New research indicates that modern sharks are not replicas of their ancient ancestors. In fact, sharks evolved and changed from more than expected over millions of years, according to revolutionary new research.
A fossil skull indicates that all jawed vertebrates have an ancestor with a bony skeleton, not a cartilage-based skeleton. This ancestor lived in the Paleozoic era about 325 million years ago. It seemed to have possessed some bony fish traits and shark traits, according to the study published Wednesday in Nature.
Scientists had long thought that all modern animals with bony skeletons evolved from a shark-like animal with a skeleton made of cartilage. Bony skeletons evolved later. Modern-day sharks and rays had been thought to resemble their jawed ancestor, having remained basically unchanged over hundreds of millions of years.
This new study adds to the school of thought that sharks lost their bony skeletons and evolved into cartilage-framed deep-sea hunters over tens of millions of years.
Conventional thinking held that this was not the case. Scientists and the public have been thinking of sharks as “living fossils,” animals essentially unchanged since they first appeared, said lead author Alan Pradel of the American Museum of Natural History.
Scientists thought of sharks as being essentially like their ancient ancestors because their skeletons are made of cartilage, like the skeletons of jawless fish, which are assumed to be more primitive than bony fish. Thus, sharks seemed to represent the modern sharks represent the ancestors of all jawed vertebrates.
The team studied a well-preserved fossil of an animal called Ozarcus mapesae first discovered by Royal Mapes and Gene Mapes and subsequently donated by Ohio University to the American Museum of Natural History. A closer look at that fossil revealed that the animal was less advanced than modern-day sharks.
The fossil skull studied by Pradel’s team is more like a bony fish than like a modern shark skull. This suggest that the ancestor of all jawed vertebrates looked more like one of today’s bony fish, Pradel said.
Sharks were thought to be the most primitive surviving jawed vertebrates. Most textbooks indicate that the internal structure of a modern shark’s jaw is very much like the stricture of primitive shark-like fish. Pradel said that modern sharks are actually specialized, adapted, and not primitive. It now looks like sharks are more evolved than scientists would have expected.
The heads of all fish species, including sharks, consist of jaws and a series of arches that support the jaw and gills. Sharks, jawless fish like the hagfish and lamprey, and bony fish all have different skull structures.
However, when Pradal and his team examined the O. mapesae fossil, they found arches that are quite different from those in modern sharks. The big difference suggests that evolution advanced more than previously thought.
Shark skeletons have not been studied in such detail before because the cartilage that serves as a frame is so fragile. Gravity, soil, and time tend to collapse skeleton and jaw.
The discovery that sharks have evolved should not be unexpected, as sharks have existed for almost 420 million years.
Examination of this specimen indicates that sharks evolved more than expected. This finding may require serious new thinking about the relationships between modern bony fishes, sharks, and other vertebrates with jaws.
By Chester Davis