LATE CRETACEOUS ACARODOMATIA REVEAL THE ANTIQUITY OF PLANT-MITE MUTUALISMS

Mutualisms – cooperative associations between species – have been critical in the ecological rise of flowering plants (angiosperms) during the Cretaceous Period (145—66 Ma). Pollination and seed dispersal are commonly studied in modern and deep time communities, but mutualisms between angiosperms and mites are less well known despite the importance of these interactions today. Acarodomatia, or mite domiciles, are located on the underside of leaves and often positioned in the vein axils. Acarodomatia protect the mite and its eggs from desiccation and provide protection from other predators. These structures in turn facilitate tri-trophic interactions between the host plant, the fungus or herbivore adversary, and the fungivorous or predaceous mites inhabiting the acarodomatia. Today, acarodomatia are widespread and have been found on over 2,000 species of angiosperms, with the vast majority occurring on eudicots with woody growth habits. However, evidence for the evolutionary history of plant-mite mutualism is rare in the fossil record despite the abundance of mite domiciles found on leaves in modern ecosystems. Previously, the oldest documented acarodomatia were described from the Cenozoic Era, in two Eocene Epoch deposits dating to 49 and 42 Ma. Here, we report the first occurrence of Mesozoic Era acarodomatia in the fossil record from Upper Cretaceous fossil leaves found in the Kaiparowits Formation (76.6 – 74.5 Ma) in southern Utah of North America. The antiquity of this plant-mite mutualism provides important constraints on the evolutionary history of acarodomatia on woody angiosperms.