The study of Orthoptera from the Eocene deposits of the Isle of Wight was initiated by T. D. A. Cockerell (Reference Cockerell1921), who described the following four species: Gryllotalpa prima, Gryllus vetus, Gryllus oligocenus, and Poekilloptera melanospila. Cockerell included these species in the Recent genera of Orthoptera (first three species) and Hemiptera (the latter species). This study was continued by F. E. Zeuner, who described two genera and four species from the same locality (defined by him as Middle Oligocene): Eneopterotrypus chopardi, Proschistocerca oligocaenica, Gryllus schindewolfi, and Pteronemobius? anglicus (Zeuner Reference Zeuner1937). Zeuner included the latter two species in two Recent genera. Later, he redescribed the majority of these taxa, including some of Cockerell's (Zeuner Reference Zeuner1939), and added several additional specimens to the series of imprints of P. oligocaenica (Zeuner Reference Zeuner1941). For a long time after these works, the Orthoptera from this locality were not revised except for the description of a new genus (Pterotriamescaptor) for G. prima by Gorochov (Reference Gorochov1992a, Reference Gorochovb) and the clarification of the systematic position of Poekilloptera melanospila by Nel et al. (Reference Nel and Ross2008). However, a revision of this material in accordance with modern views on the taxonomy and palaeontology of Orthoptera is very useful for the further study of insect evolution during the Caenozoic, which is most important for the understanding of the formation of Recent faunas.
1. Material and methods
The material studied here is deposited in the collections of the Natural History Museum in London (NHMUK) and Maidstone Museum & Bentlif Art Gallery in Maidstone (MNEMG). Types of the majority of the species are kept at the NHMUK, and the counterpart of the holotype of a single species is preserved in the Sedgwick Museum of Earth Sciences, University of Cambridge (CAMSM). All of these specimens are preserved in Insect Limestone from the Insect Bed (Bembridge Marls) of the Isle of Wight, southern England. The age of these deposits is probably latest Eocene (Ross & Self Reference Ross and Self2014). The specimens were examined in the NHMUK during my visit to London in 2004 and 2005, as well as in the Zoological Institute in St Petersburg, using a Leica MZ 16 microscope acquired with the help of an International Association for the Promotion of Co-operation with Scientists from the New Independent States of the Former Soviet Union (INTAS) grant.
The nomenclature of wing venation is given after Sharov (Reference Sharov1968) and Gorochov (Reference Gorochov1995). This paper also contains the description of a new genus (Proeneopterotrypus gen. nov.) from the Palaeocene/Eocene of Denmark (Fur Formation) based on the publication by Rust (Reference Rust1999). This addition is necessary for clarification of the systematic position of the genus Eneopterotrypus.
2. Systematic palaeontology
Order Orthoptera Olivier, 1789
Suborder Ensifera Chopard, Reference Chopard1920
Superfamily Grylloidea Laicharting, 1781
Family Gryllotalpidae Leach, 1815
Subfamily Gryllotalpinae Leach, 1815
Remarks. The family Gryllotalpidae (Fig. 1a–j), consisting of a few Recent and fossil genera, is known from the Early Cretaceous (Martins-Neto Reference Martins-Neto1995, Reference Martins-Neto1997; Perrichot et al. Reference Perrichot, Neraudeau, Azar, Menier and Nel2002) to Recent. The Recent Gryllotalpidae are divided into two subfamilies: Scapteriscinae, with only two articulated digging spurs on the fore tibiae (as in Cretaceous genera), and Gryllotalpinae, with 3–4 digging spines on the fore tibiae (two articulated spurs and 1–2 additional processes on these tibiae). The appearance of these processes is probably an autapomorphy of Gryllotalpinae. The most ancient representatives of Gryllotalpinae from the Eocene of Italy (Secretan Reference Secretan1975; Capra Reference Capra1977) and USA (Gorochov & Labandeira Reference Gorochov and Labandeira2012) had a single additional digging process on the fore tibiae. This primitive structure is preserved in a Recent genus from New Zealand (Triamescaptor Tindale, Reference Tindale1928). Moreover, in the Recent Gryllotalpinae with two additional digging processes (Fig. 1h), the youngest nymph has only a single additional process (Fig. 1g).
Gryllotalpidae. (A–C) Pterotriamescaptor primus (Cockerell, Reference Cockerell1921), Isle of Wight: (A) male tegmen, holotype In.24327 with addition from In. 17460; (B) female tegmen, IL 58; (C) fragments of female body with wings, In. 17461. (D–H) Gryllotalpa sp., Recent (from Gorochov Reference Gorochov1992a, Reference Gorochovb, Reference Gorochov1995): (D) male tegmen; (E) female tegmen; (F) basal area of male tegmen; (G) fore tibia of youngest nymph; (H) fore tibia of young nymph, but older than that in (G). (I, J) Basal area of male tegmen, recent: (I) Neocurtilla sp.; (J) Scapteriscus sp. Abbreviations: 1A = first branch of anal vein; 2A = second branch of anal vein; CuA = cubitus anterior; CuP = cubitus posterior; d=diagonal vein; M = medial vein; o=oblique vein; p=plectrum; s=stridulatory vein; Sc = subcostal vein.
