Epilogue to the tale of the Triassic amphipod: Rosagammarus McMenamin, Zapata and Hussey, 2013 is a decapod tail (Luning Formation, Nevada, USA)

Abstract

M. A. S. McMenamin and colleagues described a unique crustacean fossil from the Triassic Luning Formation in west-central Nevada as a new genus and species of amphipod, Rosagammarus minichiellus McMenamin, Zapata and Hussey, 2013. The amphipod identity caused an over-150-million-year ghost range connecting R. minichiellus with the next-oldest fossil amphipods in Eocene amber. The amphipod identity nevertheless failed to stand up to closer scrutiny. Additional preparation of the fossil, coupled with examination using polarized light and electron microscopy revealed that many podomere boundaries of the original interpretation were either artifacts of incomplete exposure of the fossil, or they represented places where the fossil arthropod cuticle had flaked off. The polarized light showed the weathered remnants of the cuticle to be more extensive than originally realized. Examination with a scanning electron microscope and energy-dispersive X-ray analysis showed the cuticle to be at least partially replaced with silica in stark contrast to the calcium carbonate matrix. Elemental mapping revealed that some supposed boundaries between adjacent legs actually had a strong silica signal, suggesting that the silica-replaced cuticle continues beneath the matrix connecting the two legs laterally. The amphipod identity for R. minichiellus is therefore rejected. The fossil is instead proposed to represent the right half of a tail belonging to a lobster-like decapod, with two broad, flat, uropods preserved. Other decapods are known from the Luning Formation, but none with enough detail to meaningfully compare with R. minichiellus. The relatively broad ecological distribution of decapods does not shed any new light on the unique paleoecology of the Luning Formation.