Plant-feeding mite diversity in Triassic amber (Acari: Tetrapodili)

Carnian-aged amber (c. 230 Ma) from north-eastern Italy contains the first pre-Cretaceous inclusions of arthropods, plant remains and microorganisms. Here, we report further recovery of mites from this Late Triassic amber, supporting the prediction of a diversity of arthropods to be found in this oldest known fossiliferous resin. Two new genera and species of the Tetrapodili lineage, Minyacarus aderces and Cheirolepidoptus dolomiticus, are described. They indicate, along with the two previously described taxa of these mites, Triasacarus fedelei and Ampezzoa triassica, from the same source, a quite flourishing group of already highly specialized, four-legged, phytophagous mites in those remote times. The diversity of character states found in these Triassic mites challenges some conceptions of polarities inferred from modern four-legged mites. A hierarchical distinction is made between the Tetrapodili as a higher category of mites, and two constituent superfamilies, the Eriophyoidea embracing c. 300 extant genera and 3500 species, and the new superfamily Triasacaroidea, accommodating the four Triassic taxa. Varied forms and sizes of bodies and mouthparts amongst these Triassic mites indicate different feeding strategies in adapting to specialize on the same host plant of the Cheirolepidiaceae, for which we first report entire shoots from this amber outcrop. The cheliceral stylets of Triasacaroidea are generally blunt, indicating that, unlike extant Eriophyoidea, they were less able to pierce surface plant cells. Rather, we suggest that they may have fed on mesophyll cells by access through leaf stomata, whose density and appropriate dimensions are revealed by our study of plant cuticles. Further findings of small arthropods from this source of amber are increasingly probable and of great potential interest in adding knowledge about their early evolution.