Oftentimes, the most identifiable feature of animals or even other humans, is the face. An international team of scientists from France and Sweden has now published a new paper presenting fossil evidence regarding the evolution of the face.

The team of researchers from France and Sweden has presented fossil evidence from an armored fish called Romundina, which may provide clues to the evolutionary origins of the jawed vertebrates. Romundina is a fish that swam in ancient oceans 410 million years ago. The scientists working on the study used micron resolution X-ray imaging provided by the European Synchrotron Radiation Facility (ESRF) to reconstruct a possible step-by-step roadmap of the evolution of the jaw. The team published their findings as well as the three dimensional reconstruction images in the online journal Nature.

"Without the intense X-rays produced at the ESRF, we would not have been able to create a virtual representation of the internal structures of the skull," says ESFR scientist Sophie Sanchez.

There are generally two types of vertebrates, Agnatha and Gnathostomata. Agnatha are vertebrates that do not posses jaws while Gnathostomata are jawed vertebrates. The jaw is one of the most defining features of the face, which is why people often see jawless vertebrates, such as hagfishes and lampreys, as alien-like and strange. Jawed vertebrates, on the other hand, are both numerous and familiar. Humans, dogs, cats as well as most fishes belong to the infraphylum Gnathostomata. However, scientists have long been pondering over the origin of the jaw.

"In effect, Romundina has the construction of a jawed vertebrate but the proportions of a jawless one", says co-lead author Per Ahlberg from the Uppsala University. "This shows us that the organization of the major tissue blocks was the first thing to change, and that the shape of the head caught up afterwards."

When looking at jawless vertebrate embryos, forward tissue growth is pronounced in the areas to the left and the right sides of the brain. The tissues meet up in the middle of the head forming a large upper lip. Another characteristic seen in jawless vertebrates is the presence of a single, vertical "nostril." For jawed vertebrates however, the same pronounced tissue growth occurs in the middle line to the lower part of the forward section of the brain. The tissues push between two nasal sacs that open up into two orifices giving jawed vertebrates their distinctive twin nostrils. The fact that jawed vertebrates have more elongated brains compared to jawless vertebrates also results in nostrils that are positioned in the more forward section of the face.

While the features of both jawless and jawed vertebrates are well known to scientists, little is known about the period of evolution when jawless vertebrates evolved into jawed vertebrates. This is where Romundina comes in.

Like modern jawed vertebrates, Romundina have two nostrils. However, they are positioned in a manner similar to jawless vertebrates.

"This skull is a mix of primitive and modern features, making it an invaluable intermediate fossil between jawless and jawed vertebrates", says co-lead author Vincent Dupret, who is also from the Uppsala University.

The scientists used the high-resolution X-ray images of a Romundina skull and the skulls of other fossil fishes in different stages of jaw evolution to complete their step-by-step evolutionary roadmap.

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