The vertebrate story lies in the fossil of a slimy fish

Physiologists are fascinated by the link between animal form and function. Some try to understand how animals use this link to adapt to their changing environments, while others have made it their career goal to uncover how an animal has evolved a specific function. But what if that evolutionary road is not as clear, perhaps because there are missing links in the evolutionary tree and competing theories about how that animal has come to be the way it is today? Or, what if an animal is so different from the rest that it is difficult to tell how it relates to other groups of animals?Undeterred by such challenges, Tetsuto Miyashita, working out of University of Chicago, Illinois, USA, and University of Alberta, Alberta, Canada, along with an international group of researchers from two different continents, set out to resolve the evolutionary history of a very peculiar creature: the hagfish. It is the only animal to have a skull, but no vertebral column or spine – even though it is classified as a vertebrate – and it produces slime when threatened. Miyashita and his colleagues investigated the creature from a unique perspective by studying the features of a hagfish fossil (Tethymixine tapirostrum) from the Late Cretaceous period found in Lebanon, which they obtained from the Black Hills Institute of Geological Research in South Dakota, USA. To add to their challenge, the team tried to reconcile two competing theories about the place of the hagfish in the evolutionary history of the vertebrates: (1) that hagfish are a primitive group of vertebrates, or (2) that they are a separate, specialized group of vertebrates all-together.Using a sophisticated technique for studying minerals in soft tissues, called synchrotron rapid-scanning X-ray fluorescence (SRS XRF), the authors examined the gill and mouth structures of the fossil. Because the fossil lacked paired fins, jaws, a mineral skeleton and features such as an oral sucker, cartilage and dorsal fin, the researchers concluded that it was not an ancestor of either the eel or the lamprey, which are similar; it was certainly a hagfish fossil. Next, the authors examined the fossil's slime glands, which are unique to hagfish. The group determined that it had 133 glands on the right side, which is more than most living hagfishes, but the animal was smaller in size. Using complex statistical analysis and historical data on the structure and evolution of lampreys and eels, the team was then able to assign the hagfish fossil to its rightful place in the evolutionary history of vertebrates. They classified it as the missing link between the last common ancestor of all vertebrates and the living soft-bodied round-mouthed fishes, thus filling a 100 million year gap in evolution.Miyashita and his colleagues’ study is revolutionary, because it shows that features such as lack of vertebral column and the round mouth of living hagfishes, which were previously believed to be primitive, are actually quite specialized, and these ancient animals seem to have evolved new adaptive features, such as their slime glands. The team's findings appear to agree with the second theory about the location of the hagfishes in the tree of life, classifying them as a specialized group rather than a primitive vertebrate. In addition, the team of researchers propose that the last common ancestor of all vertebrates is not a soft-bodied hagfish-like animal, as is the current theory. However, what that animal is, what it looked like and how closely it relates to the modern animals we see today awaits discovery.

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