First checklist of the oribatid mites (Arachnida: Oribatida) of the Gran Chaco region (South America) with new records

The South American Gran Chaco ecoregion extends through great areas of Argentina, Paraguay, Bolivia, and Brazil. The region includes vast forests that serve as an important reservoir of tropical and subtropical diversity. In spite of its extent and high biodiversity, it remains an unexplored area regarding Oribatida. We made bibliographic research about the oribatid mites previously registered. In the bibliography of Argentina and Paraguay, a total of 53 species are cited for this region. From our own works in the Argentinean provinces of Santiago del Estero and Chaco, we add some new records of Oribatida in the area. We provide a list of the species registered to date, including 48 firstly recorded in this work, as well as new localities for other 11 species previously known for the area. This paper almost doubles the number of registered species for this highly diverse but poorly known ecoregion, adding information towards the true diversity of Oribatida in the Chaco region.


Introduction
Oribatid mites (Arachnida: Acari: Oribatida) are one of the most abundant and diverse groups of soil fauna, including more than 11,500 species recorded worldwide (Subías 2022). They are commonly found in soils around the world, but many species dwell on vegetation (Salavatulin 2019), including canopy soils on trees, bromeliads, mosses, or even lichens (Behan-Pelletier and Walter 2000). They are small and very abundant in almost all ecosystems and play a key role in decomposition, participating in the comminution of dead organic matter, and increasing the available surface for decomposers; they also help in bacteria and fungi proliferation, by external and internal transport of spores and other structures (Behan-Pelletier 2003).
The South American Gran Chaco ecoregion extends through great areas of Central and Northern Argentina and Western Paraguay, with small areas in Eastern Bolivia and South-Western Brazil (Bucher and Huszar 1999) (Fig. 1). The region is generally divided into a humid Eastern area and a dry Western area (Bianchi and Gibbs 2000). This region includes large areas of forests that serve as an important reservoir of tropical and subtropical diversity.
In spite of its large extent and high biodiversity, relatively few efforts have been made to explore the oribatid fauna of the Gran Chaco region. A complete catalogue of global oribatid  Table 1. mites can be found in Subías (2022); although very complete and frequently updated, this list does generally provide scarce information about the distribution, such as country (e.g., Argentina) or wide region (e.g., Neotropics). A more detailed list of species and records is presented by (Fredes 2018), although only for Argentina; in her work, she includes a compilation of the species cited for the Argentinean Chaco region, mainly in the southernmost part, around the "Sierras de Córdoba" area, with two individual records for the Chaco province. A single work citing oribatid species (23 spp.) for the Paraguayan Chaco is recorded  and none from the Bolivian and Brazilian areas.
In the present work, we compile and provide a first checklist of Oribatida for the Gran Chaco region.

Material and methods
The collection in the new prospected sites reported in this work come from an expedition to the Santiago del Estero-Chaco border led by Dr. Matías Mastrangelo in August 2017, and another to "El Impenetrable" National Park (Chaco) led by researchers from the Universidad Nacional del Nordeste (UNNE) in November 2021.
At the first site, three fields were sampled. All fields had native forests until the late 90's but then the forests were partially cut off between 1999 and 2012 for pasture or agriculture. Soil and litter samples were taken to the laboratory and processed in Berlese funnels for 12 days. Material was deposited at the Oribatida collection of the "Universidad Nacional de Mar del Plata" (UNMP).
At "El Impenetrable" National Park, two forest sites were sampled, which differ in their conservation status, based on modified categories proposed by Roig et al. (2015). This public reserve was established in 2014 from previous private lands. Most of its forests were relatively untouched for decades, but illegal deforestation, hunting and livestock were frequent, leaving some degraded areas. One of the sites is a well conserved forest area and the other one corresponds to a forest with intermediate level of conservation. Litter samples were taken to the laboratory and processed in Berlese funnels for 10 days. Material was deposited at the Arthropods collection of the "Universidad Nacional del Nordeste" (CARTROUNNE).
Additional to these new data, a search of bibliography about Oribatida in the Gran Chaco ecoregion was performed. Coordinates of all localities and sources of data are shown in Table  1. Some of the coordinates of sites from bibliography are approximate due to the lack of accuracy of data provided in the papers. A map of the Gran Chaco region including localities of Oribatida species cited in this paper is shown in Figure 1.

Results
Based on bibliography and new data we present an updated checklist of the Oribatida of the Gran Chaco. Supra-generic classification follows that of Schatz et al. (2011); these categories are presented in phylogenetical order following the same authors. Genera and species follow the nomenclature in Subías (2022) and are presented in alphabetical order. Locality number codes correspond to those in Table 1.

Final remarks
Though large and diverse in habitats and species, the Gran Chaco remains a poorly prospected area. Few previous data registered 53 species of Oribatida from the region, concentrated in the southernmost and eastern areas Mahunka 1981; Fredes 2018). Our data from the central part of the Dry Chaco (West) add 48 species newly recorded for the ecoregion and new localities for 11 species previously recorded in other areas. This constitutes a big step forward in the knowledge of soil biodiversity in the region, and Oribatida in particular, a group of the fauna that is important in food webs but whose actual diversity remains unknown. Still, there is a huge gap in knowledge of Oribatid mites of the Gran Chaco, with many large areas not prospected, such as the diverse Iberá wetlands in the East and the rich forests in the North, and many species still to be described, including some found in the works presented here, which could not be assigned to any known species.