Aenigmatias kishenehnensis Brown, 2019

Dale E. Greenwalt, Daniel J. Bickel, Peter H. Kerr, Gregory R. Curler, Brian V. Brown, Herman de Jong, Scott J. Fitzgerald, Torsten Dikow, Michal Tkoč, Christian Kehlmaier & Dalton De Souza Amorim, 2019, Diptera of the middle Eocene Kishenehn Formation. I. Documenting of diversity at the family level, Paleontologia Electronica 22 (2), No. 50, pp. 1-56 : 38-40

Aenigmatias kishenehnensis Brown , sp. nov.

Figure 35 zoobank.org/ 28A4C2F4-40E2-45AF-83A4-FADAD0EBD616 Etymology. The specific epithet denotes the geological Formation in which the specimen was preserved.

Holotype. USNM 625132 , deposited in the Department of Paleobiology , National Museum of Natural History (NMNH), Smithsonian Institution, Washington, District of Columbia, USA.

Type horizon. Middle Eocene Coal Creek Member, Kishenehn Formation.

Type locality. The Spring site, Middle Fork of the Flathead River (Pinnacle, Montana, USA).

Differential diagnosis. This species of Aenigmatias is distinguished by the presence of wings, very short C, R 1 and Rs veins, the absence of R 2+3 and an apparently nonlimuloid body shape.

Description

Body length (head to the end of tergite 6) 1.5 mm. Female (Figure 35.1).

Head. Four large setae present at vertex and at least six large setae on frons (all frontal setae absent in modern species, often reduced in num- ber and size in fossils). Postpedicel rounded. Palpus with well-developed setae.

Thorax. Most details of thorax not visible, but not appearing to be highly limuloid. One pair of short setae visible (possibly posterior dorsocentral setae) and one longer pair (scutellars?) near base of wing.

Wings. C short, but costal setae long. R 2+3 absent. Base of radial veins possibly with row of setulae.

Legs. Foretibia not preserved. Midtibia with one large seta near base. Hind legs preserved, but details not visible.

Abdomen and genitalia. Abdomen unmodified, unlike flattened modern species. Abdominal segment 7 with dense striation and blunt thick setae. (Figure 35.2, bts; as in modern species, Figure 35.3).

Allotype. Male unknown.

Syncompressions. None.

Remarks

The family Phoridae consists of 302 genera and about 4,300 species. The fossil record of the family is quite good, with 103 fossil species (47 genera), 95% of which are in amber (about half of these in Baltic amber). Many extant phorids exhibit a parasitoid lifestyle and several genera are myr- mecophilous ( Brown, 2018). The 14 extant species of the genus Aenigmatias are presumed to be larval parasitoids of ant pupae. Members of the genus are markedly sexually dimorphic, with the female wingless and limuloid in shape, the latter an adaptation that provides defense against attack by ants ( Brown, 2017; Brown et al., 2017). Brown (1999), in an examination of fossil phorids in Baltic and Fushun amber, concluded that many of the specimens assigned to extant genera actually belong to more primitive stem-groups. Protophorites fimbriatus Brues, 1939 , was synonymized with Protoplatyphora tertiaria Brues, 1939 (holotype lost), which was thought to be a stem group of a clade with the modern Aenigmatias species, based on a less limuloid body shape, the presence of wings ( Protophorites fimbriatus is a female) and the presence of large setae on the vertex and scutellum. Brown (2017), in a phylogenetic analysis of the fossil Aenigmatias and related genera, transferred Protoplatyphora tertiaria and Chaetopleurophora multisetosa Brown, 2007 to Aenigmatias and described three new species from Baltic amber. With Aenigmatias kishenehnensis , there are currently a total of six fossil species in the genus: A. tertiarius Brues, 1939 , A. bisetosa Brown, 2007 , A. longicornis Brown, 2017 , A. primitivus Brown, 2017 and A. nigeroticus Brown, 2017 . Only A. tertiarius, Brues 1939 , A. kishenehnensis and an additional specimen (LACM 159804) are females.

Aenigmatias kishenehnensis can be differentiated from A. tertiaria in having a much shorter costa (0.24 mm vs. 0.5 mm), R 2+3 vein absent, and the frons with large setae; from A. bisetosa in having a much shorter body length (1.5 mm vs. 4.3– 4.4 mm), a much shorter costa and R 2+3 absent; from A. longicornis in having a rounded first flagellomere (vs. elongate) and a much shorter costa (0.24 mm vs. 0.43 mm); from A. primitivus in having a much shorter costa (0.24 mm vs. 0.48 mm); and from A. nigeroticus in having a much shorter costa (0.24 mm vs. 0.48 mm), R 2+3 absent and the frons with large setae. Aenigmatias kishenehnensis is also differentiated from Aenigmatias sp. indet. (LACM ENT 159804; Brown, 2017), a female, in having a much shorter costa (0.24 mm vs. 0.71 mm). Apparently, the set of extinct species of Aenigmatias do not constitute a clade sister to the extant species of the genus, but rather a grade.

The 21 fossil phorid flies of the Kishenehn formation are intriguing. With the exception of A. kishenehnensis , they are difficult to place to any modern group. None of the specimens exhibit proclinate supra-antennal setae, a condition found in most phorids of the subfamily Metopininae , which is the numerically dominant group today. The earliest undoubted metopinines are known from Baltic amber, and even there they are a smaller portion of the fauna than today, suggesting that the metopinine radiation is indeed an evolutionarily recent (i.e., post-Eocene) event. Other character states visible in the Kishenehn phorids are not considered synapomorphic of any modern groups, with a single exception of A. kishenehnensis .

Modern Aenigmatias , and most fossil species of the genus ( Brown, 2017), have a series of setulae along the radial vein, more than four scutellar setae and longitudinal, irregular rows of tightlypacked setulae (setal palisades) on the hind tibia in addition to the limuloid body form. None of these character states was convincingly observed in A. kishenehnensis , but the much more distinctive structure of the female terminal segments is clearly visible: the ovipositor has heavily striate membrane and thick, peglike setae. This structure is known from at least one fossil species, A. tertiarius , and all examined modern species, but no other phorids. The life history of two of the 14 modern species has been studied and both found to be parasitoids of Formica Linnaeus 1758 ant pupae ( Donisthorpe, 1927). No studies have been done on the function of this peculiar ovipositor, which is unlike those of other parasitoid phorids that attack adult ants, but it is likely related to the parasitic lifestyle.

Figures

FIGURE 35. Aenigmatias kishenehnensis sp. nov., USNM 625132. 1, Habitus; 2, Terminalia, (bts = blunt thick setae); 3, Terminalia of an extant female Aenigmatias sp. Scale bars equal 1.0 mm (1) and 0.2 mm (2).