Modern sharks have undergone many more evolutionary changes than previously thought and are not the "living fossils" of their prehistoric ancestors, a new study has found.
The skull of a newly discovered 325-million-year-old shark-like species suggests that early cartilaginous and bony fishes have more to tell us about the early evolution of jawed vertebrates - including humans - than do modern sharks, as was previously thought.
The study, led by scientists at the American Museum of Natural History shows that living sharks are actually quite advanced in evolutionary terms, despite having retained their basic "sharkiness" over millions of years.
"Sharks are traditionally thought to be one of the most primitive surviving jawed vertebrates. And most textbooks in schools today say that the internal jaw structures of modern sharks should look very similar to those in primitive shark-like fishes," said Alan Pradel, a postdoctoral researcher at the museum and the lead author of the study.
"But we've found that's not the case," said Pradel.
The modern shark condition is very specialised, very derived and not primitive," said Pradel.
The study is based on an extremely well-preserved shark fossil collected by Ohio University professors, where an ocean basin once was home to a diverse marine ecosystem.
Jon Mallatt, a co-author and associate professor of biological sciences at Washington State University (WSU), helped determine what the fossil says about the origin and evolution of jaws in vertebrates.
"The first fish had no jaws and were probably filter feeders but the appearance of these mouth parts, for biting and grasping prey animals, was revolutionary. It let the early jawed vertebrates become predators high in the food chain and led to the evolution of earth's largest animals. Without this evolutionary leap, we would not be here," said Mallatt.
The heads of all fishes - sharks included - are segmented into the jaws and a series of arches that support the jaw and the gills. These arches are thought to have given rise to jaws early in the tree of life.
Pradel imaged the specimen with high-resolution X-rays to get a detailed view of each individual arch shape and organisation.
"We discovered that the arrangement of the arches is not like anything you'd see in a modern shark or shark-like fish. Instead, the arrangement is fundamentally the same as bony fishes," he said.
The research was published in the journal Nature.
