Ancient fossil fish reveals key step earlier than evolutionary increase

A fossil the dimensions of a fingernail is rewriting the opening chapter of vertebrate historical past.

An worldwide crew led by scientists from the Canadian Museum of Nature and the University of Chicago have reconstructed the mind, coronary heart and fins of a long-extinct fish known as Norselaspis glacialis. Using superior imaging methods, they discovered proof of change towards a fast-swimming life-style guided by sharp senses, properly earlier than jaws and tooth arose to raised seize meals. 

The discovery gives a uncommon glimpse into how important traits emerged earlier than certainly one of evolution’s most transformative leaps—the rise of jawed vertebrates.

“These are the opening acts for a key episode in our own deep evolutionary history,” mentioned Tetsuto Miyashita, a analysis scientist with the museum and lead writer of the brand new examine published in the journal Nature on August 6.

Fish have been round for half a billion years. The earliest species lived near the seafloor, however after they developed jaws and tooth, every little thing modified. By 400 million years in the past, jawed fishes dominated the water column. Ultimately, limbed animals—together with people—additionally originated from this explosion of vertebrates.

However, it has lengthy been a thriller how this pivotal occasion occurred. The commonplace idea holds that jaws developed first and different physique components underwent modifications to maintain a brand new predatory life-style.

“But there is a large data gap beneath this transformation,” mentioned Michael Coates, professor and chair of organismal biology and anatomy at UChicago and a senior writer of the examine. “We’ve been missing snapshots from the fossil record that would help us order the key events to reconstruct the pattern and direction of change.”

The new examine flips the “jaws-first” concept on its head. 

“We found features in a jawless fish, Norselaspis, that we thought were unique to jawed forms,” mentioned Miyashita, who was previously a postdoctoral fellow in Coates’ lab in Chicago. “This fossil from the Devonian Period more than 400 million years ago shows that acute senses and a powerful heart evolved well before jaws and teeth.”

An beautiful digital atlas

The fossil of Norselaspis the crew studied is so exquisitely preserved in a fraction of rock that they had been in a position to scan it and see impressions of its coronary heart, blood vessels, mind, nerves, inside ears and even the tiny muscular tissues that moved the eyeball. 

The fossil was hidden in certainly one of hundreds of sandstone blocks collected throughout a French paleontological expedition to Spitsbergen, an island in Norway’s Arctic archipelago, in 1969. 

Sorting by means of these rocks a long time later, the examine’s coauthors Philippe Janvier and Pierre Gueriau cut up one open, revealing a superbly preserved Norselaspis skull barely half an inch lengthy. The crew took the fossil to a particle accelerator on the Paul Scherrer Institute in Switzerland to scan it with high-energy X-ray beams.

The end result was jaw-dropping. Slice by slice, the X-ray photos revealed delicate movies of bone that enclosed the fish’s organs with astonishing element. 

At a hundredth of a millimeter extensive, these tissue-thin bones seize the ghosts of organs previously held by the skeleton. Back in Chicago, digital imaging specialist Kristen Tietjen labored with Miyashita and Coates to digitally dissect and sew collectively the fish’s anatomy by means of hundreds of display screen hours.

“With this exquisite digital atlas, we now know Norselaspis in greater anatomical detail than many living fishes,” Miyashita mentioned. 

For instance, the fish had seven tiny muscular tissues to maneuver its eyeballs, whereas people have six. It had outsized inside ears, an unlimited coronary heart and vessels organized like freeway bypasses to hold extra blood.

“If Norselaspis was to our scale, its inner ears would be each the size of an avocado, and its heart would be as large as a cantaloupe melon,” Miyashita mentioned.

Fish use their inside ears in a lot the identical means that we use ours, to sense vibration, orientation and acceleration. The capacious coronary heart and larger blood move offers extra horsepower for the animal. 

“One might even say Norselaspis had the heart of a shark under the skin of a lamprey,” Miyashita mentioned.

The fish additionally sported a pair of tilted, paddle-like fins behind the gills, which Coates defined would have been helpful for making sudden stops, bursts and turns. These anatomical improvements made Norselaspis one thing of a sportscar among the many typically sluggish jawless fishes of its time.

Action packed anatomy

Such “action-packed” anatomy possible developed for evading predators moderately than for chasing prey. But what triggers fast escape responses in jawless fish would in flip give jawed fish a bonus to do the alternative—detecting and catching meals effectively. 

“When jaws evolved against this background, it brought about a pivotal combination of sensory, swimming and feeding systems, eventually leading to the extraordinary variety and abundance of Devonian fishes,” Coates mentioned.

But the earliest jaws had been most likely higher tailored for sucking up meals together with water and dirt than for snapping at passing prey. 

“It wasn’t as simple as marching straight from a bottom feeder to an apex predator,” Miyashita mentioned.

The new examine additionally challenges the concept shoulders and arms in trendy tetrapods—an unlimited lineage of limbed creatures—developed from modified gill buildings. 

The crew traced the nerve going to the shoulder in Norselaspis and noticed that it was separate from the nerves going to the gills—clear proof that one didn’t come from the opposite. Instead, the crew argues that the shoulder developed as an entirely new construction with a brand new area, the neck, separating the top the from the torso.

“A lot of these evolutionary changes have to do with how the head is attached to the trunk,” Miyashita mentioned. 

In primitive jawless fishes, the top is steady from the torso, whereas jawed vertebrates have a neck and throat to separate the 2 areas. Norselaspis is within the center—its head is instantly hooked up to the shoulder with no neck, nearly as if our arms had been protruding behind the cheeks. But the organs at this interface, like inside ears, shoulders and a coronary heart, are enhanced or reorganized for larger talents to navigate its surroundings. 

Paleontologists are nonetheless investigating what ignited this transformation. Some imagine the lineage of Norselaspis arose within the time of the so-called Nekton Revolution, when marine organisms had been starting to maneuver up within the water column. The recreation then was about getting quicker, smarter and extra maneuverable.

“For a historical event, we often emphasize one or two symbolic moments to the point of becoming a cliché. In this sense, the evolution of jaws is like a gunshot in Sarajevo starting World War I in 1914,” Miyashita mentioned. “But it is imperative we understand the context. With Norselaspis, we can really find it in its heart.”

—This story was originally published on the Biological Sciences Division website.