Cosmobunus sagani, Palencia & Peñalver & Prieto & Poyato-Ariza, 2019

Palencia, Lorena, Peñalver, Enrique, Prieto, Carlos E. & Poyato-Ariza, Francisco José, 2019, First fossil harvestmen (Arachnida: Opiliones) from Spain and notes on the fossil record of Opiliones, Palaeontologia Electronica (5 A) 22 (1), pp. 1-18 : 4-10

Cosmobunus sagani nov. sp.

Figures 2–5

zoobank.org/ B970A094-E9E0-4E57-BA9F-978513452047

Diagnosis. Fossil species of Cosmobunus with the following unique combination of features: pedipalps with smooth tarsal claw and rows of laterodistal granules in the tibiotarsal articulation, smooth ocularium, and trilobate coxal denticles.

Material. Holotype ( MPV-2417-RM ), adult individual, most likely a male. Specimen present in a large slab of oil-shale measuring 37 x 20 x 1 cm. Specimen appears in dorso-ventral position, slightly lateral (tilted towards the right), nearly complete and well preserved, having lost only the main part of the right leg I; the distal part of the left leg IV is displaced close to the body. The same slab also contains a few indeterminate plant remains. Type specimen housed at the Museo de Ciencias Naturales de Valencia, Valencia, Spain.

Derivation of name. In memory of Carl Edward Sagan (1934–1996), creator of the award-winning television series Cosmos : A Personal Voyage, for his popularization of science that stimulated generations of school children and university students.

Type horizon. Lacustrine oil-shales from the Early Miocene (early Burdigalian) of Rubielos de Mora Basin (Iberian Chain).

Type locality. "Río Rubielos" outcrop, Rubielos de Mora municipality, Province of Teruel, Spain.

Description. Body length: 5.3; body width: 4.1 (artefactual due to the lateral shift, estimated 3).

Prosoma: Carapace length: 1.2. Cuticle with granulate ornamentation. A postocular line suggests the presence of a metapeltidium. Supracheliceral lamina not visible. Ozopores not discernible. Ocularium globular ( Figure 4.2 ), as long as wide (0.5 x 0.5), distant from frontal edge, smooth, without granules.

Opistosoma: Anterior edge of scutum smooth. Scutum with conspicuous granulate ornamentation ( Figure 4.6 ); granules roundish, 40 µm in diameter, tightly arranged together. Color pattern: dark lateral regions and a lighter longitudinal dorsal band; the left region appears considerably wider and darker. Three free tergites granulate as the scutum.

Ventral: Posterior coxapophysis preserved, with distal pilosity, observable by transparency.

Appendages: Chelicera not visible, hidden beneath body. Right pedipalp poorly preserved ( Figure 4.1 ). Left trochanter and basal part of femur hidden beneath body. Distal part of femur with some acute granules. Patella wide, with acute granules. Tibia longer and higher than patella, covered by trichomes, scattered long sensillae, and sharp granules; distal end lined with sharp granules ( Figure 4.4 ). Tarsus covered with stiff sensillae and small trichomes; medioventral area darker. Tarsal claw smooth. Measurements: Patella (left) 0.4, tibia (left) 0.8, tarsus 1.5.

Leg measurements: Leg I: Length; Femur 5.3, Patella 1.3; Tibia 4.5; Metatarsus 2.3; Tarsus 9.8. Leg II: Length; Femur (left) 9.1, Patella (left) 1.4; Tibia 8.6; Metatarsus (left) 8.6; Tarsus (right) 19.1. Leg III: Length; Femur (left) 5.3, Patella (right) 1.4; Tibia (right) 4.4; Metatarsus 4.9; Tarsus (left) 7.9. Leg IV: Length; Femur 6.5, Patella (left) 1.6; Tibia (right) 5.6; Metatarsus (right) 8.4; Tarsus (right) 10.2.

