B
A new study, led by scientists at the American Museum of Natural History, shows that living sharks are actually quite advanced in evolutionary 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 primitive surviving jawed vertebrates（脊椎动物）. 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 and the lead author of the study. "But we've found that is not the case. "
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. The heads of all fishes -- sharks included -- are segmented into the jaws and a series of arches that support the jaw and the gills（鳃）. These arches are thought to have given rise to jaws early in the tree of life.
Because shark skeletons are made of cartilage（软骨）, not bone, their fossils are very fragile and are usually found in flattened parts, making it impossible to study the shape of these internal structures. But the Ozarcus mapesae specimen（标本）was preserved in a nearly three-dimensional state, giving researchers 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 and one of the authors on the study. "There's enough depth in this fossil to allow us to scan it and digitally dissect 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 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.
What does the underlined word “that” in Paragraph 1 refer to?

A．Living sharks are quite advanced in evolutionary terms.

B．Sharks have kept their basic “sharkiness” over millions of years.

C．The jaw structures of modern sharks are similar to those in fishes.

D．Shark’s jaws have remained almost the same over millions of years.

It is hard to study the internal jaw structures of ancient sharks because         .

A．there are only a few shark fossils

B．shark skeletons are made of bones

C．their fossils are often in poor condition

D．their fossils are found in a nearly three-dimensional state

According to Paragraph 4, the shark fossil used in the study is         .

A．the only complete shark fossil

B．the oldest complete shark fossil

C．the only Ozarcus mapesae specimen

D．the biggest Ozarcus mapesae specimen

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