These data allow one to propose a hypothesis about the presence of two generic groups of Gryllotalpinae: an ancestral one widely distributed in the first half of the Caenozoic and having a single additional process on the fore tibiae (apterous Triamescaptor is possibly a Recent relict of this group), and its descendant, a group of Gryllotalpinae widely distributed in the second half of Caenozoic and possessing two such processes (Gorochov Reference Gorochov1992a, Reference Gorochovb). In accordance with this hypothesis, the mole-cricket from the Isle of Wight (with unknown structure of fore tibiae) may be more closely related to Eocene Gryllotalpinae than to one of the Recent genera (except Triamescaptor), as they have a more primitive structure of tegminal venation than in all Recent winged Gryllotalpinae. In the male of Scapteriscinae (Fig. 1j) and that from the Isle of Wight (Fig. 1a), there is only a weak additional longitudinal vein between the branches of 2A in the proximal part of tegmina, but in the male of Recent winged Gryllotalpinae (Fig. 1f, i), this additional vein is usually strong and the area between the proximal parts of first branch of anal vein (1A) and second branch of anal vein (2A) is narrower than in the previous Gryllotalpidae.
For early Caenozoic Gryllotalpinae, having such primitive tegminal venation and possibly three digging spines on fore tibiae, the genus Pterotriamescaptor was described (Gorochov Reference Gorochov1992a, Reference Gorochovb). In view of the insufficiently complete material of this genus, it is a somewhat tentative taxon, but its position may be clarified after new finds of Eocene and Lower Oligocene Gryllotalpinae.
Genus Pterotriamescaptor Gorochov, Reference Gorochov1992a, Reference Gorochovb
Type species. Gryllotalpa prima Cockerell, Reference Cockerell1921.
Other probable included species. Gryllotalpa tridactylina Secretan, Reference Secretan1975 from the Middle Eocene of Monte Bolca, northern Italy; Pterotriamescaptor? americanus Gorochov & Labandeira, Reference Gorochov and Labandeira2012 from the Middle Eocene of Green River, WY, USA.
Diagnosis. Area between 1A and 2A in basal part of male tegmen very wide, distinctly wider than in all known Gryllotalpinae with wings (Fig. 1a); area between fore branches of 2A in the same part narrow, somewhat narrower than in other winged Gryllotalpinae; additional longitudinal vein between these parts of 2A branches weakly developed, usually noticeably thinner than that in other Gryllotalpinae (for comparison, see Fig. 1a, d, f, i). Fore tibiae possibly with only three digging teeth: two articulated spurs and a single additional process (almost as in Fig. 1g).
Comparison. This genus differs from all Recent Gryllotalpinae in having developed wings with the aforementioned characters of male tegmina and possibly in the presence of only three (not four) digging teeth in the fore tibiae. From apterous Triamescaptor having three such teeth, it may be distinguished by the presence of well-developed wings.
Pterotriamescaptor primus Cockerell, Reference Cockerell1921
(A–C) Pterotriamescaptor primus (Cockerell, Reference Cockerell1921): (A) IL 58b, non-type, female tegmen; (B) In. 24327a, holotype, male tegmen; (C) In. 17461, non-type, female body with wings. (D) Ontogryllus rossi sp. nov., In. 25806, holotype, male tegmen. (E) Ontogryllus oligocenus (Cockerell, Reference Cockerell1921), In. 24329, holotype, male tegmen. (F) Ontogryllus vetus (Cockerell, Reference Cockerell1921), In. 24328, holotype, male tegmen. Scale bars=5mm.
1921 Gryllotalpa prima Cockerell, p. 474, fig. 39.
1939 Gryllotalpa prima: Zeuner, pp. 238, 239, pl. 36, fig. 2, pl. 58, fig. 9.
1992 Pterotriamescaptor primus: Gorochov, p. 97, fig. 1.
Holotype. NHMUK In. 24327a, b (Figs 1a, 2b) (Hooley collection); Insect Limestone, NW Isle of Wight; incomplete male tegmen.
Other material. NHMUK In. 24776 (Hooley collection), In. 17460a, b and In. 17461 (Figs 1c, 2c) (Smith collection); Insect Limestone, NW Isle of Wight; fragments of male tegmina (first two) and female body (third). MNEMG IL 58a, b (Figs 1b, 2a) (D. Bone collection); Insect Limestone, NW Isle of Wight; incomplete female tegmen.
Diagnosis. Morphological differences between this species, Pterotriamescaptor? tridactylinus and P.? americanus are unclear, as tegminal venation of the two latter species is unknown, and the structure of the fore legs in P. primus is also unknown. However, the ages of these species are different: latest Eocene for P. primus and Middle Eocene for the others. The female of this species may be distinguished from that of Recent Gryllotalpinae by the somewhat wider cells around the diagonal vein and narrower subproximal part of the cell between cubitus posterior (CuP) and 1A (for comparison, see Fig. 1b, e) in the tegmina.
Description. Size rather small for Gryllotalpinae. General shape of body and its known parts (pronotum, wings, hind legs) typical of this subfamily. Tegmina in both sexes well developed (not shortened), comparatively narrow, with strongly and almost angularly curved subcostal vein (Sc). In male, tegmina with very long cells between cubitus anterior (CuA) and oblique vein, between stridulatory and diagonal veins, and between proximal part of CuA2, diagonal vein and CuP; proximal part of these tegmina with very narrow area between stridulatory vein and 1A, distinctly S-shaped proximal part of 1A, and rather strongly curved parts of 2A near plectrum. In female, tegmina with rather wide cells between medial vein (M) and diagonal vein, between proximal part of CuA2, diagonal vein and CuP, and between middle parts of CuP and 1A; cell between CuA2 and distal part of diagonal vein long and narrow; area between CuP and 1A near their anastomosis (behind it) rather narrow; 1A in this part of tegmina weakly curved. Colouration of body and tegmina slightly darkened, but with light interspaces between Sc branches. Length of imprint of holotype 6.6mm; probable length of tegmina 12–14mm; length of pronotum 5.1mm.
