Abstract: Opilioacarida are among the most primitive and enigmatic lineages of mites, retaining numerous plesiomorphic characters critical to understanding the early evolution of Parasitiformes. Here, we describe a new extinct family, Plesioacaridae fam. nov., from mid-Cretaceous Kachin amber, representing the second known family within Opilioacarida. The fossil specimen, here named Plesioacarus minutus gen. et sp. nov., was investigated using high-resolution confocal laser scanning microscopy, which revealed an exceptional suite of morphological features that clearly distinguish it from the extant family Opilioacaridae. These include dendritic prodorsal setae, a strongly U-shaped prodorsal furrow, a fused sternapophysis, a pair of setae posterior to the genital plate, a three-clawed palpal apotele, and a three-segmented tarsus I. The morphology of the palp apotele and tarsus I provides new insights into the evolution of appendage segmentation and specialization in early mites. To evaluate its systematic position, we conducted the morphological phylogenetic analysis for Opilioacarida based on 36 discrete characters scored across 22 extinct and extant taxa. The analyses consistently recovered Plesioacaridae as the sister group to all other opilioacarids. This discovery expands the known diversity of Mesozoic Opilioacarida and contributes important evidence for reconstructing the ancestral morphological groundplan and early evolutionary history of Parasitiformes.

Key words: Cretaceous, mites, morphological evolution, new family, Parasitiformes