THE FIRST FULLY SILICIFIED ASSEMBLAGE FROM THE EARLIEST TRIASSIC

The recovery following the late Permian mass extinction event was a key interval in the evolution of those marine invertebrate groups that characterize our oceans today. Diversification of bivalves, gastropods, and other clades is typically described as occurring in the Middle Triassic, ~5 Myr after the extinction event. The Early Triassic fossil record is, however, notoriously poor, with an unusually high number of ‘Lazarus taxa’, a typical moldic preservation of shells, and a near absence of silicified fossil assemblages. The latter has been attributed to the disappearance, or movement offshore, of siliceous sponges due to climate change. Partly silicified assemblages have been described in Lower Triassic rocks, but no fully silicified example preserving formerly aragonitic shells in high fidelity has hitherto been documented. The assemblage described herein was discovered in central Spitsbergen, Svalbard, in the lower Deltadalen Member of the Vikinghøgda Formation. Conodonts, ammonoids and magnetostratigraphy confirm an earliest Induan (Hindeodus parvus Zone) age, which makes this assemblage the oldest fully silicified fauna of the Triassic. The diverse assemblage comprises bivalves and gastropods, and the extremely fine preservation, especially of the internal characters of bivalve shells and gastropod protoconchs, provides important new insights into the taxonomy and evolution of a number of key Permian-Triassic mollusk taxa. Five families are identified in the earliest Induan for the first time, with one confirmed to have survived the extinction. Several records represent the earliest known occurrences of their respective families, extending the family ranges back into the Induan and implying a greater hidden diversity in the Early and Middle Triassic than previously appreciated. The assemblage also has important palaeoecological implications, supporting previous analyses that suggested that the northern extratropical paleolatitudes housed major ecological diversity in the hothouse earliest Triassic. These new data, combined with recent evidence that crinoids and other bivalve clades had already begun to radiate by the earliest Induan, provide further evidence that the immediate aftermath of the late Permian extinction was as a key interval in the evolution of benthic invertebrates.