Over the past century and a half, the taxonomic placement of Eriophyoidea has been in flux. For much of this period, this group has been treated as a subtaxon within Trombidiformes. However, the vast majority of recent phylogenetic analyses, including almost all phylogenomic analyses, place this group outside Trombidiformes. The few studies that still place Eriophyoidea within Trombidiformes are likely to be biased by incomplete taxon/gene sampling, long branch attraction, the omission of RNA secondary structure in sequence alignment, and the inclusion of hypervariable expansion–contraction rRNA regions. Based on the agreement among a number of independent analyses that use a range of different datasets (morphology; multiple genes; mitochondrial/whole genomes), Eriophyoidea are almost certain to be closely related to Nematalycidae, a family of vermiform mites within Endeostigmata, a basal acariform grade. Much of the morphological evidence in support of this relationship was apparent after the discovery of Nematalycidae in the middle of the 20th century. However, this evidence has largely been disregarded until very recently, perhaps because of overconfidence in the placement of Eriophyoidea within Trombidiformes. Here, we briefly review and identify a number of biases, both molecular- and morphology-based, that can lead to erroneous reconstructions of the position of Eriophyoidea in the tree of life.

Determining the chelicerate tree of life has been the subject of much research, with deep phylogenetic divergences and high evolutionary rates complicating both morphological and molecular approaches (Noah et al., 2020; Ontano et al., 2021; Sharma et al., 2021; Ballesteros et al., 2022). Within Acari, similar challenges exist—even the basic question of whether Acari is mono- or diphyletic remains unresolved (Pepato & Klimov, 2015; Lozano-Fernandez et al., 2019; Van Dam et al., 2019; Ontano et al., 2021). Several recent molecular studies have attempted to resolve these issues using large multi-gene datasets (Pepato et al., 2022; Klimov et al., 2018), mitochondrial genomes (Ban et al., 2022), and transcriptomes (Lozano-Fernandez et al., 2019). Ultraconserved elements (Faircloth et al., 2012) have recently shown great promise for constructing phylogenies for both deeply- and shallowly-diverged taxa (Zhang et al., 2019), including for Chelicerata (Starrett et al., 2017) and for Arrenuridae within Acari (Shoop, 2019), but have not yet been used across the Acari.