Introduction

The family Mymaridae (Hymenoptera: Chalcidoidea) is represented by 103 genera and about 1400 nominal extant species in all terrestrial habitats and a few fresh water habitats (Huber 1986, Noyes 2010). Fossil Mymaridae have been described from amber inclusions from various localities, including five extinct genera and species from Cretaceous amber (Yoshimoto 1975, Huber and Poinar 2011) and eight genera (seven still extant, one extinct) from Tertiary amber, mostly from the Samland Peninsula, Kaliningrad district, Russia, and Chiapas, Mexico. Probably because of their small size, no compression fossils of Mymaridae have been discovered until now. Here we describe the first ones, representing three genera and six species. These and the Tertiary amber fossil genera are discussed and compared with extant genera.

Methods

A total of 17 specimens of fossil Mymaridae, all females, were collected in 2009 and 2010 at six sites (e.g. 113°42.173'W, 48°23.476'N) along the Middle Fork of the Flathead River in northwestern Montana between Paola and Coal Creeks in accordance with USFS Authorization HUN281. Fossil specimens were collected from the middle sequence of the Coal Creek member of the Kishenehn Formation, which has been estimated to be 46.2 +/- 0.4 myo (Lutetian) by 40Ar/39Ar analysis and 43.5 +/- 4.9 myo by fission-track analysis (Constenius 1996). Although the paper/oil shales of the middle sequence are thin (<1 mm to several mm), they were often easily split into even thinner pieces so as to expose unweathered surfaces on which the fossil insects reside.

The compression fossils were immersed in 95% ethanol for examination and photography. For figures 1–10 and 13–18, specimens were photographed using a Zeiss AxioCam MRc5 digital CCD camera mounted on a Zeiss Discovery V20 microscope and Zeiss AxioVision EDF software in Ottawa, ON. Measurements were generated using Zeiss AxioVision software. For figures 11 and 12, specimens were photographed using an Olympus SZX12 microscope, DP-25 camera and DPM imaging software in Washington, DC, and measurements were taken with the DP2-BSW software. All measurements are in micrometers (mm). If a measurement could not be made (e.g., scape length) it is represented by a dash.

Abbreviations used: flx = funicle segment x. Measurements were taken as accurately as possible but given that the beginning and end points of a structure were not always clear or were hidden, the measurements may not be accurate. Appendage measurements are the most accurate except the wing bases cannot be clearly determined. Consequently, wing lengths were taken from the visible edge of the mesosoma and therefore they and length/width ratios are slightly smaller than they should be.

Fossils examined are in the following institutions:

AMNH American Museum of Natural History.

IPMGö Institute and Museum für Paläontologie, Georg-August University, Göttingen, Germany.

NMNH National Museum of Natural History, Washington, D.C.

OSU Oregon State University, G. Poinar, Jr. collection, Corvallis, Oregon, USA.

SVT S. V. Triapitsyn private collection, California, USA.

UCRC University of California, Riverside, California, USA.

ZMUC Zoological Museum, University of Copenhagen, Denmark.

The compression fossils are housed in NMNH. Baltic amber fossils were examined from the remaining institutions.

Systematics Discussion

Compression fossils in sedimentary rock are less informative morphologically than amber fossils. Fine details such as body setation and sculpture are not visible (wing setae may be visible), and body flattening during fossilization usually results in distortion. Measurements of body parts may therefore not be accurate though they can sometimes be more easily measured because they are flat. Appendages are the best source of useful diagnostic characters because some (legs, antenna) are less and some (wings) are not distorted. Because of the preservation method, compression fossils cannot be easily compared to amber fossils or extant genera and species. Nonetheless, they can be diagnosed moderately well and differentiated with reasonable certainty from each other and from other extinct and extant genera.

