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A Primitive Earwig in Cretaceous Amber From Myanmar (Dermaptera: Pygidicranidae)

Posted on: Thursday, 9 September 2004, 06:00 CDT

INTRODUCTION

THE EARWIGS comprise an order (Dermaptera) of polyneopterous insects allied to the Dictyoptera and Grylloblattodea (Giles, 1963; Hennig, 1969; Boudreaux, 1979). Dermaptera have at times in the past been allied with the Phasmida, Embiodea, and Plecoptera (e.g., Blacklith and Blacklith, 1968; Jamieson, 1987) but these relationships do not seem to be congruent with the overall body of data for Polyneoptera phylogeny and the monophyly of a Grylloblattodea + Dermaptera + Dictyoptera clade currently has perhaps more evidence than competing hypotheses (e.g., Carpenter and Wheeler, 1999; Wheeler et al., 2001). However, revised morphological and paleontological evidence may suggest that Dermaptera are basal within Anartioptera-a clade consisting of Plecopterida, Orthopterida, Grylloblattodea, and Mantophasmatodea, as well as the earwigs (e.g., Engel and Grimaldi, 2004). The earwigs are certainly autapomorphic, like many polyneopterous insects, and their definitive placement remains elusive.

The order comprises approximately 1,900 species of distinctive insects with relatively flattened bodies, tegminous forewings without venation, an enlarged subgenital plate (sternum 7) in females, trimerous tarsi, and elongate forcipate cerci. Species are fairly mobile and generally prefer damp, concealed habitats such as under bark or stones. The order has a cosmopolitan distribution but is principally found in tropical or warm temperate regions. Two unusual groups, the traditional suborders "Arixeniina" and "Hemimerina," are apomorphic lineages that are highly specialized for their epizoic lifestyles and at least the latter group has at times been excluded from the order (e.g., Popham, 1973) owing to its peculiar, paedomorphic differences from other earwigs. The five species of arixeniids live in the roosts of molossid bats on Southeast Asia islands (Nakata and Maa, 1974), while the 10 species of hemimerids live under the fur of rodents in Africa (Rehn and Rehn, 1935). The arixeniids have been demonstrated to be derived from the forficuloids, perhaps even within the spongiphorids (Popham, 1985) and the suborder is therefore incorporated into the superfamily Forficuloidea and treated as a family therein (see Table 1). Similarly, increasing evidence suggests that the hemimerids are merely autapomorphic, perhaps derived from the Apachyidae (Klass, 2001: Table 1). Despite concerted efforts on the part of systematists, a consistent phylogenetic classification of the order has remained elusive. The current hierarchical classification of the order is outlined in Table 1.

Although the earwigs are clearly of ancient origin, their fossil record is particularly scattered and the best understood paleofauna for the order (albeit also deserving of revision) is the relatively recent material from the middle Eocene Baltic amber (Burr, 1911). Herein we describe a new subfamily, genus, and species of pygidicranid earwig preserved in Middle Cretaceous amber from Myanmar. The "Burmese amber" mines have been shrouded in mystery for many years and until recently the only available material from these deposits has been a historical collection maintained in the Natural History Museum, London (NHM) (Rasnitsyn and Ross, 2000). New material from Myanmar, however, has revived these "lost" mines and thousands of new specimens are now available for study (Grimaldi et al., 2002). Material in the NHM had previously revealed a single, poorly preserved earwig and this specimen was tentatively described by Cockerell in the modern genus Labidura (Cockerell, 1920). Examination of Cockerell's species reveals that it is certainly misplaced as to genus and is reassigned herein. Its definitive placement as to family remains uncertain owing to the poor state of preservation but is tentatively retained in Labiduridae pending the future discovery of more complete material.

