Micro-CT can take us back in time to the dawn of jaws

Synchrotron studies show fine details on the jaw of one of the earliest jawed vertebrates

Acanthothoracids are generally considered to be the most primitive placoderms, an ancient group of armoured fish that first appeared during the early Silurian period, approximately 440 million years ago, and went extinct during the Late Devonian, about 360 million years ago. Placoderms were among the earliest jawed vertebrates, and many features of their anatomy can still be seen in modern fish and other animals. During this period, the skeletons of many animals were comprised of cartilage, which doesn’t preserve as well as bone. As a result, our understanding of placoderms is largely gleaned from small pieces of incomplete skeletons. The structure of their jaws and jaw hinges is poorly understood. In work recently published in Royal Society Open Science, an international team of researchers used X-ray micro-computed tomography to examine a near-complete acanthothoracid upper jaw discovered in western Mongolia. Their results suggest jaw morphology was phylogenetically conserved across most placoderms, and bring a step closer to understanding the origin and evolution of jaws and teeth in vertebrates.

A Well-Preserved Jaw

More than 99% of living vertebrate species, including ourselves, are jawed vertebrates (gnathostomes). However, how and when jaws and teeth evolved remains a contentious issue. Studying the jaws of placoderms, and comparing them to other early jawed fishes, offers some clues as to what their ancestors – and, by extension, our ancestors – would have looked like. The discovery of a near-complete acanthothoracid upper jaw is therefore a significant find. Studying it, though, presents a challenge.

Dr Martin Brazeau from Imperial College, London explains,

Some of the oldest jawed fish fossils come from this particular location in Mongolia. We found a bed of rock there that is full of pieces of fish fossils. But there’s a problem with the way that they’re preserved. Normally a palaeontologist will either chip away the surrounding rock to expose a fossil, or etch out the bone to leave an impression, from which it’s possible to make a rubber peel. Unfortunately, neither of those techniques is very successful at this site.

Read more on the Diamond website

Image: Upper jaw complex in virtual three-dimensional rendering from synchrotron tomography