The first description of a microtrombidiid mite (Actinotrichida: Prostigmata, Microtrombidiidae) from Baltic amber, with notes on related extant genera and species

Discovery of parasitengone mites (Acari) in the Gulf of Gdańsk deposits of Baltic amber (“Blue Earth” sediment) resulted in the first description of a fossil representative of Microtrombidiidae. The new species, based on larvae, displays affinity to recent members of Montenegtrombium Saboori and Pešić, 2006, Persianthrombium Sedghi, Saboori and Hakimitabar (in Sedghi et al. 2010) and Porttrombidium Haitlinger, 2000, known from the southwestern Palaearctic. A comparison with related genera and species places the newly described taxon in Porttrombidium (as Porttrombidium gedanense sp. nov.). Montenegtrombium is regarded as a junior synonym of Porttrombidium.


Introduction
Baltic amber, also known as succinite, is widely distributed in central-eastern Europe. The richest and the oldest deposits are located within the Gulf of Gdańsk, at the mouth of the hypothetical Eridanus river, which brought the resin from the primary amber forest (Weitschat and Wichard 2002).
Research on amber inclusions has a history of more than 200 years (Perkovsky et al. 2007;Szwedo and Sontag 2009). Recent studies on the taxonomic grouping of zooinclusions in randomly selected pieces of Baltic amber revealed that mites, constituting more than 20 % of all zooinclusions, are one of the best represented groups in succinite, giving precedence only to Diptera, which account for ca. 40 % of inclusions (Sontag 2003). Despite the abundance of mites in Baltic amber, the knowledge of Eocene acarofauna is scant.
The cohort Parasitengona, comprising ca. 11,000 of the species described to date (ca. 5,000 terrestrial and ca. 6,000 aquatic), constitutes one of the most speciose groups of mites, with relatively scarce knowledge of fossil taxa. The first mention of terrestrial parasitengone amber inclusion originates from 1845 (Berendt 1845;Dunlop et al. 2015;Judson 2012), and relatively few species have been described till now Konikiewicz and Mąkol 2014; Bartel et al. 2015). A summary of hitherto knowledge of fossil terrestrial parasitengones has been recently provided by Bartel et al (2015). The present work describes Porttrombidium gedanense sp. nov., based on larvae. It is the first representative of Microtrombidiidae found in the fossil record. In sample preparation we followed the protocol provided by Sidorchuk (2013). The amber pieces containing inclusions were pre-cut to the following dimensions (mm): 3.3 9 2.6 9 0.1 (MAI 896), 3.0 9 2.0 9 0.1 (MAI 1343) and 2.9 9 2.6 9 0.1 (MAI 3048), using a Dremel 300 rotary tool with a flexible drive and diamond disc. After polishing with a portable USB-powered MiniPolly2 polishing machine, the pieces were placed in cavity slides in thymol and distilled water solution and sealed with a cover glass. The inclusions were examined using a Nikon Eclipse E-600 light microscope, equipped with a DIC, drawing tube, and DS-Fi1 camera, at magnifications of 4009 and 10009. Raw drawings were graphically processed with the GIMP software, whereas the in-focus images were produced using CombineZP software. Measurements are given in micrometers. The samples are stored in Eppendorf vials filled with a solution of thymol and distilled water. The terminology follows Wohltmann et al. (2007) and Mąkol et al. (2014). For the purpose of comparison the type material of the following recent species was studied: Porttrombidium sebastiani Haitlinger, 2000 (holotype) and Montenegtrombium baloutchi Masoumi, Saboori and Seiedy, 2016 (four paratypes: MP 1256) deposited at the Museum of Natural History, University of Wrocław, Poland.

