Ancient shark fossil reveals new insights into jaw evolution
|This photo shows the exceptionally well-preserved fossil of Ozarcus mapesae from two|
different lateral views. The scale bar is 10 millimeters [Credit: AMNH/F. Ippolito]
The new study is based on an extremely well-preserved shark fossil collected by Ohio University professors Royal Mapes and Gene Mapes in Arkansas, where an ocean basin once was home to a diverse marine ecosystem. The fossilized skull of the new species, named Ozarcus mapesae, along with similar specimens from the same location, were part of a recent donation of 540,000 fossils from Ohio University to the Museum.
The heads of all fishes—sharks included—are segmented into the jaws and a series of arches that support the jaws and the gills. These arches are thought to have given rise to jaws early in the tree of life. Because shark skeletons are made of cartilage, not bone, their fossils are very fragile and are usually found in flattened fragments, making it impossible to study the shape of these internal structures. But the Ozarcus mapesae specimen was preserved in a nearly three-dimensional state, giving researchers a rare glimpse at the organization of the arches in a prehistoric animal.
|A 3D reconstruction of the skull of Ozarcus mapesae. The braincase is shown in light grey,|
the jaw is shown in red, the hyoid arch is shown in blue, and the gill arches are
shown in yellow [Credit: ©AMNH/A. Pradel]
Working with scientists at the European Synchrotron, the ESRF, Pradel imaged the specimen with high-resolution x-rays to get 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 authors say it’s not unexpected that sharks—which have existed for about 420 million years—would undergo evolution of these structures. But the new work, especially when considered alongside other recent developments about early jawed vertebrates, has significant implications for the future of evolutionary studies of this group. “Bony fishes might have more to tell us about our first jawed ancestors than do living sharks,” Maisey said.
Source: American Museum of Natural History [April 16, 2014]