A New Opilioacarid Mite in Baltic Amber
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European Arachnology 2008 (W. Nentwig, M. Entling & C. Kropf eds.), pp. 59–70. © Natural History Museum, Bern, 2010. ISSN 1660-9972 (Proceedings of the 24th European Congress of Arachnology, Bern, 25–29 August 2008). A new opilioacarid mite in Baltic amber JASON A. DUNLOP 1, CAROLIN SEMPF 2 & JÖRG WUNDERLICH 3 1 Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity at the Hum- boldt University Berlin, Invalidenstrasse 43, D–10115 Berlin, Germany (Corresponding author: [email protected]) 2 Freie Universität Berlin, Institut für Biologie, Chemie & Pharmazie, Evolution und Systematik der Tiere, Königin-Luise-Str. 1–3, D–14195 Berlin, Germany ([email protected]) 3 Oberer Häuselbergweg 24, D–69493 Hirschberg, Germany ([email protected]) Abstract The second fossil opilioacariform mite (Acari: Anactinotrichida: Opilioacarida) is described as ?Opilioacarus aenigmus sp. nov., from Baltic amber (Palaeogene: Eocene). Compared to the previously described amber opilioacarid, the new fossil reveals more of the genital region, as well as an unusual pattern of long setae towards the distal end of the first leg. Unlike the previous species, the new fossil bears only two pairs of eyes and the fourth pair of legs are shorter in relation to the body. This clearly indicates a new fossil taxon. Recent work on the phylogenetic position of the mites – and their constituent subgroups – is briefly reviewed, together with a summary of the fossil record of anactinotrichid mites in general. INTRODUCTION to be described is Paracarus pristinus Dunlop, Opilioacarids (Acari, Anactinotrichida, Opil- Wunderlich & Poinar, 2004 from Eocene Bal- ioacarida) are a rather rare group (ca 20 tic amber. The well-preserved holotype (and species) of mites, sometimes referred to by only known specimen) was assigned to a re- the alternative names Opilioacariformes cent genus with a Central Asian distribution or Notostigmata. In older literature opilio- today. Here, we describe only the second fos- acarids were often treated as ‘primitive’ mites, sil example of an opilioacarid mite (Figs 1–5), largely because they retain a number of ple- a fossil again originating from Baltic amber. siomorphic characters, such as multiple lat- The ventral surface of this new specimen is eral eyes and externally visible opisthosomal particularly well preserved, including some segmentation. They have even on occasions details not observed in Dunlop et al.’s (2004) (e.g. Reuter 1909) been excluded from Acari original study. The new specimen also ex- sensu stricto. Their current placement within presses a prominent group of very long se- the Anactinotrichida mite lineage (Parsiti- tae towards the distal end of the first pair of formes s. l. in some schemes) is now strongly legs. This, and other characters – such as eye supported (see below). Valuable morphologi- number and leg proportions – differ substan- cal accounts of opilioacarids include With tially from the condition seen in P. pristinus (1904), Grandjean (1936), van der Hammen and indicate that the present fossil repre- (1968, 1976, 1977, 1989), Lehtinen (1980), Klom- sents a new taxon, which we tentatively as- pen (2000) and Vázquez & Klompen (2002). sign here to the Recent Mediterranean genus The first, and so far only, fossil opilioacarid Opilioacarus With, 1902. 6 0 European Arachnology 2008 MATERIAL AND METHODS The idiosoma is broadly oval in outline. Dor- The fossil described here derives from the sally there is a subtriangular ‘anterior area’ Wunderlich collection (F1882/BB/AC/CJW) (cf. Klompen 2000), alternatively called the and will eventually be transferred either to prodorsum (sensu van der Hammen 1976, the Senckenberg Museum (Frankfurt am 1989) and perhaps equivalent to the carapace Main) or the Görlitz Museum. It was pho- of other arachnids. Due the emulsion film, tographed using a Leica Systems camera ar- details such as setae are largely equivocal rangement attached to a stereomicroscope, (Fig. 1). However, on the right side two eye which generates a series of images through lenses can be resolved (Fig. 3), suggesting the specimen at different focal planes. The that the animal had two pairs of eyes (see ca 20 individual images per picture were chap. Systematic Palaeontology). The ante- combined into final composites (Figs 1, 2) rior area is delimited posteriorly by a trans- using the software package Auto Montage©. verse disjugal furrow. Behind this, further A Canon Eos Digital Camera attached to a furrows hint at up to four additional, short Nikon compound microscope was used for dorsal segments on the remainder of the close-up work (Fig. 5). The specimen was idiosoma. However, expected details (setae, drawn by CS using a stereomicroscope with lyrifissures, spiracles, etc.) are indistinct due a camera lucida attachment. (Figs 3–4) Mor- to the emulsion film and the fact that much phological terminology was adopted from of the posterior