单项选择题

  Line      Conodonts, the spiky phosphatic remains (bones
            and teeth composed of calcium phosphate) of
            tiny marine animals that probably appeared about
            520 million years ago, were once among the most
    (5)    controversial of fossils. Both the nature of the
            organism to which the remains belonged and the
            function of the remains were unknown. However,
            since the 1981 discovery of fossils preserving not
            just the phosphatic elements but also other remains
    (10)    of the tiny soft-bodied animals (also called conodonts)
            that bore them, scientists’ reconstructions of the
            animals’ anatomy have had important implications
            for hypotheses concerning the development of the
            vertebrate skeleton.
    (15)      The vertebrate skeleton had traditionally been
            regarded as a defensive development, champions of
            this view postulating that it was only with the much
            later evolution of jaws that vertebrates became
            predators. The first vertebrates, which were soft-
    (20)    bodied, would have been easy prey for numerous
            invertebrate carnivores, especially if these early
            vertebrates were sedentary suspension feeders.
            Thus, traditionalists argued, these animals developed
            coverings of bony scales or plates, and teeth were
    (25)    secondary features, adapted from the protective
            bony scales. Indeed, external skeletons of this
            type are common among the well-known fossils of
            ostracoderms, jawless vertebrates that existed from
            approximately 500 to 400 million years ago.
    (30)    However, other paleontologists argued that many of
            the definitive characteristics of vertebrates, such as
            paired eyes and muscular and skeletal adaptations
            for active life, would not have evolved unless the
            first vertebrates were predatory. Teeth were more
    (35)    primitive than external armor according to this view,
            and the earliest vertebrates were predators.
              The stiffening notochord along the back of the
            body, V-shaped muscle blocks along the sides,
            and posterior tail fins help to identify conodonts as
    (40)    among the most primitive of vertebrates. The lack of
            any mineralized structures apart from the elements
            in the mouth indicates that conodonts were more
            primitive than the armored jawless fishes such as the
            ostracoderms. It now appears that the hard parts that
    (45)    first evolved in the mouth of an animal improved its
            efficiency as a predator, and that aggression rather
            than protection was the driving force behind the origin
            of the vertebrate skeleton.   It can be inferred that on the basis of the 1981 discovery of conodont remains, paleontologists could draw which of the following conclusions