Coxa I: Separated from prosoma, slightly displaced, and connected to leg I. At least anterior rows of coxal denticles present; coxal denticles small (9–10 per 0.5 mm) and weak, slightly trilobate. Trochanter not visible. Femur with conical denticles ( Figure 4.3 ). Patella presents the least amount of small denticles. Tibia considerably wide; microtrichiae present. Tarsus multisegmented, with ventrodistal spines. Tarsal claw simple, curved and smooth.

Remarks. The presence/absence of protuberant ozophores bearing ozopores and the morphology of the tarsal claws of legs III–IV are important characters for the subordinal assessment of Opiliones (Pinto-da-Rocha and Giribet, 2007). These features allow the exclusion of Cyphophthalmi (having ozophores, lacking median eyes) and Laniatores (having ramified or two tarsal claws on legs III–IV, and raptorial pedipalps with robust spines), but these characters are insufficient by themselves. Both Eupnoi and Dyspnoi suborders, grouped together in Palpatores, have single simple tarsal claws and graceful antenniform pedipalps. In the absence of cheliceral features (shape and size of chelal teeth) or genitalic features, the relative length of pedipalpal tibia and tarsus is the most reliable character.

A tarsus longer than the tibia (1.5 vs. 0.8 in the Rubielos de Mora specimen) in the pedipalp is typical of most taxa of the suborder Eupnoi (except Protolophidae ), although this difference may not be considered significant due to the deformations that could occur during lithostatic compression, and also considering that the specimen rested in the lake bottom substrate tilted laterally. In addition, the pedipalp tarsal claw is lacking or rudimentary in Dyspnoi, being a short peg at most, but it is well developed in Eupnoi .

The suborder Eupnoi comprises two superfamilies, the monotypic Caddoidea and Phalangioidea, with four families (Kury, 2017; Fernández et al., 2017). Only two living families are found in the Western Palearctic Region. However, a member of the Caddidae , currently restricted to North America, Chile, Japan, Australia, New Zealand, and South Africa has been reported from Baltic and Bitterfeld ambers (Dunlop and Mitov, 2009). Therefore, the other taxa with current North American distributions ( Protolophidae ) cannot be excluded a priori in Europe in the past. However, the monotypic Protolophidae present very dimorphic pedipalps (long patellar apophysis in females, huge curved tibia and shorter tarsus in males) and short legs (Cokendolpher in Pinto-da-Rocha and Giribet, 2007). In turn, the monotypic Caddidae has tiny bodies with huge ocularium and eyes, and spiny pedipalps (Dunlop and Mitov, 2009). Therefore, both families can be discarded.

A relevant distinctive anatomical feature between Sclerosomatidae and Phalangiidae is the tarsal claw of the pedipalps, which is normally pectinate in the former and smooth in the latter, although with exceptions in both families (Pinto-da-Rocha and Giribet, 2007). The fossil specimen presents smooth claws, but other characters suggest an affiliation to Sclerosomatidae , especially the presence of rows of coxal denticles and the absence of rows of denticles or setae along the edges of the femur and the tibia.

Within Sclerosomatidae , four subfamilies are currently regarded: the Pantropical Gagrellinae, the Holarctic Leiobuninae , and the Palearctic Gyinae and Sclerosomatinae (Pinto-da-Rocha and Giribet, 2007). The Gyinae, lacking coxal denticle rows, must be transferred to the Phalangiidae , as shown by the molecular analyses of Hedin et al. (2012) and Fernández et al. (2017). Among the other three subfamilies, the Sclerosomatinae present sclerotized bodies with a tuberculate opisthosoma and short legs, so it can be excluded. This leaves the tropical Asian/American Gagrellinae and the Holarctic Leiobuninae as remaining possibilities. Distinction between them is complex; most Gagrellinae have femoral nodules (at least one in femur II), but, other than this, only biogeographical considerations can be used, and, as a consequence, their taxonomic validity has been questioned (Martens, 1987; Hedin et al., 2012). The presence of leiobunines, with absence of gagrellines, in the European amber fossil record (Dunlop, 2006; Dunlop and Mitov, 2009) could be due to an ancient vicariant event, thus allowing assessment of the Rubielos de Mora specimen to the Leiobuninae on the basis of the presence of rows of coxal denticles. The European Leiobuninae are formed by three genera. Nelima Roewer, 1910 , is a genus based only in the absence of coxal denticles, which could be due to heterochronic processes (Hedin et al., 2012). This leaves Leiobunum Koch, 1839 and Cosmobunus as the only possibilities.