Family Gryllidae Laicharting, 1781
Subfamily Gryllinae Laicharting, 1781
Remarks. The Recent representatives of Gryllinae are divided into three tribes (Gorochov Reference Gorochov1995) or more (Otte Reference Otte1994). The latter author divided these crickets into two subfamilies: Gryllinae and Brachytrupinae Saussure, Reference Saussure1877. However, all these crickets are very similar to each other in morphology, including wing venation. It is impossible to determine their tribal and even generic position without the study of male genitalia. In this connection, the fossil specimens of Gryllinae known mainly from fragments of their tegmina and legs (or from their body outlines) may not usually be classified further than this subfamily. However, Palaeogene representatives of Gryllinae have some peculiarities of venation or colouration allowing them to be grouped into several more or less formal genera. These cannot be directly compared with Recent genera, as the venation and colouration in numerous Recent genera may be rather diverse and inclined towards convergence. In this regard, a new, almost formal, genus is described here for the Palaeogene representatives of Gryllinae with similar tegminal venation.
Genus Ontogryllus gen. nov.
Type species. Ontogryllus rossi sp. nov.
Other probable species. Gryllus oligocenus Cockerell, Reference Cockerell1921 and G. vetus Cockerell, Reference Cockerell1921 from the latest Eocene of the Isle of Wight, southern England.
Diagnosis. Size rather small for Gryllinae. Male tegmina (Fig. 3a–d) with well-developed stridulatory apparatus: stridulatory vein rather long (not shorter than diagonal vein); oblique veins not numerous (2–4) and weakly or moderately S-shaped; diagonal vein moderately long (approximately equal to chords in length or slightly shorter); chords weakly curved and connected with mirror by two cross-veins; mirror well developed, not large, angular, more or less oblique, and divided by a single dividing vein or by part of this vein. Apical area of these tegmina developed, and their lateral part with rather sparse branches of Sc not parallel or only partly parallel to tegminal radial vein (R) and M.
Gryllidae, male tegmen, Isle of Wight. (A) Ontogryllus rossi sp. nov., holotype In. 25806. (B, C) Ontogryllus oligocenus (Cockerell, Reference Cockerell1921): (B) holotype In. 24329/In. 26045; (C) specimen 2018.6.1147. (D) Ontogryllus vetus (Cockerell, Reference Cockerell1921), holotype In. 24328/I.8889. Abbreviations: a=apical area; c=chords; d=diagonal vein; dv=dividing vein; m=mirror; o=oblique veins; s=stridulatory vein; Sc = subcostal vein.
Comparison. This genus is practically indistinguishable from many of the Recent genera distributed over the different regions of the Earth, as these Recent genera are indistinguishable from each other by their venation. However, the representatives of the new, almost formal, genus described here differ from the majority of representatives of Recent genera by the characters listed in the aforementioned diagnosis. The Middle Eocene genera Pronemobius Scudder, Reference Scudder1890 and Eogryllus Gorochov & Labandeira, Reference Gorochov and Labandeira2012 (Green River, USA) may also belong to Gryllinae. Pronemobius is characterised by the spotted colouration of the legs, but its wing venation as well as legs of the new genus are unknown; comparison between these genera is not possible at present. From Eogryllus, the new genus differs in the male tegmina with the mirror clearly more angular and oblique (vs. almost oval and not oblique) as well as with the branches of Sc less parallel to R and M.
Ontogryllus rossi sp. nov.
Derivation of name. The species is named in honour of the British palaeoentomologist A. J. Ross for organising the investigations on the Eocene insects of the Isle of Wight.
Holotype. NHMUK In. 25806 (Smith collection); Insect Limestone, NW Isle of Wight; incomplete male tegmen.
Diagnosis. The new species differs from Ontogryllus oligocenus in the longer stridulatory apparatus (especially mirror and area containing oblique veins), oblique veins more numerous, absence of additional vein in the long cell between chords and mirror, and branches of Sc less parallel (for comparison, see Fig. 3a–c); from O. vetus in the much longer stridulatory apparatus, oblique veins less S-shaped, and branches of Sc less parallel (for comparison, see Fig. 3a, d); and from the majority of Recent representatives by the characters listed in the generic diagnosis.
Description. Male tegmen not shortened, with long stridulatory apparatus and possibly long apical area; stridulatory vein slightly oblique; area between this vein, diagonal vein, and CuA strongly elongate; four oblique veins, two longest of them S-shaped, two shortest ones almost arched; chords weakly curved; mirror strongly elongate, much longer than its width, with well-distinct dividing vein in middle part; long cell between lateral chord and mirror without additional longitudinal vein; branches of Sc almost not parallel. Colouration of this tegmen is light. Length of imprint 6.5mm; hypothetical length of tegmina 8–9mm.
Ontogryllus oligocenus Cockerell, Reference Cockerell1921
1921 Gryllus oligocenus Cockerell, p. 475, fig. 41.
1939 Gryllus oligocaenicus (unjustified emendation) Zeuner, p. 204, pl. 58, fig. 4.
Holotype. NHMUK In. 24329/In. 26045 (Figs 2e, 3b) (Hooley collection); Insect Limestone, NW Isle of Wight; part and counterpart of fragment of male tegmen with stridulatory apparatus.
