A new species of Brachychthoniidae (Acari: Oribatida) from the Eastern Central Alps (Austria, Tyrol), with the proposal of a new genus

A new species of the family Brachychthoniidae (Acari: Oribatida), Arcochthonius roynortoni n. sp., found on the summit of the Festkogel mountain (3030 m a.s.l.) in the Central Alps (Obergurgl, Tyrol, Austria), is described and illustrated, based on adults. Arcochthonius n. gen. is proposed for this species. The relationships to the closely related genus Synchthonius Hammen, 1952 and other genera of Brachychthoniidae are discussed. The new species is unique among Brachychthoniidae in having arched ridge-like sculpturing on thickened anterior edges of medial and posterior notogastral sclerites, and differs by a combination of morphological character states. Some notes on ecology and biology are given.


Introduction
Prior to the present study 174 species of the oribatid family Brachychthoniidae Thor, 1934(including 12 spp. inquir., Schatz et al. 2011 were described and grouped into 11 genera using morphological characters of the adult. Brachychthoniidae occur in all terrestrial zoogeographic regions. About two third of all species are recorded from the Holarctic region (Subías 2004(Subías , electronic update 2020, with 39 species known from Austria (Krisper et al. 2017). They are common inhabitants of forest litter and moss, but occur also in xeric grassland or in tundra with lichens, as well as in raw humus and soil, and in some localities, they were found with considerable abundances (Schweizer 1956, Moritz 1976a, b, Leonov 2020. Brachychthoniidae are among the smallest oribatid mites, the length of most adults varies between 120-250 µm. They are frequently overlooked and apparently underrepresented in several faunistic and ecological investigations. The late Konrad Thaler, Innsbruck, forwarded me collections of oribatid mites from high mountains of the Eastern Alps, in particular from the summit of the Festkogel mountain. This material contains numerous remarkable findings which will be discussed elsewhere (Schatz in prep.). In the present paper a new species is presented and described. Its generic position is discussed, which leads to the conclusion that this species represents a new genus.
The Festkogel (3035 m a.s.l.), situated southwest of the locality Obergurgl, is the highest summit on the mountain ridge that descends from the Granatenkogel, between the Gaisberg valley and Ferwall valley. The summit area is on the upper edge of the alpine grassland belt (mainly formed by loose stands of Carex curvula) with the beginning small cushions of subnival vegetation (e.g. Androsace, Cerastium), lichens and mosses.
Etymology -The genus name has two meanings: The prefix "Arco-" derives from the arched (latin arcus) clamp-like ridges on the anterior edges of notogaster sclerites. It is also a homage to the municipality of Arco/Trentino (Northern Italy), my second home, where I prepared part of this work. It is compounded with "-chthonius", a common generic root for Pseudoscorpions and enarthronote genera (from ancient Greek χθόνιος, meaning "underworld" or "subterranean").
Notogaster (Figs 1-3, 4F) -Shape subquadrangular in dorsal view, lateral edges almost parallel. Anterior edges of medial and posterior notogaster sclerites with distinct and broad (12-15) transverse thickenings, spanned by arched clamp-like ridges, formed by raised sculpturing. Number of ridges variable (NM 11/12, NP 14/15). On both sclerites one single ridge medially, proximal two ridges posteriorly connected, others partially with posterior appendices, also formed by raised sculpturing. Two medial bands of embossed sculpturing traversing notogaster longitudinally, at level of setal row c 1 -d 1 on anterior notogastral sclerite (NA), at level of e 1 on medial (NM), and at level of row f 1 -h 1 on posterior sclerite (NP). Additional plate-like sculpturing at base of most setae, posterolateral of c 1 and along posterior edges of sclerites NA and NM. Regular medial field groups on all notogastral sclerites. Some maculae on sclerite NA, mainly around insertion plate of seta d 2 . Notogastral setae smooth, stout, attenuating, setae of rows c, d, e, and seta f 1 and h 1 on distinct sculptured plates, seta p 1 and p 2 on common plate, f 2 , h 2 , h 3 , p 3 not on sculptured plates. All setae inserting on tubercles and of medium length  lateral to notogastral sclerites NA and NM, with thin suture at level of scissure between NA and NM. Lyrifissures as small cupules, in usual position (cf. Moritz 1976a).

