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ORDOVICIAN PERIOD

488-444 million years ago

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In the Late Cambrian, the ancestors of conodonts and higher vertebrates evolved larger eyes, dentine, enamel, a larger cerebellum, and a second semicircular canal.

The first euconodonts are known from the Late Cambrian. The basal tissue of euconodont elements seems equivalent to dentine (Donoghue, 2000; Fedonkin, 1990).
Euconodont elements are now known to be the teeth of some of the most primitive known vertebrates most of which were about 3-10 cm long. The few fossils which preserve the soft anatomy include eyes, and external eye muscles, trunk muscles with chevrons, and fin rays. One genus, Promissum, might have reached 40 cm in length. They had eyes, teeth with enamel and dentin, a notochord, a dorsal nerve chord, calcified cartilage, external eye muscles, cellular bone, and segmented muscle. Their body shape was similar to that of the most primitive known fish, the hagfish. (Ohno, S., from Muller, 1998; Gabbott, 1995; Donoghue, 2000). Conodonts had enamel and a calcified dermal skeleton (Smith, from Ahlberg, 2001). The microwear on conodont teeth indicate that they were used to crush and shear food, suggesting that the first vertebrates were active predators (Purnell, 1995).

The traits which seem to have been present in conodonts and gnathostomes which are absent in lampreys and hagfish include an olfactory tract, a larger cerebellum, pretrematic branches of branchial nerves, a flattened spinal cord, and a vertical semicircular canal (Donoghue, 2000).

conodont

conodont elementconodont element

conodont elementconodont element