If you wanted to consult the geologic time scale for when the first vertebrates evolved

Ordovician period

Explanation:

The vertebrate animals are those which are comprised of a backbone or a spinal cord. These organisms started to evolve initially during the time of the Ordovician period, which forms the second contrasting period of the Paleozoic era.

The Ordovician period belongs to the time, about 450 million years back. This period lasted for about 45 million years and a lot of events has taken place during this time. The oldest vertebrate fossils belonging to the Ordovician period are the ostracoderm that includes the jawless and armored fishes.

Thus, the first vertebrate fossils were evolved during the Ordovician period.