MATERIAL AND METHODS

The piece of amber that entombs the newly discovered earwig originates from the amber mines of the Hukawng valley, northern Myanmar. The dating of this amber has been considerably problematic and for a long time it was considered to be of relatively recent origin; e.g., Miocene (Noetling, 1892). Despite the foresighted estimate of a Late Cretaceous age for Burmese amber by Cockerell (1917: based solely on the entrapped arthropod remains), work on the dating of the amber-bearing strata suggested an Eocene age (Stuart, 1923; Chhibber, 1934) and Cockerell later reversed his position (Cockerell, 1922). Modern detailed studies on both the organismal inclusions as well as the geology of Myanmar, however, demonstrate that the amber is truly Cretaceous in origin (Lower Cenomanian, but perhaps even uppermost Albian: e.g., Cruickshank and Ko, 2003) and has perhaps been redeposited in Tertiary strata at least once during its long geological history (Zherikhin and Ross, 2000; Grimaldi et al., 2002). The paleoenvironment under which Burmese amber was produced was perhaps the warmest tropical climate of any Cretaceous amber forest (Grimaldi et al., 2002; Engel and Grimaldi, 2002), with a rich biotic diversity (e.g., Rasnitsyn and Ross, 2000; Grimaldi et al., 2002). Arthropods are particularly abundant in the deposit, providing a wealth of information not only for descriptive and phylogenetic systematics but also for paleoenvironmental and paleoecological studies.

The amber piece with the newly discovered earwig was first encased in epoxy following the vacuum-embedding procedure outlined by Nascimbene and Silverstein (2000) prior to trimming and polishing. Measurements were made with an ocular micrometer on an Olympus SZX-12 stereomicroscope.

SYSTEMATIC PALAEONTOLOGY

Order DERMAPTERA de Geer, 1773

Suborder NEODERMAPTERA Engel, 2003

Family PYGIDICRANIDAE Verhoeff, 1902

Subfamily BURMAPYGIINAE new subfamily

Type genus.-Burmapygia new genus.

Diagnosis.-Small earwigs (ca. 6 mm); somewhat dorsoventrally compressed. Compound eyes well developed, ovoid, length greater than postorbital length of head; ocelli absent. Scape short, distinctly shorter than length of compound eye; flagellar segments distinctly longer than wide, tapering toward their base. Posterior border of head slightly concave. Pronotum broad, lateral and posterior margins rounded. Tegmina present, not greatly shortened or reduced to lateral flaps, instead elongate, covering anterior portions of abdomen (Fig. 2); tegmina without venation, symmetrical. Body, particularly tegmina, densely setose; setae not bristlelike. Femora compressed (particularly profemora), with strong ventral, lamellate keels; tarsi trimerous; basal tarsomere elongate, nearly as long as combined length of second and third tarsomeres; second tarsomere greatly shortened, not extending distally beneath third tarsomere; arolium present, large; pretarsal ungues (=claws) slender, simple. Lateral margins of abdomen roughly parallel; ultimate abdominal tergum simple, wider than long (Blandicinae have large tubercles overlying base of forceps). Cereal forceps in females relatively slender, about twice length of ultimate tergum, well separated at base (separated by at least basal width of individual forcep), with a few minute teeth along inner margin (Fig. 2). Female valvulae present, elongate, discernible extending beyond apex of subgenital plate (Fig. 2).

TABLE 1-Hierarchical classification of living and fossil earwigs (updated from Engel, 2003). Subordinal names such as "Forficulina" are not employed here owing to confusion with the standard suffices (i.e., -ina) for subtribal names in zoological nomenclature.

Discussion.-Like other primitive earwigs, this group retains the distinctly compressed and ventrally keeled femorae and like many pygidicranids the female valvulae extend beyond the apex of the subgenital plate and the bases of the cerci. However, the tenth tergum is not elongate (longer than wide in living groups while wider than long in Burmapygiinae). In some Echinosomatinae the distal tergum can be nearly wider than long but in this subfamily the setae form distinct bristles dorsally, the femora are not keeled, and the flagellar segments are not elongate.

Genus BURMAPYGIA new genus

Type species.-Burmapygia resinata new species.

Diagnosis.-As for the subfamily (see above).