Other material. MNEMG 2018.6.1147 (Fig. 3c) (Mitchell collection); Insect Limestone, Thorness Bay, Isle of Wight; positive and negative imprint of small fragment of male tegmen.
Diagnosis. This species differs from Ontogryllus rossi in the shorter stridulatory apparatus (especially mirror and area containing oblique veins), less numerous oblique veins, presence of additional longitudinal vein in the long cell between chords and mirror, and more parallel branches of Sc (for comparison, see Fig. 3a–c); and from Ontogryllus vetus in the tegmina not shortened, with the oblique veins less numerous and less S-shaped (for comparison, see Fig. 3b–d).
Description. Male tegmen not shortened, with moderately short stridulatory apparatus and possibly long apical area; stridulatory vein almost not oblique; area between this vein, diagonal vein, and CuA weakly elongate; two oblique veins, longest of them S-shaped, shortest one almost straight; chords weakly curved; mirror weakly elongate, slightly longer than its width, with weakly distinct dividing vein in middle part; long cell between lateral chord and mirror with additional longitudinal vein; branches of Sc weakly parallel. Colouration of these tegmina slightly darkened, but with darker regions of chords and diagonal vein. Length of imprint 4.8mm; hypothetical length of tegmina 5–5.5mm.
Ontogryllus vetus Cockerell, Reference Cockerell1921
1921 Gryllus vetus Cockerell, p. 474, fig. 40.
1939 Gryllus vetus: Zeuner, pp. 203, fig. 2, pl. 58, fig. 5.
Holotype. NHMUK I. 8889/In. 24328 (Brodie/Hooley collections); Insect Limestone, NW Isle of Wight; part and counterpart of incomplete male tegmen.
Diagnosis. This species differs from Ontogryllus rossi and O. oligocenus in the shorter stridulatory apparatus (especially mirror), strongly shortened apical area, and more curved chords; from the first species, it is probably distinguished also by the more parallel branches of Sc, and from the second one by the more numerous oblique veins and absence of additional vein in long cell between chords and mirror (see Fig. 3a–d).
Description. Male tegmen distinctly shortened, with short stridulatory apparatus and strongly shortened apical area; stridulatory vein distinctly oblique; area between this vein, diagonal vein, mirror, and CuA moderately elongate; four oblique veins, two longest of them S-shaped, two shortest ones almost straight; chords distinctly curved; mirror transverse, somewhat shorter than its width, with well-distinct dividing vein in distal part; long cell between lateral chord and mirror without additional vein; branches of Sc weakly parallel. Colouration of this tegmen rather dark, but with light proximal part (excepting lateral area), cell between two lateral chords, region of oblique veins, and area between chords and diagonal vein. Length of imprint 5.3mm; hypothetical length of tegmina 5.5–5.7mm.
Gryllidae incertae sedis
Remarks. The fossils of Gryllidae considered in this section may be divided into three parts: (1) well-preserved imprints that probably belong to the group of subfamilies related to Phalangopsinae; (2) well-preserved imprints that belong to an unknown group of subfamilies; and (3) poorly preserved imprints that may be determined only as belonging to Gryllidae.
First group (probable representatives of the group of subfamilies related to Phalangopsinae). This group of subfamilies was proposed for four Recent subfamilies: Phalangopsinae, Cacoplistinae (=Cachoplistinae, unjustified emendation), Phaloriinae, and Pteroplistinae (Gorochov Reference Gorochov2001; group ‘Phalangopsidae' = Phalangopsinae subfamily group). It is characterised by the presence of two or more dividing veins in the tegminal mirror of the male. This character is probably plesiomorphic, but its presence over a very long time (from Cretaceous to recent time) in the majority of representatives of this group may be considered a certain kind of synapomorphy – the strong fixation of this character in a general ancestor of this group. All other subfamilies of Gryllidae (excepting Gryllomiminae, probably related to Gryllinae) usually have a single dividing vein in the mirror of the male, if this mirror is well developed. However, sometimes their mirror is without any dividing veins, or some additional dividing veins may appear as a result of the partial reduction of the mirror or in rare aberrant specimens.
In this connection, the male tegmina of fossil Gryllidae with two dividing veins in the mirror (Fig. 4a–c) may be included in this group of subfamilies. There is only a problem with the Gryllomiminae. This subfamily, including a single genus, is characterised by the presence of two approximate dividing veins (Fig. 4i), and its mirror is very similar to that of Pteroplistinae (Fig. 4h). Moreover, in extinct genera of Gryllomiminae, the mirror might be similar to that of the Palaeocene/Eocene species pictured in Figure 4a, and the venation of the latter species is also similar to the hypothetical venation of the ancestor of the Phalangopsinae subfamily group. In Recent Phaloriinae and Cacoplistinae, the dividing veins are more or less distant from each other (Fig. 4f, g); in Recent Pteroplistinae, they are more approximate (Fig. 4h); and in Recent Phalangopsinae with well-developed wings, these veins are usually more numerous. Therefore, it is reasonable to determine this Palaeocene/Eocene specimen as a probable primitive representative of the Phalangopsinae subfamily group or as a possible primitive representative of the subfamily Gryllomiminae (or of an extinct subfamily ancestral to the Gryllinae subfamily group (Gorochov Reference Gorochov1995; group ‘Gryllidae') consisting of the following Recent subfamilies: Gryllomiminae, Itarinae, Gryllomorphinae, and Gryllinae).
