrates (thus ‘rudimentary’).

“Our new study suggests that these are degenerative – rather than rudimentary, these structures were lost or simplified as hagfish adapted to their distinct lifestyle,” says Miyashita.

Another bone of contention in the science world is the ancestral relationship between lampreys and hagfish. The new study supports the theory that they are closely related, and that their common ancestor split away from proper vertebrates, and degenerated – but then they went their different ways.

Cousin lamprey, according to a new study that appeared in December, also has a single canal in the inner ear, like hagfish. But lampreys have taste buds, eyes and spines. “Overall, hagfish appear to be more specialized than lampreys,” says Miyashita, driving home the point that hagfish seem to have evolved from proper fish backward, while the lampreys took less of a journey.

Some people eat hagfish, chiefly in Korea but really nowhere else. Whatever the belief, their flesh does not have aphrodisiac properties. It was never a reasonable supposition that a spineless throwback to the Cretaceous could stiffen flaccid vertebrate organs.

How to gross out a plesiosaur

Now, when hagfish feel threatened – or just as a default in order to protect themselves from predators as they preoccupy themselves with boring headfirst into dead things on the seafloor in order to eat them – they emit copious amounts of slime from special glands along their eely bodies. The slime gums up the predator’s gills and chokes it.

In fact, it was the slime signal that gave away the fossil as being a hagfish, the scientists explain.

Tetsuto Miyashita / University

It can be hard enough to identify slime in a teenager’s bedroom after a week. How did they identify fossilized mucus after 100 million years?

Soft tissue and slime do not normally fossilize, but they may leave behind molecular traces. Miyashita used synchrotron scanning at Stanford University to identify chemical traces of soft tissue remaining in the limestone as the hagfish fossilized. He picked up a signal for keratin – a protein found, for instance, in our hair and nails, and in hagfish slime.

Hagfish secretions are actually packaged, coiled keratin fibers lubricated by mucus. “When these packets hit seawater, the fibers explode and trap the water within, turning everything into shark-choking slop. The fibers are so strong that when dried out they resemble silk threads,” the team explains.

So, the team found keratin concentrations along the fossil body. “The ancient hagfish probably evolved its slime defense when the seas included fearsome predators such as plesiosaurs and ichthyosaurs that we no longer see today,” they write. Indeed.

The bottom line is that Miyashita’s work reconciles the genetic and paleontological evidence, concluding that hagfish and lampreys should be grouped separately from the rest of fishes. Their apparent primitiveness is actually a specialized condition, sums up co-author Michael Coates.

Miyashita stresses that their study is not the final word on the “seemingly endless debate” over the origin of the hagfish features. “We showed that different types of data (morphology and molecules) support one view – but converging (molecular and fossil) evidence does not guarantee that it is the ultimate answer,” he tells Haaretz. In short, the fight continues. Stay tuned.

skip all comments

Comments

Sign in to join the conversation.

Required field
Required field

By adding a comment, I agree to this site’s Terms of use

  1. 1