The genus Leiobunum currently includes more than 100 species from the Holarctic region (about twenty in Europe), but it has been shown to be polyphyletic (Hedin et al., 2012). In turn, Cosmobunus is a monotypic genus (spurious species removed) geographically restricted to Iberia and Morocco. The diagnostic feature between both genera is the absence of pectination on the tarsal claw of the pedipalp in Cosmobunus , a feature that fortunately can be checked in both appendages of the fossil specimen.

A smooth tarsal claw in the pedipalp and leg coxae without denticle rows were the characters used by Banks (1901) to erect the Leptobuninae for some North American genera, but Roewer (1910) dismissed the second character to include Cosmobunus and Protolophus Banks, 1893 . The subfamily was then considered polyphyletic and, therefore, modified by Cokendolpher (1985), who transferred Cosmobunus (after removing spurious North American species) and the North American Leuronychus Banks, 1900 to the Leiobuninae , and the remaining genera to other subfamilies of Phalangioidea. The only paper devoted to Cosmobunus granarius was that of Rambla (1970); she redescribed the genus, considered Cosmobunus unicolor Roewer, 1910 as senior synonym of Cosmobunus unifasciatus Roewer, 1923 , outlined the geographical distribution of the genus according to many new records and added biological information.

Since the presence of coxal denticles is an adult feature, the issue about the sex of the studied individual still remains. Sexual dimorphism in Leiobuninae is reduced to a slightly longer leg length and hardiness in males and a broader opisthosoma in females, occasionally with dimorphic color pattern (Tourinho in Pinto-da-Rocha and Giribet, 2007). However, in absence of genital features, sex can be inferred by comparison only; Rambla (1970) stated that in Cosmobunus granarius both sexes are alike. A secondary sexual character in Phalangioidea is the presence of a ventromesal row/belt of microgranules along the pedipalpal tarsus (Pinto-da-Rocha and Giribet, 2007). This feature seems to be present in the studied specimen as a darker strip in the corresponding area, thus being a male.

The legs of the fossil specimen are notably shorter than the measurements provided by Roewer (1923: 42, 64, 40, 55) and Rambla (1970: 45, 78, 46, 60), for males of Cosmobunus granarius , reaching 50–60% (up to 73% for leg II) of the recorded leg lengths of the extant species. However, there is no information about its individual variability to consider the significance of such difference. As shown by Rambla (1970), C. granarius has marginal denticle rows in all leg coxae except for the back margin of the coxa III. This character is shown by some European Leiobunum , although there are species with a reduced dotation of the coxal rows (Prieto and Fernández, 2007). Only three forward denticle rows of the fossil specimen are exposed, so that coxal configuration cannot be determined. Coxal denticles of C. granarius are usually trilobate (Rambla, 1970); same goes for many species of Leiobunum , some of them with a characteristic shape within the broad variability (Prieto and Fernández, 2007). Although the coxal denticles of the fossil specimen are clearly trilobate, they are smaller and weaker than those of C. granarius (6 denticles per 0.5 mm) ( Figure 6.3–4 ).

The ornamentation of the dorsal surface of the fossil specimen consists of rounded granules 40 µm in diameter and densely packed. This is similar to that of Cosmobunus granarius . Moreover, the color pattern is congruent with the dorsal pattern of C. granarius , a light, broad median band flanked by lateral dark areas ( Figure 6.1 ).

