Researchers have disagreed about （4） ____ when the earliest bites evolved.Molecular （分子的） studies based upon estimations of genetic changes have （5） ____ that the first jaws and teeth evolved about 450 million years ago.Until now，the oldest fossil of jawed vertebrates-or gnathostomes （6） ____ to about 425 million years ago，a significant gap.As Institute of Vertebrate Paleontology and Paleoanthropology researcher Min Zhu and his colleagues describe in a series of Nature papers published today，the new （7） ____ from South China sit in the middle of this gap and offer a startling look at a critical moment in evolutionary history. Zhu and coauthors note in their research，the fish they uncovered were small and had （8） ____ bones.That means these fish were generally （9） ____ to become fossils except in truly outstanding circumstances where they could be buried quickly and shielded from scavengers （食腐动物） that would otherwise eat them.That an exceptional stroke of luck was required to preserve such fish likely explains the difference between the molecular （10） ____ and fossil record.

A.dated   B.unlikely   C.set   D.fossils   E.estimate   F.delicate G.takes   H.finds   I.exactly   J.advocated   K.tracked

Haul of Fossil Fish Pushes Back the Origin of Teeth and Jaws    Our teeth and jaws are incredibly ancient.They're older than dinosaurs，older than arms and legs，older than trees adaptations that paleontologists （古生物学家） have （1） ____  to our distant，fishy relatives that thrived in the seas around 425 million years ago.But a collection of carefully-preserved vertebrates （脊椎动物） found in China has （2） ____   a new date for the earliest record of jaws and teeth.The early vertebrates include some of the earliest fish，one of the earliest relatives of sharks，and，at about 436 million years old，the most ancient （3） ____  yet found of fish with teeth and jaws.
    Researchers have disagreed about （4） ____  when the earliest bites evolved.Molecular （分子的） studies based upon estimations of genetic changes have （5） ____  that the first jaws and teeth evolved about 450 million years ago.Until now，the oldest fossil of jawed vertebrates-or gnathostomes （6） ____  to about 425 million years ago，a significant gap.As Institute of Vertebrate Paleontology and Paleoanthropology researcher Min Zhu and his colleagues describe in a series of Nature papers published today，the new （7） ____  from South China sit in the middle of this gap and offer a startling look at a critical moment in evolutionary history.
    Zhu and coauthors note in their research，the fish they uncovered were small and had （8） ____  bones.That means these fish were generally （9） ____  to become fossils except in truly outstanding circumstances where they could be buried quickly and shielded from scavengers （食腐动物） that would otherwise eat them.That an exceptional stroke of luck was required to preserve such fish likely explains the difference between the molecular （10） ____  and fossil record.