Among the 17 Kishenehn mymarids found, Eoeustochus is the most common, followed by Gonatocerus and Eoanaphes. It is surprising that no males were found; in extant genera males are less common than females but they are rarely almost completely absent. The compression fossils are of middle Eocene (Lutetian) age, as indicated above, and so are most Baltic amber fossils (Weitschat and Wichard 2010), i.e., about 41–49 my old. Incidentally, Meunier (1901), quoting A. Jentzsch, had noted that their age was lower Eocene, not lower Oligocene (yet the latter was stated again by Meunier 1909 and Doutt 1973). The Eocene had a warm humid climate, significantly warmer than the present (Zachos et al. 2001), allowing for development of subtropical and tropical rain forests to which the Baltic amber forests belong (Weitschat and Wichard 2010). Baltic amber forests were apparently rich in lentic water, flood plains, ponds, lakes with littoral habitats and temporary micro waters. Aquatic insects make up 25% of Baltic amber inclusions (Weitschat and Wichard 2010). The Middle sequence of the Coal Creek member of the Kishenehn Formation consisted of lacustrine sediments of a large permanent and shallow lake and associated lakeside and/or marsh, and has a high proportion of aquatic insect fossils (e.g., > 60% of all the fossil insects are Corixidae and Chironomidae). Because the climate and habitat of both Baltic and Kishenehn fossils was apparently similar it is not surprising that the genera of Mymaridae found in each are similar.

Meunier (1901) described most Baltic amber fossils of Mymaridae. Doutt (1973) described a few species from Mexican amber (Simojovel, Chiapas), dated as 15–20 my, and Thuróczy (1983) described one species. There is no indication from morphology that any of these or the mymarid compression fossils form a link between Cretaceous and Tertiary/Quaternary genera and species. Instead, they add support to the analysis by Rasnitsyn and Kulicka (1990) that showed Hymenoptera assemblages from Baltic amber seem to be more similar to the extant fauna than to the Late Cretaceous fauna.

One clear morphological tendency from Cretaceous to Tertiary and Quaternary (present) species can be seen — a reduction in the number of funicle or claval segments in females. A 3-segmented clava occurs in three of five (60%) Cretaceous genera, four of ten (40%) previously reported Tertiary genera from amber and shale (reported above), and 13 of 103 (about 8%) currently recognized extant genera. An 8-segmented funicle occurs in three of five (60%) Cretaceous genera, one of the 10 (10%) previously reported Tertiary genera, and five of the 103 (about 5%) extant genera. Except for Gonatocerus, which has eight funicle segments but an entire clava, all other Tertiary fossils have six (rarely fewer) funicle segments in females.

Spahr (1987) catalogued the literature and listed 11 mymarid genera from amber, three of them Cretaceous and eight Tertiary. He followed previous authors by including genera and species now correctly classified in Mymarommatidae (Gibson et al. 2007). Witsack (1986) described Palaeopatasson for one species from Dominican amber.

The senior author examined 16 Baltic amber pieces (SVT, UCRC) in 2005, 13 pieces (AMNH) in 2011, and three pieces (IPMGö., ZMUC) in about 2005. Only a few specimens could be referred confidently to an extant genus. These were four specimens of Gonatocerus, and (less confidently) three specimens of Anaphes and two of Stethynium. If correctly identified, five more genera, all known from the extant fauna, are reported here for the first time as amber fossils. Having examined these 32 additional amber specimens and realizing that most of them cannot definitely be classified in an extant genus, the senior author has doubts about the correct generic placement of at least some of the specimens studied by past workers. None of those specimens were examined, however. Each Tertiary fossil genus is discussed briefly below.

Anagroidea (1 female, C.V. Henningsen, B-1 1956, ZMUC, examined) is no longer known from extant species in Europe and is rare in the Holarctic region (eastern Asia and south east USA).

Alaptus is known from at least two species in Miocene amber (Doutt 1973) that are probably correctly identified to genus.

Anaphes was known from three specimens in Baltic amber. Anaphes splendensMeunier is probably incorrectly classified. According to Meunier the ovipositor extends appreciably beyond the apex of the metasoma, unlike any extant Anaphes. The fore wing with a slight but distinct ventral lobe, its overall shape, the venation with a distinct stigmal vein, and the antenna with fl1 almost as long as fl2 also do not fit the genus. Anaphes schellwieniens Meunier is described from a male so it is impossible to determine its placement. Doutt (1973) examined a male from Chiapas amber and tentatively placed it in Anaphes. Three specimens examined here are fairly confidently placed in Anaphes (1 female, UCRC; 2 females, SVT). Additional specimens (2 females, SVT) examined are tentatively placed in Anaphes.