The ocularium of the fossil specimen is smooth, as in Leiobunum and most Sclerosomatidae genera. In the specimens of Cosmobunus granarius used for comparison, a great deal of variability has been observed in the sclerotization of the rows of granules of the ocularium; that is, these rows of granules can be more conspicuous or less conspicuous. Therefore, the body of the fossil specimen presents a dense granulate ornamentation, trilobate coxal denticles, and a body coloration similar to those of C. granarius ( Figure 6 ).

The tibiae of the pedipalps present acute denticles in the fossil specimen, as in Cosmobunus granarius . The coxal denticles have a distribution in marginal rows (anterior and posterior), and are slightly trilobate. For the coxal denticles, only simple and trilobate denticles have been described, but there is variation in their relative development among species and even within the same row (see Prieto and Fernández, 2007).

The fossil specimen shows characters of both Leiobunum and Cosmobunus , but the characters typical of the latter have a greater taxonomic significance. The tarsal claws of the pedipalps, the dark lateral coloration with a clear central band, and the granular cuticle that covers the whole body are features unique to Cosmobunus within Sclerosomatidae . The smooth ocularium and the similar thickness of all femora distinguish the fossil specimen from the extant species Cosmobunus granarius indicating it is a new species.

The current geographical distribution of Cosmobunus granarius ranges from Barcelona to Algarve along the Iberian Mediterranean coast, plus inner Andalusia and southern regions of Extremadura and Castilla-La Mancha (Rambla, 1970; unpublished records), reaching as far north as Madrid. Interestingly, Rubielos de Mora is close to the boundary of this current distribution.

Paleobiology. The analogous extant species Cosmobunus granarius is lapidicole, active at dawn. It abounds on walls and tree trunks, and also on the ground, seeking for prey (spiders, mites, dipterans and isopods) (Rambla, 1970). During the day, the individuals of this species rest in dark places such as tree holes, caves, tunnels and under bridges. They usually come together in compact and massive aggregations, taking the aspect of a bunch of roots. Solitary individuals rest in a distinctive manner on walls. Its life cycle is annual, with birth in early spring, growth until summer, maturation, copula and oviposition in late summer, and death in autumn, thus being stenochronous univoltine (Belozerov, 2012), at least in non-arid regions such as Catalonia. Such a life cycle can be modified in caves, where Rambla (1970) reported the presence of egg batches, young nymphs, and coexisting nymphs and adults. The egg batches have 16– 78 eggs (commonly 35–50), with average egg size of 1.14–1.25 mm; eggs are laid into the ground, then covered by vegetal debris.

Figures

FIGURE 2. Photomicrograph under ethanol of Cosmobunus sagani nov. sp. (Sclerosomatidae), from the Miocene of Rubielos de Mora Basin. Holotype (MPV-2417-RM), most likely a male.

FIGURE 3. Cosmobunus sagani nov. sp. (Sclerosomatidae), holotype (MPV-2417-RM), most likely a male. 1, Camera lucida drawing of the habitus. 2, Detail of the body.

FIGURE 4. Microphotographs of some anatomical details of Cosmobunus sagani nov. sp. (Sclerosomatidae), holotype (MPV-2417-RM), most likely a male, taken under ethanol. 1, pedipalps. 2, smooth ocularium. 3, granules on leg I. 4, row of laterodistal granules in the tibiotarsal articulation of the pedipalp. 5, rows of trilobate denticles on two coxae (arrows point some of the denticles). 6, cuticular granulate ornamentation on the opistosoma. Image 3 made with some pictures taken at successive focal planes, as explained in the text.

FIGURE 5. Microphotographs of the pedipalpal tarsal claw. 1, Extant Leiobunum rotundum (pectinated claw). 2–3, Fossil Cosmobunus sagani nov. sp. and extant Cosmobunus granarius (both with smooth claw). All to the same scale.

FIGURE 6. Photographs and microphotographs of extant Cosmobonus granarius for comparison. 1–2, Photographs of the body in dorsal and lateral views. Note the coxal denticles (images electronically made with consecutive pictures taken at successive focal planes). 3–4, Photomicrographs of two rows of trilobate coxal denticles in coxa I (3) and coxa IV (4).