The skull1 of a newly discovered 325-million-year-old shark-like species suggests that early cartilaginous（软骨的） and bony fishes have more to tell us about the early evolution of jawed3 vertebrates -- including humans -- than do modern sharks, as was previously4 thought. The new study, led by scientists at the American Museum of Natural History, shows that living sharks are actually quite advanced in evolutionary5 terms, despite having retained their basic sharkiness over millions of years. The research is published today in the journal Nature. Sharks are traditionally thought to be one of the most primitive6 surviving jawed vertebrates. And most textbooks in schools today say that the internal jaw2 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 weve found thats not the case. The modern shark condition is very specialized7, very derived8, and not primitive.The new study is based on an extremely well-preserved shark fossil collected by Ohio University professors Royal Mapes and Gene9 Mapes in Arkansas, where an ocean basin once was home to a diverse marine10 ecosplayystem11. The fossilized skull of the new species, named Ozarcus mapesae, along with similar specimens13 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 jaws14 and a series of arches that support the jaw and the gills（鱼鳃）. 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 flattened15 fragments, making it impossible to study the shape of these internal structures. But the Ozarcus mapesae specimen12 was preserved in a nearly three-dimensional state, giving researchers a rare glimpse at the organization of the arches in a prehistoric16 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 Museums Division of Paleontology and one of the authors on the study. Theres enough depth in this fossil to allow us to scan it and digitally dissect17 out the cartilage skeleton.Working with scientists at the European Synchrotron, the ESRF, Pradel imaged the specimen with high-resolution x-rays to get a detailed18 view of each individual arch shape and organization. We discovered that the arrangement of the arches is not like anything youd see in a modern shark or shark-like fish, said Pradel. Instead, the arrangement is fundamentally the same as bony fishes.The authors say its 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.
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1 skull n.头骨；颅骨 参考例句： The skull bones fuse between the ages of fifteen and twenty-five.头骨在15至25岁之间长合。 He fell out of the window and cracked his skull.他从窗子摔了出去，跌裂了颅骨。
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