The fossil record of early vertebrates is particularly patchy, largely due to the fact that most of these primitive creatures were soft bodied and, as a result, didn’t readily fossilise. On the rare occasions when squishy early vertebrates did become fossils, their remains were so squashed that their original state became almost unrecognisable.
This has made the study of early vertebrates particularly difficult, obscuring what is, arguably, the most important period in the history of life on Earth. However, new fossil imaging techniques are beginning to offer a rare glimpse into this period and shed some light on our soft-bodied ancestors that lived more than 440 million years ago.
As part of a study published today in Proceedings of the Royal Society B, a team of scientists from the University of Manchester used a technique known as synchrotron X-ray fluorescence imaging to examine the geochemical makeup of fossils belonging to two species of ancient jawless fish, known as Jamoytius and Lasanius.
What the study found was remarkable. Not only did the X-ray imaging reveal some of the oldest evidence of advanced eyes in vertebrates, it also found evidence of bone-like tissues. These results reshape our understanding of how the vertebrate body first evolved and suggest these tiny fish weren’t quite as ‘primitive’ as was once thought.
“We decided that transitional fossils, from the earliest stages of vertebrate evolution, would be perfect to look at with our new methods,” explained co-author Roy Wogelius. “We captured images of…camera-type eyes. These eyes preserve even the small notch where the optic nerve connected - features that form the basis of modern vertebrate eyes today.”
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The fossils of Jamoytius and Lasanius were found at a site near Lesmahagow, just south of Glasgow, and dated to 443 million years ago. Both are extinct species of primitive, eel-like fish that are believed to be distant ancestors of hagfish and lamprey. Like these living animals, Jamoytius and Lasanius didn’t have jaws - a peculiarity amongst living vertebrates.
In order to look at these Scottish fossils in more detail, the University of Manchester-led team sent them to Stanford Synchrotron Radiation Lightsource (SSRL) in California, USA, where they were scanned by University of Manchester alum Nick Edwards. Here, X-ray imaging identified traces of zinc and copper in the eyes of Jamoytius, revealing the structure of the retina and the pigment layer.
The team also found evidence of calcium and phosphorus in both Jamoytius and Lasanius - a sign these early vertebrates possessed bone-like tissues.
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“It’s been amazing to see just how much new information we can recover from fossils, which are usually too poorly preserved to be useful, using these new technologies,” said lead author Jane Reeves. “Our findings help resolve scientific debates that have been running since the Victorian era. They point to a very early origin of bones and eyes in vertebrate history, probably even predating the group appearing altogether.”
Reeves’ comment is particularly pertinent as just last week another study focussed on early vertebrate evolution presented even older evidence of camera-like eyes, this time in a group that may be ancestral to vertebrates. This study, published in Nature, examined two species of myllokunmingids found in China and dated to 518 million years ago.
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Unlike later vertebrates, which have just two eyes, these myllokunmingids had four eyes - two either side of their head and two on top. The presence of a retina in each of these upwards-pointing eyes suggest they were capable of creating visual images, as well as just sensing light. The loss of this extra pair of eyes in later vertebrates serves as another interesting example of reverse evolution, like the loss of limbs in snakes.
“I love how our findings fit with work from other groups; when you put our respective research together, they connect to fill in a lot of gaps in our knowledge, creating a really well supported picture of the early evolution of vertebrates,” said Reeves when asked about this other study.
The discovery of advanced, camera-like eyes in Jamoytius, Lasanius, and two species of myllokunmingids has changed the way we look at some of our earliest ancestors. They may look wildly different to us, but when we look into their eyes we see elements of ourselves staring back. A lot has happened in the last half billion years, though some things - such as eyes - have stayed pretty much the same.
The study was published in the journal Proceedings of the Royal Society B and conducted by the University of Manchester’s Jane Reeves, Roy Wogelius, Phil Manning, and Robert Sansom, and the SSRL’s Nick Edwards.
Top image: A figure from the latest study highlighting concentrations of zinc and copper in the eyes of Jamoytius. Credit: University of Manchester
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