part of the dorsal idiosoma the literature, principally van der Hammen has collapsed to leave a large hole in the dor- (1976), but also Klompen (2000). The pre- sal surface. cise locality from which the specimen was recovered is not recorded, but much of the Gnathosoma. In ventral view (Figs 2, 4, recently collected Baltic amber originates 5a) the ventral surface (infracapitulum) of from the Kaliningrad area on the coast of the the gnathosoma is quite well preserved and Baltic sea. It is generally dated at Palaeogene partly overlain by the sternapophyses (see (Eocene), with an approximate age of 45–50 below) which cover the expected position of Ma. the subcapitular gutter. Laterally, the dark, sclerotised rutella insert on this infracapitu- MORPHOLOGICAL INTERPRETATION lum. As in modern taxa, the rutella are den- The specimen is a largely complete inclu- tate on their inner (mesal) surface and we sion, preserved at the base of a subtriangu- were able to resolve five teeth in total. We lar piece of translucent, yellow amber with could not identify either With’s organ – ex- maximum dimensions of ca 12 x 6 mm. The pected immediately mesal of each rutellum specimen is best seen in ventral view. A hole – or the adjacent lateral lips and regard these in the posterior dorsal part of the body, to- as equivocal for the fossil. Between the rutel- gether with a thin, white emulsion over the la, the anterior margin of the infracapitulum entire upper body makes interpretation of is slightly recurved and presents a series of the dorsal surface more difficult. Limbs and quite elongate, anteriorly-projecting setae mouthparts are preserved in some detail (Figs 4–5a). In van der Hammen’s termin- (see below) and in general the fossil closely ology these probably comprise circumbuccal resembles living opilioacarid species. All and paralabial setae; whereby the rutella and measurements are in mm. With’s organ are often interpreted as modi- fied paralabial setae. Specific patterns of se- Dorsal surface. The mite body is trad- tae position on the infracapitulum could not itionally divided into a gnathosoma (i.e. the be resolved. The bases of the chelicerae are pedipalps and mouthparts) and the idio- hidden in the ventral view, but as in modern soma comprising the remainder of the body. opilioacarids there are three articles, the dis- Dunlop et al.: Opilioacarid in Baltic amber 61 tal two forming a small chela. The fixed fin- solved in this area, but a distinct pattern is ger is slightly dentate, the free (movable) fin- absent. The pregenital area is bordered pos- ger is smooth. The pedipalps are fairly short teriorly by a small sclerite – segment VII sen- and pediform. The proximal articles are su van der Hammen – which is procurved largely concealed above the gnathosoma, but anteriorly, but has a more or less straight the terminal tarsus can be seen more clear- posterior margin. Further lineations, or sul- ly. It is covered distally with fine setae and ci, within segment VII can also be resolved. bears a pair of very fine claws (the apotele). Segment VII is flanked by the genital ver- rucae, a prominent pair of raised, wart-like Ventral idiosoma. The ventral idiosoma structures bearing three setae. In juvenile reveals more detail. At the anterior end, a pair instars this plate-like structure is absent of slender, elongate sternapophyses project (e.g. van der Hammen 1966, fig. 5B), thus we forwards and underlie the gnathosoma (Figs interpret this fossil as an adult. Determin- 2, 4, 5a). Setal patterns on the sternapophysis ing gender is more difficult. In some extant can be useful taxonomically, but the pres- taxa there are differences in the number of ence of any particular pattern in the fossil is pregenital papilliform setae: typically five difficult to resolve. We found evidence for at or more in males as compared to two in fe- least one lateral seta on each sternapophy- males. These pregenital setae should occur sis; possibly the latero-ventral, antaxial one directly in front of segment VIII (van der sensu van der Hammen (1989). Flanking the Hammen 1966, fig. 5B) and arenot the longer sternapophyses are the sternal verrucae; hairs more widely distributed pregenitally rounded, wart-like valves of uncertain func- as alluded to above. We were not able to ob- tion. Genital structures are detailed below. serve any papilliform pregenital setae in the The post-genital ventral idiosoma preserves fossil. However, we should note Grandjean’s few features of interest; only a couple of iso- (1936) observation that females of Opilioaca- lated setae. A series of eight weakly defined rus segmentatus lack pregenital setae alto- transverse furrows, which do not extend all gether; thus we favour the possibility that the way across the ventral surface, probably this specimen could be a mature female. In- indicate segment boundaries, but patterns ternal features such as a female ovipositor or of lyrifissures (slit sense organs), which can male accessory glands are equivocal.