The beginning of the Devonian is marked by the sudden occurrence of abundant vertebrate faunas, often associated with detritic facies of Old Red Sandstone type. A few groups, already represented in the Silurian and even in the Ordovician, pass through the Silurian-Devonian boundary and seem to undergo an important diversification at this particular moment of their history. The case of the Osteostraci, a group of fossil jawless vertebrates, is examined in detail to show whether or not its phylogenetic pattern displays a radiation that coincides with the extension of the Old Red Sandstone facies over the North Atlantic continent. It appears that this geological event may have triggered the radiation of the cornuate osteostracans, although this monophyletic group must have already begun to diversify in the Middle Silurian. A review of the analyses of three osteostracan-bearing localities (Welsh Borderland, Spitsbergen and Saaremaa) is presented, and it is concluded that the Osteostraci may have consisted of both fresh water and marine species. When marine, the environment of the Osteostraci may have been vast intertidal zones, hardly having any Recent equivalent. The difficulty in assigning the Old Red Sandstone (from Britain and elsewhere) to any clear-cut environment may be because the physical constants of the Earth or the amplitudes of the tide were not the same in early Devonian times as they are now.