The most ancient vertebrates fossils are of the jawless fish. The oldest rocks in which these fossils are found are strata of the Ordovician period, just above the Cambrian. It must, therefore, be true that:

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Animals with gill slits, such as the ancient jawless fish found in Ordovician strata, are indicative of early vertebrate life. The presence of these fossils in the Ordovician period suggests that animals with such features were indeed living during that time.

The Most Ancient Vertebrates Fossils Are of the Jawless Fish