Mitostoma gruberi, Dunlop & Mitov, 2009

Dunlop, Jason & Mitov, Plamen, 2009, Fossil harvestmen (Arachnida, Opiliones) from Bitterfeld amber, ZooKeys 16, pp. 347-375 : 355-356

Mitostoma gruberi sp. n.

urn:lsid:zoobank.org:act:F6F51FD0-9D10-4094-975F-3C202B48BDB9

Figs 3 , 9

Material. MfN, MB.A. 1654. Bitterfeld amber. Probably from the site of the Goitsche Open Cast Mine near Bitterfeld, Sachsen-Anhalt, Germany; Palaeogene (Oligocene: Chattian).

Diagnosis. Fossil Mitostoma with a specific form of the apophysis on the first cheliceral segment and specific form of the palpal tarsal segment.

Etymology. In honour of Dr Jürgen Gruber (Vienna) in recognition of his extensive studies on harvestmen, and nemastomatids in particular.

Description. MB.A. 1654 comprises a well-preserved body and pedipalps in lateral view, with a number of slightly disarticulated leg femora. Body length, 1.5 mm. Ocular tubercle distinct, eye diameter 0.20 mm. Opisthosoma with hints of segmentation and slight tuberculation on the dorsal surface towards the posterior half of the body. Chelicerae largely obscured, but dorsal part of first segment appears inflated forming an apophysis whose form is close to that of the Italian endemic Mitostoma orobicum ( Caporiacco, 1949) (see Tedeschi and Sciaky 1997: fig. 11). Pedipalps long and slender, article lengths in mm: trochanter, 0.27; femur, 1.42; patella, 1.63; tibia, 0.80; tarsus, 0.48. Tarsus slightly swollen at its distal end. Pedipalps setose, many palpal setae, particularly towards the distal end, with rounded tips (i.e. clavate setae). Trochanters of legs oval, femora long and slender, but more distal parts of the legs equivocal. Femora with scattering of short, thorn-like spines and one femur (femur 4?) with at least three short pseudoannulations, beginning about 0.8 mm along its length.

Remarks. This fossil can be assigned to Mitostoma , as opposed to Nemastoma C. L. Koch, 1836 , on the basis of the proportions of the pedipalp articles, while the cheliceral apophysis indicates a male specimen. Whether it is juvenile or adult is less clear, but strong parallels can be drawn with the widespread European species M. chrysomelas ( Hermann, 1804) if the fossil is juvenile, or with the Sardinian troglobitic endemic M. patrizii Roewer, 1953 if adult (cf. Fig. 5 ). Juveniles of the former and adults of the latter have small tubercles on the scutum – see especially Roewer (1953: fig. 1) and Tedeschi and Sciaky (1997: fig. 1) for M. patrizii – although the scutum of the fossil specimen is better developed than in comparable juveniles of M. chrysomelas (cf. Fig. 4 ).

Note that an existing fossil from Baltic amber has already been assigned to Mitostoma – M. denticulatum ( Koch & Berendt, 1854) – transferred here from Nemastoma by Staręga (1976a); an interpretation followed by Dunlop (2006). On reflection, this assignment to Mitostoma may be incorrect and the situation is not helped by the dorsal surface of the holotype being partially broken and the apparent loss of the type material of its putative junior synonym N. succineum Roewer, 1939 . In our revised opinion, the legs of M. denticulatum express lengths more consistent with modern Carinostoma Kratochvíl, 1958 or Mediostoma Kratochvíl, 1958 species. These (and other) nemastomatid genera with comparable dorsal ornament may be closer to the Baltic amber fossils. The status of M. denticulatum will be reassessed in a future study of Baltic amber harvestmen. Thus for the reasons outlined above we are confident that our new Bitterfeld species can be assigned to Mitostoma and is probably not conspecific with Koch and Berendt’s Baltic amber taxon.

Figures

Figures 1-5. Nemastomatids. 1?Histricostoma tuberculatum (Koch & Berendt, 1854), MB.A. 1652 2?H. tuberculatum, MB.A. 1653 3 Mitostoma gruberi sp. n. MB.A. 1654 (holotype) [all from Bitterfeld amber (Palaeogene: Oligocene)] 4 Juvenile Mitostoma chrysomelas (Recent harvestman for comparison) 5 Male Mitostoma patrizii (Recent harvestman for comparison). Scale bars equal 1.0 mm (1-2, 5), 0.5 mm (3) and 0.35 mm (4).

Figures 6-9. Camera lucida drawings of the specimens shown in 1-3. 6?Histricostoma tuberculatum (Koch & Berendt, 1854), MB.A. 1652 7 the same – detail of the apophysis at the tip of the first cheliceral segment (right chelicerae, dorso-lateral view) 8?H. tuberculatum, MB.A. 1653 9 Mitostoma gruberi sp. n. MB.A. 1654 (holotype). All from Bitterfeld amber (Palaeogene: Oligocene). Scale bars equal 1.0 mm (6, 8) and 0.5 mm (9).