Gryllidae. (A) Proeneopterotrypus danicus (Rust, Reference Rust1999), male tegmen, holotype (from Rust Reference Rust1999), Palaeocene/Eocene, Denmark. (B, C) Eneopterotrypus chopardi Zeuner, Reference Zeuner1937, male tegmen, Isle of Wight: (B) holotype I. 8855/In. 26058; (C) specimen I. 9835. (D) Achetomorpha irregularis sp. nov., female tegmen, holotype I. 9335, same locality. (E) Electrogryllus septentrionalis (Chopard, Reference Chopard1936), male tegmen, holotype (from Chopard Reference Chopard1936), Upper Eocene, Baltic amber. (F–I) General view of male, recent (from Chopard Reference Chopard1969 and Gorochov Reference Gorochov1995): (F) Trellius sp.; (G) Cacoplistes sp.; (H) Pteroplistes sp.; (I) Gryllomimus sp.
Genus Proeneopterotrypus gen. nov.
Type species. Pteroplistes danicus Rust, Reference Rust1999 (Palaeocene/Eocene of Fur, Denmark).
Other probable species. Only the type species is included in the genus.
Diagnosis. After Rust (Reference Rust1999, figs 1, 2). Male tegmina (Fig. 4a) with not very numerous, weakly parallel, and slightly curved oblique veins; diagonal vein rather short, much shorter than mirror; mirror large, with rounded distal part and two dividing veins arched and weakly approximate to each other; chords rather short and distinctly curved (lateral one angular); cross-veins between mirror and lateral chord comparatively short; distal part of mirror bordered by very long, narrow, and arched cell; apical area of tegmina long and provided with six or more branches; all these branches roundly curved not far from their bases, and their distal parts directed more or less backwards.
Comparison. The presence of the short cross-vein between the mirror and the lateral chord indicates similarity of this genus to Recent Phaloriinae, Cacoplistinae, Phalangopsinae, and even Gryllomiminae (Fig. 4f, g, i), but not to Recent Pteroplistinae, which has a distinctly longer cross-vein (Fig. 4h). From all known genera of Phaloriinae and Cacoplistinae, the new genus differs in the dividing veins of the mirror being more approximate or arched (not curved almost angularly); from those of Gryllomiminae and Pteroplistinae, in the less approximate dividing veins; and from those of Phalangopsinae, in the less numerous and/or arched dividing veins (or not reduced tegmina). There are also four fossil genera probably belonging to this group. From Electrogryllus Gorochov, Reference Gorochov1992a, Reference Gorochovb (Baltic amber, Eocene; Fig. 4e), the new genus is distinguished by the distinctly shorter diagonal vein and the area with oblique veins in male tegmina, as well as a less strongly curved proximal dividing vein in the mirror (distal dividing vein in Electrogryllus is probably not preserved); and from Eotrella Gorochov & Labandeira, Reference Gorochov and Labandeira2012 (Middle Eocene of USA), by the tegminal mirror, with the dividing veins transversally rounded, and with some oblique veins somewhat S-shaped (vs. these dividing veins strongly angularly curved, and all the oblique veins practically straight). Differences from Eozacla Gorochov & Labandeira, Reference Gorochov and Labandeira2012 (Middle Eocene of USA) are unclear, as the tegmina of the latter genus is not preserved. The differences from Eneopterotrypus are presented in the following section.
Genus Eneopterotrypus Zeuner, Reference Zeuner1937
Type species. Eneopterotrypus chopardi Zeuner, Reference Zeuner1937.
Other probable species. Only type species is included in this genus.
Diagnosis. Male tegmina (Fig. 4b, c) with numerous, parallel, slightly curved oblique veins; diagonal vein as in Proeneopterotrypus (much shorter than mirror); mirror large and with two dividing veins S-shaped and very close to each other; chords long and weakly curved (lateral one hardly arched); cross-vein between mirror and lateral chord long, much longer than in Proeneopterotrypus; distal part of mirror bordered by cell somewhat similar to that of Proeneopterotrypus; apical area of tegmen long and provided with six or more branches; one of these branches almost angularly curved not far from its base, but three distal branches with weakly curved proximal parts; distal parts of all these branches directed backwards.
Comparison. The genus is similar to Pteroplistinae in the long cross-vein between the mirror and lateral chord, two very close dividing veins of the mirror, and sloping chords in the male tegmina, but it differs from all known genera of this subfamily in the shorter diagonal vein, more numerous oblique veins, and longer cross-vein between the mirror and lateral chord. From Recent Phaloriinae, Cacoplistinae, Phalangopsinae, and Gryllomiminae with unreduced tegmina, as well as from fossil Electrogryllus and Proeneopterotrypus, this genus is distinguished by the distinctly longer aforementioned cross-vein, more sloping chords, and two very close dividing veins in the mirror.
Eneopterotrypus chopardi Zeuner, Reference Zeuner1937
(Figs 4b, c, 5a, b)
(A, B) Eneopterotrypus chopardi Zeuner, Reference Zeuner1937, I. 8855/In. 26058, holotype, male tegmen, part and counterpart. (C) Achetomorpha irregularis sp. nov., I. 9335, holotype, female tegmen. (D) Achetomorpha? schindewolfi (Zeuner, Reference Zeuner1937), In. 24774, holotype, lateral part of tegmen (sex unknown). (E–G) Proschistocerca oligocaenica Zeuner, Reference Zeuner1937: (E) In. 26028, non-type, hind femur of ?nymph (proximal half is imprint of outer side); (F) In. 24578, holotype, tegmen (sex unknown); (G) In. 17406, paratype, hind wing (sex unknown). Scale bars=5mm.
