You're looking at a 3D facial reconstruction of a 410 million year-old armoured fish called Romundina. It's one of the first creatures on this planet to feature a characteristic many modern animals take for granted — a face.
Above: The internal structures of Romundina's face show a mixture of features of both jawless and jawed vertebrates. External bones of two different kinds in orange and pink grey, nerves and cranial cavity in yellow, arteries in red, veins in dark blue and inner ears in light blue; anterior part of the bone rendered semitransparent. Image and caption credit: Vincent Dupret, Uppsala University
Backboned animals, or vertebrates, come in two basic types: Those who have jaws, and those who don't. Today, there are over 50,000 vertebrates with jaws (including humans), while jawless vertebrates have dwindled down to just two species, lampreys and hagfishes. There was a time in evolutionary history, however, when jaws, and by consequence faces, didn't really exist.
We know that jawed vertebrates evolved from jawless ones, but scientists aren't entirely sure how it happened. Thanks to fossilized evidence — including the tiny Romundina — a clearer picture is now emerging. It involved a process that quite literally turned faces inside out.
When a jawless vertebrate is still in the embryonic stage, it features blocks of tissue on each side of the brain that meet at the midline in the front. This creates a big upper lip surrounding a single midline "nostril" that rests in front of the eyes. But in jawed vertebrates, these same chunks of tissue grow forward, pushing between the left and right nasal sacs. This, say scientists, is why we have two nostrils instead of a single hole in the center of our face. And because the front part of our brains are much longer, our noses are positioned at the front of the face instead of sitting between our eyes.
Okay, this much we knew — but using micron resolution X-ray imaging, a team of French and Swedish researchers determined the precise evolutionary steps required to create this strange transformation. It appears that Romundina was an intermediary species in this evolutionary process — a creature with a skull that featured a mix of primitive and modern features.
Specifically, Romundina had separate left and right nostrils that sat far back behind an upper lip. Its skull housed a brain with a short front end similar to those of jawless vertebrates. So, it had the construction of a jawed vertebrate but the proportions of a jawless one.
This shows that the organization of the major tissue chunks was the first thing to change, and that the shape of the head caught up afterwards.
The new study now appears at Nature.