The effect of environmental changes on the evolution and extinction of Late Devonian trilobites from the northern Canning Basin, Western Australia

Abstract

Abstract The Frasnian–Famennian Virgin Hills Formation represents fore-reef facies deposited as part of the extensive Late Devonian reef system that fringed the SW Kimberley Block in Western Australia. It contains a rich trilobite fauna dominated primarily by proetids and, to a lesser extent, harpetids, phacopids, scutelluids and odontopleurids. To date, 49 taxa have been described, 40 of these being restricted to the Frasnian. Herein five Frasnian taxa are described, three in open nomenclature, and two the new species Telopeltis intermedia and Otarion fugitivum . Evolutionary trends in the Virgin Hills trilobites are dominated by a reduction in body size and eye size and, to a lesser extent, a reduction in exoskeletal vaulting. Although recording no sedimentological signature, the fauna was strongly affected by the two globally recognized Kellwasser extinction events. The first, at the end of conodont Zone 12, affected taxa at the species and genus level. The second, within Zone 13b, had a much greater impact on the fauna, causing extinctions at the familial and ordinal levels. Evidence is presented to suggest that evolutionary trends in the trilobites during the late Frasnian reflect selection for forms adapted to low nutrient conditions. The two intensive Kellwasser extinction episodes may reflect periodic massive inputs of nutrients from the terrestrial into the shallow-marine environment.