1937 Eneopterotrypus chopardi Zeuner, p. 156.
1939 Eneopterotrypus chopardi: Zeuner, pp. 218, 219, pl. 40, fig. 1, pl. 58, fig. 7.
Holotype. NHMUK I. 8855/In. 26058 (Figs 4b, 5a, b) (Brodie/Hooley collections); Insect Limestone, NW Isle of Wight; part and counterpart of fragment of male tegmen with stridulatory apparatus.
Other material. NHMUK I. 9835 (Brodie collection); small fragment of male tegmen (Fig. 4c).
Diagnosis. As for genus.
Description. Male tegmina with almost round mirror, very wide area between its dividing veins and distal edge of mirror, and very narrow distance between mirror and distal part of lateral chord. Colouration of these tegmina with darkened interspaces between veins and cross-veins in region of chords, apical and R–M areas, and light margins of these interspaces; other parts in preserved fragment of tegmina also light. Length of imprint of holotype 8mm; hypothetical length of tegmina 10–12mm.
Second group (well-preserved imprints belonging to unknown group of subfamilies). Imprints of the female wings in this family do not give any possibility for the determination of subfamily or even group of subfamilies. Moreover, it is usually impossible to identify these imprints with species described from male tegmina. For these reasons it is not possible to use the same classification for males and females of fossil crickets. The creation of formal taxa for such imprints is possibly something to work on in the future. However, some female tegmina have very characteristic venation, which allows one to distinguish them from those of the other gryllid genera with known females. For one such tegmen, a new genus is described in the following section.
Genus Achetomorpha gen. nov.
Type species. Achetomorpha irregularis sp. nov.
Other possible species. Gryllus schindewolfi Zeuner, Reference Zeuner1937 from the latest Eocene of the Isle of Wight, southern England.
Diagnosis. Dorsal part of tegmina possibly belonging to female (Fig. 4d) narrow, with very irregular venation in proximal part, and more or less parallel longitudinal veins in central and probably distal parts; lateral part of tegmen with rather numerous branches of Sc situated not parallel to R and M, and with more or less similar (regular) areas between Sc branches. Colouration of tegmina in female or in both sexes completely dark. Size not small (tegminal length more than 9mm).
Comparison. This genus probably does not belong to the subfamily Gryllinae, as the venation of the dorsal part of the female tegmina in all Recent species of this subfamily is more regular. The new genus is distinguished from Ontogryllus by having distinctly more numerous branches of tegminal Sc and by the areas between them (especially proximal parts of these areas) being more similar (regular) in shape; from Eogryllus, by having Sc branches not parallel to R and M; from Eotrella, by much less numerous Sc branches; and from Eneopterotrypus, by having completely dark tegminal colouration. Differences in the new genus from Pronemobius and Eozacla are unclear, as the structure and colouration of tegmina in the latter genera are unknown. From the majority of other groups of Gryllidae, the latter genus differs in its characteristic tegminal venation (with irregularity in dorsal part), size, or colouration.
Achetomorpha irregularis sp. nov.
Holotype. NHMUK I. 9335 (Brodie collection); Insect Limestone, NW Isle of Wight; incomplete tegmen possibly belonging to a female.
Diagnosis. The new species differs from Achetomorpha? schindewolfi in having more sparse branches of tegminal Sc and less numerous cross-veins between them, and from the other similar species, by the characters listed in the generic diagnosis.
Description. Tegmen with 5–6 more or less distinct longitudinal veins in proximal half of dorsal part, seven distinct branches of Sc, which are rather sparse in distal part, and a rather wide area between Sc and R. Colouration very dark in all preserved parts of tegmen. Length of imprint 9.5mm; hypothetical length of tegmina 10–11mm.
Achetomorpha? schindewolfi Zeuner, Reference Zeuner1937 comb. nov.
(Fig. 5d)
1937 Gryllus schindewolfi Zeuner, p. 156.
1939 Gryllus schindewolfi: Zeuner, p. 205, pl. 40, fig. 5, 60, fig. 1.
Holotype. NHMUK In. 24774 (Hooley collection)/CAMSM: X. 50140.92 (Smith collection); Insect Limestone, NW Isle of Wight; part and counterpart of incomplete lateral part of tegmen (sex unknown).
Diagnosis. This species differs from Achetomorpha irregularis in having less sparse branches of tegminal Sc and more numerous cross-veins between them. The differences between A.? schindewolfi and Eneopterotrypus chopardi are unclear, but the tegmen of the first species is possibly slightly larger (∼14 mm; vs. ∼11 mm).
Description. Lateral part of tegmina similar to that of Achetomorpha irregularis in the areas between Sc branches being more or less equal in shape; however, these branches are very numerous (∼12) and somewhat more parallel in relation to R and M than in A. irregularis; cross-veins between Sc branches are very numerous, and the majority of them are longitudinally oriented. Colouration of tegminal lateral part is dark with a hardly lighter distal area. Length of imprint 13.2mm; hypothetical length of tegmina about 14mm.
