脊椎动物颚骨的进化史

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 we've found that's 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 ecosystem¹¹. 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 Museum's Division of Paleontology and one of the authors on the study. "There's 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 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.

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