Type deposition
The holotype (adult female) and two paratypes (females) from the type locality, preserved in ethanol, are deposited in the collection of the Senckenberg Museum, Görlitz, Germany (SMNG). Additional material is deposited in the collection of the author, which will finally also be placed in the Senckenberg Museum, Görlitz. Rostral tectum -The small scale on the rostral tectum might contain an internal ocellus ("oeil infère", defined by its cuticular morphology) which has lost its pigment and original function (Grandjean 1963). This structure was found in several early derivative oribatid taxa, some Brachychthoniidae and other Enarthronota, Palaeosomata, and also in Endeostigmata (Grandjean 1958, Alberti & Moreno-Twose 2012. A variability in the jagged outline of the frontal rostral apex as in Arcochthonius roynortoni n. sp. (cf. fig. 4E) was also observed in Synchthonius crenulatus by Engelmann (1969 fig. 1, sub S. boschmai). 3. Cuticular structures on notogaster -The arched clamp-like ridges on the anterior edges of the medial and posterior notogastral sclerites of Arcochthonius roynortoni n. sp. are unique among Brachychthoniidae species. Nevertheless, similarities of the general notogastral sculpturing of this species with the sculptured cuticular pattern of the Synchthonius species are discernible. The illustration of Synchthonius elegans (Fig. 67 in Chinone & Aoki 1972) shows certain resemblances with A. roynortoni n. sp., especially in the embossed sclerotisation in the medial region of the notogaster. According to Balogh & Mahunka (1983) the "heavy, irregular sculpture" and "definite ornamentation" of Synchthonius with median trilobate transverse structures on the notogastral sclerites is "fundamentally different from that of all other Brachychthoniid genera". Arcochthonius roynortoni n. sp. exhibits curved lines in the median area of the notogastral sclerites, partially in similar position as the "irregular" sculptures on Synchthonius species, but neither trilobate nor with "small chitinous thickenings" (Hammen 1952).
The morphological differences of Arcochthonius roynortoni n. sp. to species of these and other Brachychthoniidae genera justify the erection of a separate genus.

Notes on ecology and biology
In the course of previous studies in the Obergurgl area, Central Alps, 12 Brachychthoniidae species were recorded (Fischer & Schatz 2013, Schatz 1979. Some species of this family were found in considerable abundances in raw soils (Schatz 2004). Remarkably, on the summit of the Festkogel mountain a further six Brachychthoniidae species beside Arcochthonius roynortoni n. sp. were encountered (Schatz in prep.). As mentioned above, this locality is only sparsely vegetated on the border between alpine grassland and subnival cushion plant formations with scarce pioneer vegetation. Numerous records of Brachychthoniidae species are reported from raw soil and lichens (e.g. Schweizer 1956, Moritz 1976a, b, Leonov 2020. Arcochthonius roynortoni n. sp. seems to be a rare species. It was detected only in two soil samples in 1999 on the summit of the Festkogel mountain despite numerous additional sampling in the same spot. Also, this species was not found in the course of intensive collecting at lower elevations on the slope of the Festkogel (Schatz 1979). Possibly it survived as preglacial relict on a nunatak or it evolved in this altitude in isolation (cf. Schatz 2020).
Little is known on the feeding preferences of Brachychthoniidae species. Schuster (1956) demonstrated a relationship between relative size of chelicerae and feeding type. Based on the morphology of the chelicera, Brachychthoniidae species can be assumed to be "microphytophagous", consuming microorganisms as fungi, bacteria and algae (Lebrun 1971). Microphytophagous oribatid species are very abundant in alpine grassland and subnival cushion plants (Schatz 1979). These observations also apply to the locality of Arcochthonius roynortoni n. sp. This species has a small movable digit with weakly developed teeth and can also be classsified as microphytophagous in the sense of the above authors.