Arescon is known from two species in amber (Meunier 1901, 1905) that are probably correctly placed because of their 5-segmented funicle, though fore wing venation length, which is important information for correct generic placement, was not given by Meunier.

?Camptoptera (1 female, SVT), examined in 2005, if correctly classified, would be the first fossil specimen for this worldwide genus.

?Dorya (1 female, UCRC) is known only from extant species in Australia and New Zealand.

?Eustochus (1 female, AMNH). The gaster appears to be petiolate. This suggests a Eustochus with a distinct 3-segmented clava (most have a 2-segmented clava) though the wing shape and antenna suggests Eoeustochus.

?Mimalaptus (3 females, SVT; 2 females, UCRC; 1 female, box G 3.910 Hymenoptera #BST03124, IPMGö.) is known from extant species in Australia and New Zealand and possibly eastern Asia.

Gonatocerus was known from only one fossil specimen, Gonatocerus henneberti (Meunier 1905), which appears to belong to either Gonatocerus (Gonatocerus) or Gonatocerus (Lymaenon) as suggested by the narrow wings. However, the “écusson semilunaire” [semilunate shield], which I interpret from Meunier’s drawing as being the dorsellum, appears to be rhomboidal in shape, which would eliminate Gonatocerus (Lymaenon) as the correct subgenus. The small size of the specimen, only 1/3 mm long, is unusual; it is much smaller than the smallest extant Gonatocerus I have seen. A specimen of Gonatocerus (Cosmocomoidea) is reported here (1 male, Poinar collection, OSU) from Dominican amber, 15–20 my.

Litus is represented by two fossil species. Litus elegans Meunier (1901) may or may not be correctly classified because the description and illustration are insufficient to place it. It apparently differs considerably from Litus mexicanus Doutt (1973), which is likely correctly classified. Extant Litus species tend to have a bi-geniculate funicle, unlike that illustrated by Meunier. Litus beneficus Meunier from copal (recent) from Madagascar also needs to be re-examined; it is unlikely to be classified correctly.

Malfatia molitorae Meunier (1901) is based on a male and from the description alone it is impossible to determine how it relates to extant genera. Its status remains uncertain.

Palaeopatasson grollei Witsack (1986) is based on a female and may be related to Anaphes.

Polynemoidea mexicana Doutt (1973) from Chiapas amber was stated to be similar to Polynemoidea domestica Girault, but the latter species is likely incorrectly placed in Polynemoidea (Lin et al. 2010). Consequently, I suspect that Polynemoidea mexicana is also incorrectly classified; it does not have a strongly exserted ovipositor as in the type species of Polynemoidea.

Stethynium (1 female, S. Triapitsyn collection), if correctly identified, confirms that the genus occurs in Baltic amber, as reported by Thuróczy (1983).

Conclusions

1. Tertiary fossils of Mymaridae are clearly more related to the extant fauna than to the Cretaceous fauna.

2. Except for Gonatocerus and the two new genera described above, tertiary fossils of Mymaridae (except perhaps Anaphes and Stethynium) are for the most part doubtfully assigned to extant genera.

3. Several Eocene amber fossils, if correctly identified to genus, represent genera that no longer occur in Europe (Anagroidea, Dorya, Mimalaptus) or are very poorly represented in most of the Holarctic region (Stethynium). These genera are now distributed mostly in the tropics or southern hemisphere, supporting observations that the Eocene climate (in parts of the Holarctic region at least) was considerably warmer than at present.

4. If correctly identified, Anagroidea, Eustochus, and perhaps Eoeustochus would be the first fossil recordsof genera belonging to the extant tribe Mymarini sensu Annecke and Doutt (1961), likely the most derived lineage in Mymaridae.