THE RISE OF FISHES AFTER THE FALL OF THE ORDOVICIAN WORLD

Fishes are an important component of marine ecosystems, both today and in the Paleozoic. However, because of the perceptions of a relatively poor record and lack of diversity compendia, little is known about the relationship between macroevolutionary transitions in early fishes and environmental change. We examined how the Late Ordovician glacial pulses (445-443 ma) and the subsequent unstable Silurian interval (443-419 ma) affected the vertebrate biosphere. We assembled and analyzed a new database of over 3,000 global occurrences for Ordovician-Devonian gnathostomes (jawed fishes and their jawless relatives). While only 16 genera are known from the Ordovician, fishes still experienced a lull in biodiversity from the Hirnantian through Aeronian (445-438 ma). This marks an extinction and recovery interval surrounding “Talimaa’s Gap,” a hiatus in the vertebrate record. The lull was followed by continuous diversification throughout the Silurian, with fish biodiversity hitting an initial peak of 164 genera only in the first stage of the Devonian (419-410 ma), after which global numbers were relatively stable. This constant growth occurred despite change in environmental conditions, sedimentation rates and rock volumes and multiple invertebrate extinction pulses. Macrofossil and microfossil fish taxa show correlated trends and similar genus counts despite the greater preservation potential of the latter. The overall pattern was initially driven by armored jawless fish, while jawed fish diversity was low and stable (~10 genera in each stage) until the Gorstian (427 ma), at which point they began to speciate rapidly in line with the agnathan groups. This change in tempo presaged the appearance of both definitive jaw fossils and articulated crown-gnathostomes in the late Silurian. Regional and ecosystem samples support these global vertebrate trends. The end-Ordovician Hirnantian event affected fish evolution much like the environmentally-similar end-Devonian Hangenberg event (359 ma) that heralded the rise of modern vertebrates. In sum, the transition to the Devonian “age of fishes” was triggered by opening of ecospace following mass extinction. Later environmental instability was associated with nested radiations throughout the vertebrate tree of life, leading to the dominance of jawed fishes.