Heterostracan vertebrates and the Great Eodevonian Biodiversification Event—an essay

Abstract

The oldest vertebrates are Early Cambrian, cephalized unossified species (craniates) from China. The oldest armoured species (euvertebrates) are Ordovician in age. After Talimaa’s Gap, vertebrates have their first adaptive radiation during the Silurian when jawless species (“ostracoderms”) are dominant and their second radiation during the Devonian when jawed species (gnathostomes), and particularly placoderms (armoured fishes), are dominant. A Lochkovian peak of diversity is registered in various Lower Devonian siliciclastic series all around the Old Red Sandstone Continent and Siberia, for ostracoderms in general, and heterostracan pteraspidomorphs in particular. It occurs at different time slices in the Lochkovian, depending on the localities, and may be followed by another smaller peak in the Pragian. Both events correspond to the rise of the Devonian Nekton Revolution as defined for marine invertebrates. This appears to be the second main biodiversification event in the Palaeozoic, following the Great Ordovician Biodiversification Event or GOBE, when euvertebrates appeared. Taking into account most recent palaeobiological studies on heterostracans that suggest they were microphagous suspension feeders or feeding upon microscopic epiphytes from filamentous algae, the origin of this Great Eodevonian Biodiversification Event (GEBE) of heterostracans is questioned. Both abiotic (sea level, tectonic events, climatic changes—oceanic oxygenation and temperature, continental surface temperature) and biotic (plankton diversity, marine primary productivity, competition with vertebrates and invertebrates, including eurypterids, macroecological turnover) factors are examined. No plausible global evolutionary scenario seems to be presently available.