Etymology.-The new genus-group name is a combination of Burma, former name of the country from which the material originates, and pygia, a common suffix for pygidicranid genera. The name is feminine.

Discussion.-Burmapygia can be readily separated from the only other Burmese amber earwig, Myrrholabia n. gen., by the characters discussed under that genus (see below).

BURMAPYGIA RESINATA new species

Figures 1, 2

Diagnosis.-As for the genus (see above).

Description.-FEMALE: Total body length 5.80 mm (including forceps); tegmen length 1.74 mm; cercus length 0.73 mm. Head quadrate, about as long as wide; compound eyes large, prominent and exophthalmic, longer than postorbital length of head; posterior margin of head gently concave medially (Pig. 1), as broad as pronotum; postfrontal and coronal sutures absent; scape slightly shorter than distance between antennal bases; pedicel quadrate, distinctly shorter than flagellomeres; flagellomeres longer than wide, bases slig\htly tapering (Fig. 2). Pronotum broadly rounded, gently and slightly convex along anterior margin, lateral margins distinctly rounded and convex, posterior margin broadly rounded and convex; median longitudinal furrow absent. Tegmina together barely wider than pronotum, thrice pronotal length (Fig. 2), lateral margins approximately parallel in anterior half, slightly convergent in posterior half; apices gently rounded, somewhat truncate; overlapping abdominal base. Abdominal terga slightly wider than long; distalmost tergum slightly wider than long, without tubercles (longitudinally cracked on right side in holotype: Fig. 2), apical margins of terga straight; sterna slightly wider than long, apical margins straight except apical margins of distal three segments concave; penultimate sternum deeply and broadly concave, about as long as wide; subgenital plate (ultimate sternum) with apicolateral processes, medially extended with short, broad lobes on either side of midpoint, lobes separated medially by broad concave margin; setae along apical margins of distal sterna longer than more sparsely distributed body setae, single particularly elongate seta arising from each lobe of subgenital plate (Fig. 2). Cereal forceps relatively simple and of equal length, bases separated by slightly more than basal width of individual forcep, relatively straight except gently curved near apex, with a few minute teeth along inner margin (Figs. 1, 2). Valvulae elongate, pulled apart in preservation, simple (Fig. 2). Body slightly dorsoventrally flattened (Fig. 1). Integument dark brown throughout, nearly black on abdominal segments, imbricate, not punctate. Body covered by short to moderate-length setae, setae simple and fine, setae more dense on thorax than on head and abdomen, particularly dense on tegmina (Fig. 2). MALE: Unknown.

FIGURE 1-Photomicrograph of holotype female of Burmapygia resinata n. sp. (AMNH Bu-274), total specimen length = 5.80 mm (including forceps).

FIGURE 2-Burmapygia resinata n. sp. (AMNH Bu-274), dorsal habitus as preserved (total length of specimen 5.80 mm) and ventral aspect of terminal abdominal segments, apices of valvulae, and cerci (scale bar for magnified, ventral view = 0.10 mm).

Etymology.-The specific epithet is a reference to the occurrence of this species in a fossil plant resin (Latin, resina).

Type.-Holotype female, Burmese amber; AMNH Bu-274 (Fig. 1); deposited in the amber collection of the Division of Invertebrate Zoology, American Museum of Natural History, New York.

Occurrence.-Amber; Myanmar (Burma), Middle-Upper Cretaceous (Albian-Cenomanian), Kachin: Tanai Village (on Ledo Rd. 105 km northwest of Myitkyna).

Family LABIDURIDAE Verhoeff, 1902

Genus MYRRHOLABIA new genus

Type species.-Labidura electrina Cockerell, 1920.

Included species.-Myrrholabia electrina (Cockerell, 1920), new combination. This species has been previously briefly discussed by Cockerell (1920), Zherikhin (1978), Keilbach (1982), Spahr (1992), Poinar (1992), and Ross and York (2000), all under the generic name Labidura.

