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.
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.
The solution is to bring sections of the rock back to the lab for CT scanning, which allows researchers to see the fossils hidden inside the rock. In this case, the team saw the jaw bone - which is only about 1cm in length - and were able to break the larger rock down to separate the intriguing chunk. However, the high density of the rock limited the details they could gather from lab-based CT scanning. Dr Brazeau continues,
We really wanted to see the specimen at very high magnification, to resolve a lot of the details. These animals didn't have bony internal skeletons; their internal skeleton was mostly cartilage, except for the thinnest sheath of bone. This thin rind of bone over the cartilage is only about 10-12 microns thick, and it's the only evidence we have of what those cartilages looked like. So it's crucial to resolve the fine details to give us a window into parts of the anatomy we would not otherwise know about.
During the summer of 2021, when some Covid restrictions were still in place, the research team sent their precious tiny fossil to Diamond, where beamline scientist Robert Atwood used X-ray micro-computed tomography (micro-CT) at the I12 beamline to examine it closely. Dr Brazeau said,
Without Diamond, we would have had to use mechanical tools like pneumatic pens to chip away the surrounding rock and expose the fossil. Not only is that extremely time-consuming, but it would also very likely have permanently damaged the specimen, as the bones on this site are so much softer than the surrounding rock.
The high-resolution micro-CT scans allowed the team to see both the actual surface of the jaw hinge and the overall shape of the cavity where the jaw muscle would have been. They could also see the course of the lateral line canals on the outside of the jaw. Although fish don't have ears, they have a network of canals running through the skin, through which they pick up waterborne vibrations. Placoderm fossils have grooves showing the placement of those canals, allowing comparisons between different species.
Placoderms were quite diverse in terms of their ecology, with some thought to be ambush predators, others free-swimming predators, and some that could have been filter feeders. However, these scans add to a body of evidence suggesting a reasonably consistent pattern of jaw closure across placoderm species, in which the upper jaw overbites the lower jaw.
It's just another piece of the puzzle, according to Dr Brazeau.
One of the big problems in trying to understand how jaws originated is that we don't have a record of their precursors in jawless fishes, as they were primarily cartilage. There's nothing we can point to in the fossil record as a definite precursor of jaws. It's a challenge to reconstruct that, and a lot depends on how we understand the anatomy of these placoderms.
With a large enough telescope, we can see back through time towards the origins of the universe. A synchrotron - a very large microscope - can also work as a time machine, allowing us to catch a glimpse of our earliest ancestors. There's so much to learn, from such tiny fossils.
To find out more about the I12 beamline or discuss potential applications, please contact Principal Beamline Scientist Genoveva Burca: genoveva.burca@diamond.ac.uk.
Brazeau MD et al. A well-preserved 'placoderm' (stem-group Gnathostomata) upper jaw from the Early Devonian of Mongolia clarifies jaw evolution. Royal Society Open Science 10.2: 221452 (2023). DOI:10.1098/rsos.221452.
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