April 17, 2014
The Evolution Of Jaws Based On A 325M-Year-Old Shark-Like Fossil
April Flowers for redOrbit.com - Your Universe Online
Despite having retained their basic "sharkiness" for millions of years, modern sharks have less to tell us about the early evolution of jawed vertebrates—including humans—than was previously thought.
A new study led by the American Museum of Natural History, based on the skull of a newly discovered 325-million-year-old shark-like species, reveals that modern sharks are quite evolutionarily advanced when compared to early cartilaginous and bony fishes. The findings were published in a recent issue of Nature.
"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. "But we've found that's not the case. The modern shark condition is very specialized, very derived, and not primitive."
The fossil skull of Ozarcus mapesae was collected by Ohio University professors Royal Mapes and Gene Mapes in Arkansas. The region was once an ocean basin home to a diverse marine system. The skull, and several similar specimens collected in the same location, were part of a recent donation of 540,000 fossils from Ohio University to the Museum.
The skulls of all fish species, including sharks, are segmented into the jaws and a series of arches. The arches support the jaw and the gills, and are thought to have given rise to jaws early in evolutionary history.
Shark skeletons are made of cartilage, not bone, making their fossils very fragile. Most shark fossils are found in flattened fragments, making it nearly impossible to study the shape of the internal structures. The Ozarcus mapesae specimens, however, was preserved in a nearly three-dimensional state, which allowed the research team a rare glimpse at the organization of the arches in a prehistoric animal.
"This beautiful fossil offers one of the first complete looks at all of the gill arches and associated structures in an early shark. There are other shark fossils like this in existence, but this is the oldest one in which you can see everything," said John Maisey, a curator in the Museum's Division of Paleontology. "There's enough depth in this fossil to allow us to scan it and digitally dissect out the cartilage skeleton."
Pradel collaborated with researchers at the European Synchrotron Radiation Facility (ESRF) to image the skull with high resolution x-rays. This allowed him to create a detailed view of each individual arch shape and organization. "We discovered that the arrangement of the arches is not like anything you'd see in a modern shark or shark-like fish," said Pradel. "Instead, the arrangement is fundamentally the same as bony fishes."
The research team was unsurprised that these structures in sharks—which have existed for around 420 million years—would have undergone evolutionary changes. When the new findings are considered alongside other recent developments about early jawed vertebrates, significant implications for the future of evolutionary studies of this group arise.
"Bony fishes might have more to tell us about our first jawed ancestors than do living sharks," Maisey said.