Ancient fish reveals how vertebrates put their heads together

Scientists are facing an evolutionary puzzle head-on.

Almost 99% of vertebrates are known as gnathostomes, with distinctive skulls that have been adapted for a wide variety of lifestyles. Their heads are believed to have evolved from jawless, poorly protected ones that only animals like hagfish and lampreys still have today.

While there have been a few theories about how this process happened, a lack of well-preserved fossils has meant they’ve been difficult to prove.

However, new research into an ancient fish called Eriptychius might finally bridge this gap. A study led by Dr Ivan Sansom at the University of Birmingham uses fossils of the species to provide the first evidence of how early vertebrates began to protect their brain using a series of mineralised cartilage elements.

Dr Richard Dearden, the first author of the new paper, says, ‘On the face of it, Eriptychius is not the most beautiful of fossils. However, by using modern imaging techniques we showed that it preserves something unique: the oldest three-dimensionally preserved vertebrate head in the fossil record.’

‘This fills a major gap in our understanding of the evolution of the skull of all vertebrates, ultimately including humans.’

The findings of the study were published in the journal Nature. 

How did vertebrate heads evolve?

The first vertebrates all evolved from filter-feeding ancestors over 500 million years ago. Unlike their ancestors, they had brains, a spinal cord along their back, and a rod-like structure called a notochord which would later form part of the backbone.

At this point, the animals were yet to develop jaws, mineralised teeth, or even bones. While a few species of very early jawless vertebrates, such as Haikouichthys, have been discovered, their identities are often controversial.

Today, the living jawless vertebrates are represented by the hagfish and lampreys, collectively known as the cyclostomes.

The Natural History Museum’s Dr Zerina Johanson, a co-author of the paper, explains, ‘Though the cyclostomes may not have jaws, they are quite specialised, and have braincases made up of multiple portions of cartilage in a very unusual arrangement.’

‘A more solid braincase begins to emerge on the path to jawed vertebrates, and is first seen in groups of jawless fossil vertebrates known as galeaspids and osteostracans. Between these two points, however, we didn’t know what was happening.’

This gap in knowledge is around 100 million years long, suggesting that a lot of evolution is currently going undetected in the fossil record. Many specimens that might be able to reveal more about the process of evolution are either flattened or haven’t preserved important parts of the head, limiting what can be learned.

However, the development of new technology which is head and shoulders above its predecessors are helping to unlock more information from specimens of species like Eriptychius. This is helping researchers to get their heads around the evolution of vertebrates.

Dr Agnese Lanzetti, a researcher at the Natural History Museum and another co-author on the paper, says, ‘The specimen of Eriptychius is very small, and would fit in the palm of my hand. As a result, the cartilage that we’re interested in is only a few millimetres in size.’

‘Studying structures this small has only become possible recently, with state-of-the-art CT scans. As our technology improves, we’ll be able to answer more and more questions about the past.

A heads up on vertebrate evolution

The team found 10 cartilage plates in the remains of the fossil, which appeared to form the front of Eriptychius’s braincase. While the plates had been squashed during the process of fossilisation, the digital scans allowed the researchers to reconstruct what the fish looked like.

Piecing together the different shapes of the cartilage suggests that Eriptychius’s head would have had eyes, olfactory bulbs to smell with and a pineal organ, which secretes hormones involved in the sleep cycle.

Hagfish, as well as the predecessors of vertebrates, only have a more basic version of the pineal organ. With no signs the fossil would have had a jaw, the presence of the pineal organ adds further evidence that the vertebrate head was still evolving in Eriptychius.

While the individual structures were interesting, the overall shape of Eriptychius’s head is unlike any other vertebrate, living or dead. Only the front of the head appears to have been mineralised, meaning that the rest of the skull, made of cartilage, did not fossilise.

Even where there was mineralisation, the cartilage elements were separate from each other, and arranged in pairs on either side of the head. It’s thought they were individually covered by a layer of thin bony armour over the head, giving it a better protected brain than its ancestors.

Later evolution would see the cartilage become more mineralised, with the paired elements replaced by a more solid braincase. This gave an even greater level of protection to the brain.

While some animals like sharks still have a cartilage skeleton today, the evolution of bony fish began a lineage that would eventually lead to modern mammals, birds, amphibians and reptiles. 

While Eriptychius helps to answer some of the questions surrounding the origins of the vertebrates, many more remain. The team hope to conduct extremely precise scans to try and answer a few more.

‘One unusual thing about this specimen is that there are quite a few canals running through the cartilage, but we don’t know what they were for at the moment,’ Zerina says.

‘As we don’t currently have any better preserved specimens of Eriptychius, we hope better resolution scans will help us to find out more. To do this, we’re hoping to use a synchrotron, a particle accelerator which is able to produce detailed images at the smallest scales.’

‘This will allow us to get even more detailed information about how the braincases of ourselves, and many other vertebrates, evolved.’