The fossil mite family Archaeorchestidae (Acari, Oribatida) I: redescription of Strieremaeus illibatus and synonymy of Strieremaeus with Archaeorchestes

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Strieremaeus is one of several oribatid mite genera proposed by Max Sellnick based on adult specimens preserved in Eocene Baltic amber. The original specimens of its type-species—S. illibatus Sellnick, 1918—were lost and the genus has received no further empirical study. For many years Strieremaeus was included in the family Eremaeidae, but recently this placement was questioned. Herein we redescribe S. illibatus based on the study of 31 non-type adult specimens from both Baltic and Rovno ambers. Among these are four Baltic specimens identified by Sellnick and currently deposited in the Kaliningrad Museum of Amber (KMA), which we designate as neotype (KMA 197-36) and paraneotypes (KMA 197-34, 197-35, and 197-37). Six immature specimens were associated with this species, of which three—one deutonymph, two tritonymphs—could be studied in detail and their characters are included in the redescription. The type specimens of a second species of Strieremaeus proposed by Sellnick—S. cordiformatus Sellnick, 1918—are also lost and two non-type specimens in the KMA seem to have been misidentified by Sellnick; therefore, we treat S. cordiformatus as a species inquirenda. A new diagnosis of Strieremaeus is presented, and the Cretaceous fossil genus Archaeorchestes is considered a junior subjective synonym, based on examination of the holotype of the type-species, A. minguezae Arillo & Subías, 2000. As a consequence, Strieremaeus is currently the sole genus in Archaeorchestidae. Strieremaeus minguezae (n. comb.) is only tentatively maintained as a distinct species, as no certain distinguishing traits could be found. Two families are reported from the fossil record for the first time: Zetomotrichidae from Baltic amber and Zetorchestidae from Rovno amber. In ancillary discussion we note how the specialized tarsal structure of S. illibatus is consistent with its likely arboreal habitat. We also discuss preservation properties and artifacts, note the dimensional discrepancy between cuticular remnants of the mite and its larger imprint in amber, and strongly recommend measuring more than the cuticular remnants themselves. Further, we provide information on different methods to observe amber inclusions, and for the first time report birefringence of fossil cuticular remnants in thin, airless preparations.