Deutonymph and adult. Not known.
Remarks. During the most recent re-examination of the type specimen of the type species (Porttrombidium sebastiani Haitlinger, 2000) we could observe that simple, setulated hypostomalae are present in the holotype, thus the character should be considered typical for the genus. The scope of differences observed between Porttrombidium and Montenegtrombium, pertaining to the chaetotaxy of the palps and chaetotaxy of tarsi, reflects the intra-generic variation known for other genera, and does not confirm their separate status. Consequently, we regard Montenegtrombium as a synonym of Porttrombidium. Masoumi et al. (2016) provided the verified characteristics of Montenegtrombium milicae Saboori and Pešić, 2006 (here regarded as Porttrombidium milicae) and pointed to the presence of 'sword-like lophotrix' in newly described Montenegtrombium baloutchi (here regarded as Porttrombidium baloutchi), whereas a sword-like seta arising at the tarsus III termination, and not being a lophotrix, is observed in the latter species but also in P. gedanense sp. nov. The lophotrix is absent in members of Porttrombidium.
Remarks the position of the specimen does not allow thorough examination of the chelicerae and setae or within the gnathosoma (the presence of or is typical of parasitengone larvae). Of three setae usually observed on the palp tibia, only one could be detected, due to the blurred surface of the segment. In the area surrounding the anus only six pseudanal setae could be observed (the actual number can depart from this value, however, the multiplication of setae can be excluded).
Keramotrombium Southcott, 1994 was erected in order to accommodate Metathrombium argentanense Bruyant, 1912. Another species, Keramotrombium talebii Karimi Iravanlou andKamali, 2001, was originally placed in the genus by Karimi Iravanlou and Kamali (2001); however, after the re-appraisal of its characters, the species was transferred to Achaemenothrombium Saboori, Wohltmann and Hakimitabar, 2010. Keramotrombium differs from Porttrombidium also in having the distinct lateral teeth (the latter absent in Porttrombidium) within the stephanostome and in the presence of multiple pseudanal setae (a state not observed in Porttrombidium). Differences between Porttrombidium and Persianthrombium are expressed in the number of solenidia on the genu and tibia I (f sol I = 0-2-2-1 in Porttrombidium, f sol I = 0-4-4-1 in Persianthrombium). Saboori et al. (2010) erected Achaemenothrombium Saboori, Wohltmann and Hakimitabar, 2010 and a new trombidioid family Achaemenothrombiidae Saboori, Wohltmann and Hakimitabar, 2010, to accommodate two species (among them Keramotrombium talebii Karimi Iravanlou and Kamali, 2001, described from Iran) with a combination of characters not known for any other family level taxon assigned to Trombidioidea: i.e. three dorsal scuta, fCx = 2-2-1, Ti I-III with eight or more normal setae, tibia I with at least four solenidia, multiple solenidia (at least four) and eupathidia (at least six) on tarsus I and multiple solenidia on tarsus II (at least two). The third species assigned to the family was described by Saboori et al (2013). Of the characters of Achaemenothrombiidae, the presence of three dorsal scuta, fCx = 2-2-1, but also the Ta III termination are shared by Keramotrombium, Persianthrombium and Porttrombidium (=Montenegtrombium syn. nov.).
Microtrombidiinae may include genera of heterogeneous origin. A discussion on constructing monophyletic groups (subfamilies) within Microtrombidiidae, supported by a critical review of the literature data, was provided by Wohltmann (2006). The ultimate answer to the question of monophyly of these groups constitutes a crucial point in further conclusions on the phylogeny of subordinate taxa and their position in the system of Microtrombidiidae.
Porttrombidium shares the characters known for Microtrombidiidae (e.g. stephanostome) and for Trombidiidae (e.g. fCx = 2-2-1, termination of tarsus III). The systematic position of Porttrombidium but also of Persianthrombium, and their relationship with other microtrombidiid genera and with Achaemenothrombiidae and Trombidiidae, should be clarified based on further evidence from biology and genetic studies. Discovery of postlarval forms, hitherto unknown both for Porttrombidium, Keramotrombium, Persianthrombium and for Achaemenothrombiidae, may shed a new light on the picture of relationships within Trombidioidea.  Terminology after Wohltmann et al. (2007) and Mąkol et al. (2014) The first description of a microtrombidiid mite from Baltic amber… 499 sebastiani available for study, and to Dr. Jolanta Jurkowska for the loan of paratypes of Montenegtrombium baloutchi. Our special thanks go to Dr. Jason A. Dunlop and Dr. Andreas Wohltmann for their helpful comments on the manuscript. The work of MK was supported by National Science Centre (Grant in Aid of research 2015/17/N/NZ8/ 02418).
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