Third group (poorly preserved imprints that may be determined only as belonging to Gryllidae). There are also several imprints of Gryllidae that are not classified here. NHMUK In. 24778 (Hooley collection) (holotype of Pteronemobius? anglicus Zeuner, Reference Zeuner1937) and I. 8906/I. 8937 (Brodie collection), which are small fragments of the tegminal lateral part; In. 24773 (specimen determined as Gryllus cf. lineiceps Walker, Reference Walker1871 by Zeuner Reference Zeuner1939); In. 24779 and In. 24780 (Hooley collection) (specimens mentioned as Gryllidarum gen. et sp. indet. by Zeuner Reference Zeuner1939), which are fragments of the tegminal dorsal part with venation typical of the majority of Gryllidae; and I. 8847 (Brodie collection), Insect Limestone, NW Isle of Wight, which is an isolated hind femur.
Superfamily Tettigonioidea Kraus, 1902
Family Tettigoniidae Kraus, 1902
Remarks. An interesting paper about a new genus of Tettigoniidae, Archepseudophylla, was published by Nel et al. (Reference Nel and Ross2008). The find of an almost complete body of A. fossilis Nel et al., Reference Nel and Ross2008 from the Early Oligocene of France allowed these authors to clarify the taxonomic position of the enigmatic Poekilloptera? melanospila, which was described from a small tegminal fragment having characteristic spotted colouration (Cockerell Reference Cockerell1921). The latter species was described from the latest Eocene of the Isle of Wight and must be mentioned in this paper. However, I have not studied this imprint, as it was originally included in a Neotropical genus of Hemiptera, and the small size of this fragment did not allow me and other specialists to understand its systematic position until this interesting find in France. The authors of Archepseudophylla included Cockerell's species in this genus, redescribed and illustrated the holotype and an additional specimen of this species, and put this genus in the subfamily Pseudophyllinae of the family Tettigoniidae.
Accordingly, it is reasonable to discuss here only the question of the systematic position of Archepseudophylla. Its authors wrote that this genus looks like some representatives of the tribes Pterochrozini, Simoderini, and Pterophyllini. The latter tribe is included by me in the subfamily Pleminiinae, and the other ones are considered by me as separate subfamilies Pterochrozinae and Simoderinae (Gorochov Reference Gorochov1995). These subfamilies are well distinguished from each other, mainly by the structure of the tympanal region in the fore tibiae and of the ovipositor. However, the aforementioned authors did not give any description of the body parts other than the tegmina, although the fore legs of the paratypes of A. fossilis are well visible in the photographs published (Nel et al. Reference Nel and Ross2008, figs 4, 5). The tympana in the latter photograph are distinct, looking like dark oval spots, and seem opened (not slit-like). If this is correct, this genus may belong to Simoderinae, having now a relict distribution (Australia and Madagascar), whereas in Neotropical Pterochrozinae and mainly Neotropical Pleminiinae, the tympana are usually slit-like, i.e., with the tympanal membranes covered by sclerotised folds of the fore tibia.
Suborder Caelifera Ander, 1936
Superfamily Acridoidea MacLeay, 1821
Family Acrididae MacLeay, 1821
Remarks. The situation with the wings of this family is similar to that of the females of Gryllidae. The primitive venation of tegmina and hind wings is distributed in many subfamilies of Acrididae. The representatives of different subfamilies with such venation may not be separated from each other with the help of only venational characters. But the presence of imprints of hind femora, which, together with imprints of wings, may belong to the same species, allows the possibility to reduce the number of possible subfamilies. For example, the fossil hind femur pictured in Figure 6a differs from that of the majority of Recent subfamilies in the presence of oblique keel-like convexities on the outer surface (all well-jumping representatives of the latter subfamilies are characterised by a feather-like relief on this surface or traces of such relief; Fig. 6f–h). Only some Recent representatives of Pyrgomorphinae have a similar relief of this surface (Fig. 6e), but the venation of their hind wings is more modified than that of the fossil species pictured in Figure 6d. Moreover, the latter hind wings are more primitive than those of all other recent subfamilies: the space between the anterior branch of medial vein (MA) and costal edge in the middle part of these fossil wings is rather wide as it includes the areas between the anterior branch of radial vein (RA) and posterior branch of radial vein (RS) as well as between the proximal branches of RS (the bases of both RS and its proximal branch are situated near the bases of the MA branches), but in Recent Acrididae with more or less primitive venation, this space is narrower (costalisation?) as the base of RS and/or bases of its proximal branches are moved in a significantly more distal position than the bases of MA branches. So, if all these fossil imprints belong to the same species or genus, it is reasonable to consider that this taxon is an extinct primitive representative of Pyrgomorphinae or a member of an extinct subfamily of Acrididae. The latter hypothesis is also permissible as these imprints are probably the earliest reliable finds of the family Acrididae.
Acrididae. (A–D) Proschistocerca oligocaenica Zeuner, Reference Zeuner1937, Isle of Wight: (A) hind femur of ?nymph (proximal half is imprint of outer side), specimen In. 26028; (B) costal-proximal piece of tegmen, specimen In. 20533; (C) tegmen, holotype In. 24578; (D) hind wing, paratype In. 17407. (E–H) Relief of outer surface of hind femur, Recent: (E) Pyrgomorpha sp.; (F) Schistocerca sp.; (G) Rhomalea sp.; (H) Thrinchus sp. Abbreviations: 1A = first branch of anal vein; 2A = second branch of anal vein; C = costal vein; CuA = cubitus anterior; CuP = cubitus posterior; MA = anterior branch of medial vein; RA = anterior branch of radial vein; RS = radius sector, posterial branch of radial vein; Sc = subcostal vein.
Genus Proschistocerca Zeuner, Reference Zeuner1937
Type species. Proschistocerca oligocaenica Zeuner, Reference Zeuner1937.