Diagnosis.-Small earwigs (ca. 5 mm); body somewhat dorsoventrally compressed, rather slender. Compound eyes well developed, ovoid, length less than postorbital length of head; ocelli absent. Scape moderate in length, longer than distance between antennal bases; flagellar segments quadrate except first flagellomere longer than wide. Posterior border of head rounded, not concave. Anterior margin of pronotum narrower than posterior margin of head. Body (where preserved) sparsely setose, setae simple. Femora apparently not compressed or keeled, not elongate (typically elongate in Labidurinae); tarsi trimerous; basal tarsomere elongate; second tarsomere greatly shortened, slightly extending distally beneath third tarsomere, only minutely widened relative to base of third tarsomere; arolium absent; pretarsal ungues (=claws) slender, simple. Preserved portion of abdomen with parallel lateral margins; ultimate abdominal tergutn wider than long, without tubercles. Cereal forceps slender, symmetrical, straight over their entire length, subcontiguous at their base, with minute denticles on inner margins. Pygidium not evident.

Etymology.-The new genus-group name is a combination of the Greek words myrrha (meaning "an aromatic gum-resin") and labis (meaning "forceps"). The name is feminine.

Discussion.-Myrrholabia generally agrees with Labiduridae and is therefore tentatively retained therein as its oldest representative. Myrrholabia can be separated from Burmapygia n. gen. by the following traits: absence of a distinct arolium (present in B. resinata n. sp.), sparsely setose body (densely setose in B. resinata), subcontiguous cereal forceps (well separated cereal forceps in B. resinata), apices of forceps straight (apices gently curved in B. resinata), second tarsomere slightly extending beneath the third tarsomere (simple in B. resinata), and femora where evident not compressed or keeled (distinctly compressed and ventrally keeled in B. resinata).

DISCUSSION

The earliest, definitive earwigs are those species of the extinct suborder Archidermaptera well known from the Jurassic and lowermost Cretaceous of Europe and Central Asia (Bey-Bienko, 1936; Whalley, 1985; Carpenter, 1992; Zhang, 1994; Coram et al., 1995) and recently recognized in the Upper Triassic as well (e.g., Jarzembowski, 1999; Rasnitsyn and Quicke, 2002; A. J. Ross, personal commun.; Engel, personal observation). These species are notable for the plesiomorphic retention of venation (albeit already quite reduced) in their tegmina, segmented cerci, and pentamerous tarsi. Primitive species of "true" Dermaptera [i.e., suborder Neodermaptera (=Forficulina, a name abandoned owing to confusion with the standardized suffix for subtribes in the family group)] are known from as far back as the Upper Jurassic and lowermost Cretaceous (Carpenter, 1992; Engel et al., 2002; Rasnitsyn and Quicke, 2002). The origin and radiation of the order was perhaps sometime in the late Permian or early Triassic when other orders of the cohort Polyneoptera appear to have differentiated from even more ancient and certainly paraphyletic (if not polyphyletic!) groups such as Protorthoptera. Although Kukalov-Peck and Peck (1993) considered the Protelytroptera to be stem-group dermapterans [following an assertion made by Boudreaux (1979)] based on the presence of tegminous forewings and an expanded anal fan in the hind wings, this extinct "order" includes entirely unrelated groups [e.g., Umenocoleidae has subsequently proven to be a lineage of roaches (e.g., Jarzembowski and Ross, 1993; Vrsansky, 1999; Engel and Grimaldi, 2000)]. While some "protelytropterans" may have affinities with Dermaptera (e.g., Protelytridae), a clear phylogenetic hypothesis is presently lacking and the origin of the order remains elusive.

