EPILOGUE TO THE TALE OF THE TRIASSIC AMPHIPOD: ROSAGAMMARUS IS A DECAPOD TAIL (LUNING FORMATION, NEVADA)

McMenamin et al. (2013) described a unique crustacean fossil from the Triassic Luning Formation in west-central Nevada as a new genus and species of amphipod, Rosagammarus minichiellus. The amphipod identity caused an over 150 million year ghost range connecting Rosagammarus with the next-oldest fossil amphipods in Eocene amber. The size of the fossil put it amongst the largest of amphipods, and its unique taxonomic affinities, suggested by McMenamin et al. (2014) carried some paleoecological implications for the Luning Formation and the nearby vertebrate assemblage at Berlin-Ichthyosaur State Park.

However, amphipod identity failed to stand up to closer scrutiny. Additional preparation of the fossil coupled with examination with polarized light revealed that many podomere boundaries of the originally 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 an SEM EDX 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.

Thus, the amphipod identity for Rosagammarus is rejected. The fossil is instead proposed to represent the right half of a decapod tail of uncertain taxonomic affinity, with two broad, flat uropods preserved. Other decapods are known from the Luning Formation, but none with enough detail to meaningfully compare with Rosagammarus. The relatively broad ecological distribution of decapods does not shed any new light on the unique paleoecology of the Luning Formation.