Other probable species. Only type species is included in the genus.
Diagnosis. Size and structure of tegmina (Fig. 6b, c) similar to those of Recent genus Schistocerca Stål (Catantopinae), but distal part of tegminal costal area becoming extremely narrow somewhat behind base of second branch of RS (in Schistocerca, somewhat before base of first branch of RS). Hind wings and hind femora, probably belonging to this genus, very characteristic: first ones with bases of both RS and its proximal branch situated near bases of MA branches, and two branches of MA not fused with R (Fig. 6d); second ones (femora) rather wide in proximal half (evidently well adapted for jumping) and with oblique and slightly S-shaped keel-like convexities on outer surface (Fig. 6a).
Comparison. This genus differs from all other Acrididae in the combination of characters of hind wings and hind femora listed in the latter diagnosis. Differences in the structure of tegmina between this genus and other large Acrididae with well-developed wings and primitive venation are not clear.
Proschistocerca oligocaenica Zeuner, Reference Zeuner1937
1937 Proschistocerca oligocaenica Zeuner, p. 158.
1941 Proschistocerca oligocaenica: Zeuner, p. 518.
Holotype. NHMUK In. 24578 (Figs 5f, 6c) (Hooley collection); Insect Limestone, NW Isle of Wight; imprint of incomplete tegmen (sex unknown).
Paratypes. NHMUK I. 10283/In. 17406 (Fig. 5g) (part and counterpart), In. 17407 (Fig. 6d), In. 17339, In. 24587, In. 25728, In. 20527, In. 24782, In. 17342, I. 8799, and I. 8633; Insect Limestone, NW Isle of Wight; fragments of hind wings (first two specimens) and of tegmina (all others) probably belonging to this species.
Other material. NHMUK In. 17346, In. 24781, In. 24777, In. 17350, In. 20533 (Fig. 6b), I. 8877, In. 24783, and In. 26028 (Figs 5e, 6a) (Hooley collection); Insect Limestone, NW Isle of Wight; small fragments of tegmina (first six specimens) and of hind wings with body (seventh), and incomplete hind femur possibly belonging to nymph (eighth).
Diagnosis. As in the genus.
Description. Tegmina with not less than seven branches of RS, and base of CuA1 situated hardly behind bases of MA branches. Hind wings and hind femora, possibly belonging to this species, characterised by the following characters: first ones with bases of proximal branches of RS situated not far from each other, and second ones (in ?nymph) 3.5 times as long as wide. Colouration of tegmina rather light with several large, hardly visible spots. Length of imprint of holotype 48mm; hypothetical length of wings 65–70mm.
3. Discussion
The discovery of the earliest fossils dependably belonging to Acrididae in the Insect Limestone implies that the fossil fauna of these deposits is somewhat younger than faunas of richer Eocene deposits such as Green River in North America (Middle Eocene; Gorochov & Labandeira Reference Gorochov2012) and Baltic Amber in Europe (Upper Eocene; Gorochov Reference Gorochov2010a–d). On the other hand, the above-mentioned fossils from the Insect Limestone show a rather primitive condition of wings and hind femora, and it is very probable that these grasshoppers are older than Middle Oligocene Acrididae from Aix-en-Provence in France and Oligocene/Miocene Acrididae from Izarra outcrop in Spain, which evidently have a feather-like relief on the outer surface of the hind femora (Zeuner Reference Zeuner1944; Arillo & Ortuno Reference Arillo and Ortuno1997). The structure of Pterotriamescaptor is also more primitive than in Gryllotalpinae from the Miocene of Germany (Zeuner Reference Zeuner1939): Neocurtilla schmidtgeni Zeuner, Reference Zeuner1937 and Gryllotalpa miocaenica Zeuner, Reference Zeuner1931. The latter mole crickets have fore legs with four digging teeth and belong to the youngest group of Gryllotalpinae. Therefore, the data on Orthoptera are consistent with the dating of the Insect Bed as latest Eocene or early Oligocene (see Ross & Self Reference Ross and Self2014).
The presence of a rather rich fauna of crickets infers that the climate of this time in southern England was much warmer than it is now: subtropical or almost tropical. The unusual abundance of imprints of the same species of Gryllotalpidae may be evidence for the wide distribution of low open territories with herbs and meadows near a large water reservoir (possibly coastal). This hypothesis is also supported by the rather numerous finds of Gryllinae and Acrididae (at present, these groups are typical of meadows near water). Rare finds of representatives of the group of subfamilies related to Phalangopsinae (Eneopterotrypus) support the hypothesis of a tropical–subtropical climate (this group of crickets is almost completely tropical now). But its Recent representatives are mainly dendrophilous and adapted to life in a forest. These data suggest that there were some forests around the banks with meadows, but these forests were situated rather far from the water reservoir, and poor-flying insects, characteristic of forests, could only rarely reach the water surface. The absence of numerous tree leaves and forest insects transferred to this water reservoir by streams can be explained by the presence of dense vegetation consisting of high herbs (Typha L.) along the banks, which may effectively have blocked the drift of leaves and insects by water.
4. Acknowledgements
I wish to thank Dr A. J. Ross (National Museum of Scotland) and Dr A. G. Kirejtshuk (Zoological Institute, St Petersburg) for their great help during my work on this theme. I also thank Mr Phil Crabb (NHMUK) for the photography of imprints, and Mr Bill Crighton (National Museum of Scotland volunteer) for checking the English. This research has been supported by INTAS (grant no. 03-51-4367).