Certainly by the Upper Jurassic the order was well established and Neodermaptera likely also occurred, although definitive evidence of their appearance by this time is lacking (putative, undescribed Neodermaptera figured by Rasnitsyn and Quicke, 2002, fig. 425). By the Lower Cretaceous Neodermaptera are known from a variety of deposits [e.g., Brazil (Popham, 1990; Martins-Neto, 1990), Korea (Engel et al., 2002), China (Zhang, 1994, 2002: misdated as Jurassic), Lebanon (undescribed amber nymph in AMNH, personal observation)]. In an odd twist of fate, the Lower Cretaceous Caririderma pilosa Martins-Neto (1990) from the now famous Santana Formation in northeastern Brazil has proved to be a staphylinid beetle (Grimaldi and Blagoderov, in press) while the "odonate nymph" Cordulagomphus santanensis Carle and Wighton (1990), from the same deposits, is a true dermapteran (Bechly, 1998). In fact, C. santanensis belongs to the genus Caririlabia Martins-Neto, 1990 where it is likely synonymous with the type species, Caririlabia brandaoi Martins-Neto (1990: if proven to be truly a synonym, then sanlanensis has priority by date over brandaoi; for now is it retained as a separate species, Caririlabia santanensis new combination, pending study of the C. brandaoi holotype).

Within the Neodermaptera, the Pygidicranoidea belongs to the most primitive group of living earwigs and is generally segregated along with Karschielloidea into its own infraorder, Protodermaptera. The pygidicranoids lack the "forficuloid-neck" seen in the Epidermaptera, whereby the cervical sclerites are enlarged and the posterior sclerite (which is no longer transverse) is much greater in size than the anterior sclerite (Popham, 1959, 1985). As noted by Haas (1995), the superfamily is perhaps paraphyletie and should be split apart into multiple groups following more comprehensive eladistic studies. The fossil record of pygidicranoids is sparse and scattered. Several genera preserved as compression fossils from the Jurassic of Kazakhstan were treated by Vishniakova (1980, 1985, 1986) as pygidicranids, and retained as such by Carpenter (1992). Willmann (1990), however, noted that all of these fossils lack synapomorphic traits with Pygidicranoidea and Haas and Kukalov-Peck (2001) removed them to the Archidermaptera. More recently Zhang (1994) described a putative genus of Echinosomatinae from the earliest Cretaceous of China. Like Vishniakova's material, this fossil does not preserve synapomorphic features that would place it definitively in the Pygidicranidae, let alone the Echinosomatinae. These same arguments apply to the Upper Jurassic fossil, Mesoforficula sinkianensis Ping, 1935, originally assigned to Forficulidae (Ping, 1935). Most of these fossils are actually eodermapterans.

The only other fossil record for the Pygidicranoidea is a nymph tentatively assigned to Pygidicrania Audinet-Serville, 1831 in Baltic amber (Burr, 1911) which is of middle Eocene age (ca. 45 Ma) (dating reviewed in Engel, 2001). Certainly, the Upper Jurassic and Lower Cretaceous Asiatic fossils figu\red by Rasnitsyn and Quicke (2002) and Engel et al. (2002) as possible pygidicranoids are tantalizing and may eventually throw the definitive age of this group back by tens of millions of years. For now, however, Burmapygia n. gen. is not only the oldest member of the Pygidicranoidea in amber but currently stands as the oldest, definitive fossil for its superfamily.

ACKNOWLEDGMENTS

We are grateful to A. J. Ross for loan of Cockerell's holotype, and to A. J. Ross, R. Willmann, and G. C. Baird for thoughtful reviews of the manuscript. Portions of this work were made possible by a Kansas Technology Enterprise Corporation/Kansas NSF EPSCoR grant (KAN29503 to MSE) while NSF grant DBI-9987372 (to DG) generously supported work on fossil insects in the AMNH. This is contribution Nr. 3346 of the Division of Entomology, Natural History Museum and Biodiversity Research Center, University of Kansas.

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ACCEPTED 11 JANUARY 2004

MICHAEL S. ENGEL1 AND DAVID A. GRIMALDI2

1 Division of Entomology, Natural History Museum, and Department of Ecology and Evolutionary Biology, 1460 Jayhawk Boulevard, Snow Hall, University of Kansas, Lawrence 66045-7523, and 2 Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York 10024-5192

Copyright Paleontological Society Sep 2004

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