Romundina gagnieri reveals early teeth in jawed fishes

A recently identified fossil fish, Romundina gagnieri, has shown that teeth grew on bony plates lining the roof of its mouth. The finding pushes the origin of teeth in jawed fishes much further back in vertebrate evolution.

This arrangement reshapes ideas about how jaws and teeth developed together in fishes, moving the origin of a defining vertebrate feature to an earlier point.

Evidence from a jawed fish skull

The tooth-bearing plates occur on pieces of an armoured skull collected from Arctic rocks. These fragments retain unusually rare evidence of an early jawed mouth.

By studying the material itself, Dr Sebastien Olive of the Royal Belgian Institute of Natural Sciences (RBINS) traced how the plates developed and enlarged as the fish grew.

In this jawed fish, teeth were added in an expanding pattern across the plate rather than progressing from one position at the rear, as had long been believed.

This limitation affects how scientists can reconstruct the earliest jaws and highlights the importance of comparing the pattern with other armoured fishes.

Introducing Romundina gagnieri

The skull fragments were discovered by field teams in northern Canada, within rocks deposited roughly 400 million years ago. They were found in 1995 on Prince of Wales Island, where former seabeds now lie exposed well above sea level.

In their study, the researchers called the fish Romundina gagnieri and classified it among the early jawed fishes. Its Arctic provenance provides an uncommon data point from an era in which fossils preserving early mouths are particularly scarce.

At that time, the oceans contained placoderms: extinct armoured fish with bony shields around their heads. In evolutionary terms, they sit close to the base of the jawed fish branch.

Whereas modern sharks and bony fish generally have biting structures along the jaw margins, many placoderms bore them on separate plates.

Scientists had previously thought that these plates did not possess genuine teeth, since fossils often retain smooth areas where teeth might otherwise be expected.

Research published in 2012 demonstrated that certain placoderms did form teeth, paving the way for more detailed comparisons.

A radial record of tooth growth

In Romundina gagnieri, the tooth plate retained a developmental record extending from its centre towards its edge.

Scans showed that older teeth lay near the centre, while younger teeth were added around them, broadening the plate over time.

This arrangement challenged the assumption that the earliest teeth consistently formed at the back of the mouth before advancing forwards.

By demonstrating that new teeth could arise in different locations, the fossil suggests a more adaptable origin for the first jawed fish lineages.

Imaging bone without damaging it

To examine the minute structures within the plate, the team used synchrotron imaging, a technique employing a particle ring that generates powerful X-rays.

The beam travelled through the fossil from numerous directions, while software combined the resulting slices into a detailed three-dimensional model.

As the technique does not damage the specimen, researchers can revisit its features when new tools and questions emerge.

Even at this level of detail, however, high-quality images cannot make up for the absence of fossils from additional species or other jaw regions.

Debate over the origins of jawed fishes

For many decades, scientists have debated whether teeth first evolved on the skin before moving into the mouth, or originated within the mouth itself.

A review from 2016 set out these competing explanations and connected them with the development of tooth tissues.

A study published in 2020 proposed that placoderm teeth largely formed at the rear of the mouth.

The Arctic fossil plates make that interpretation more difficult to sustain, as their tooth-growth record does not follow a single rule restricted to the back of the mouth.

More than teeth

The scans further revealed odontodes, mineralised hard bumps found in skin or the mouth, which covered some of the teeth in layers.

These coatings accumulated in stages and occasionally concealed fractured teeth below, indicating that the biting surface altered through time.

The researchers also identified areas of additional hard mineral on certain teeth, which may have assisted the fish in crushing resistant prey.

As the RBINS researchers were unable to establish the source of these coverings, they did not determine whether they should be considered teeth or another structure.

Teeth enabled new feeding behaviours

When jaws acquired teeth, animals could seize, cut and crush food rather than relying solely on filtering or suction feeding.

Variation in tooth form and growth can distribute chewing forces, as the hardest points encounter food first and undergo the quickest wear.

“This allowed our distant ancestors to exploit new food sources and occupy new ecological resources,” said Dr. Olive.

This greater control over feeding enabled jawed vertebrates to spread into a wide range of habitats, offering later lineages additional means of surviving in changing seas.

What Romundina gagnieri reveals

Comparing this Arctic fish with other armoured relatives led the team to suggest that certain mouth characteristics have very ancient roots.

Comparable tooth plates occur in arthrodires, a placoderm group distinguished by articulated head armour, leading the researchers to infer a shared arrangement.

“We are looking here at one of the first steps in tooth evolution,” said Dr. Olive.

Because uncertainties remain in the placoderm family tree, scientists regard this ancestral interpretation as a persuasive but testable working hypothesis.

This Arctic fish provides an unusual glimpse of early dental plates, while its pattern of tooth growth expands the account of ancient jaws.

Further discoveries and scans of related fossils may establish whether this design represents a genuine great-grandmother or merely one evolutionary branch among many.