A new whip scorpion (Arachnida: Thelyphonida) with a phoretic mite (Acariformes: Trochometridiidae) from Mid-Cretaceous Kachin amber.

Background: Mid-Cretaceous Kachin amber is a valuable resource for studying the diversity, evolution, and ecology of microarthropods, including arachnids. Its exceptional preservation offers a unique opportunity to uncover biological associations between organisms with high fidelity. Whip scorpions (Thelyphonida) are rare in the fossil record, with a few known from the Paleozoic era and Cretaceous period. However, the ecological interactions of these fossils with other organisms remain largely unexplored.

Results: Here, we describe a new whip scorpion species, Mesothelyphonus xiaoae sp. nov., from Kachin amber. This species is diagnosed by its relatively small body size, an accessory tooth on the pedipalp coxal apophysis, six teeth on the pedipalpal trochanter, and an unmodified abdominal sternite III. Notably, the fossil includes a heterostigmatic mite (Acariformes: Prostigmata: Heterostigmata: Trochometridiidae) attached to the first leg of the whip scorpion. The mite appears to have selected a densely setose area on a limb primarily used for sensory purposes by the host, which may have made it more difficult to dislodge. This association likely represents an instance of phoresy, where the mite benefits from transportation and protection provided by the whip scorpion.

Conclusion: Based on modern knowledge of Trochometridium biology and host associations, we suggest that while the whip scorpion served as an incidental host, the primary host was likely a ground-nesting Apoidea (bees or wasps). This hypothesis implies that Mid-Cretaceous ecosystems included early apoids exhibiting nesting behavior, providing an essential niche for the development of this ancient symbiosis.