Silurian atmospheric O2 changes and the early radiation of gnathostomes

The number of known Silurian gnathostome fossils has increased significantly during the last two decades, and greatly improved our understanding of the early diversification of gnathostomes (jawed vertebrates). Primitive gnathostome remains from the Silurian of China are of special interest in bridging morphological gaps between osteichthyans and non-osteichthyan gnathostome groups. A review of these early fishes shows that gnathostomes had already obtained a wide distribution and experienced an early radiation in the Middle-Late Silurian.Environmental conditions in the Silurian are inferred from recent advances in geochemistry and paleoclimate models. Atmospheric oxygen concentration, an environmental factor critical in organismal evolution, rose gradually during the Silurian and reached modern levels for the first time. Compared to the Middle Ordovician when invertebrates and agnathans underwent a great radiation, the Middle-Late Silurian is distinctive for its high atmospheric oxygen level. We suggest that the rise of the atmospheric oxygen concentration would have triggered the early radiation of jawed vertebrates in the Silurian, which paved the way for the high generic diversity of vertebrates in the Devonian.