THE phylogeny of primitive vertebrates has been investigated over the past fifteen years(1-3), and there has been a notable increase in the amount of data by which such phylogenies can be tested(4-7). Stratigraphic data should provide constraints to such analyses, and we record here descriptions of putative shark and thelodont scales from the Harding Sandstone (Caradoc, Late Ordovician) of Colorado, The fossil record of sharks is extended back from the Llandovery (lower Silurian) by some 25 million years and that of thelodonts by 10 million years from the Ashgill (Late Ordovician). The new data indicate that a major radiation of lower vertebrates took place during the Ordovician.