Lonchoptera eocenica Amorim and 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 : 36-38

Lonchoptera eocenica Amorim and Brown , sp. nov. Figures 32-34

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Etymology. The specific epithet, a Latin adjective, refers to the geologic epoch in which this species lived.

Holotype. USNM 625379 , compression fossil, 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. Spring site, Middle Fork of the Flathead River (Pinnacle, Montana).

Differential diagnosis. Similar to modern lonchopterids, but can be clearly separated from the recent species by the broader shape of the wing, slightly more rounded apically; R 2+3 not as close to R 4+5 as in modern species of the genus; R 4+5 reaching C slightly before wing tip (while in recent

species R 4+5 characteristically ends at tip of wing); probably related to the general shape of the wing, the medial fork wider, with M 1 and M 2 relatively more separated at the wing margin; the distal end of M 4, close to the margin, is almost straight, while in recent species it is slightly curved posteriorly.

Description

Female ( Figure 32 ), body dark brown, body length 2.8 mm, wing length 2.7 mm.

Head. Relatively small, flattened; flagellomere 1 apically pointed.

Thorax and wings. Thorax stout. Sc weakly sclerotized (as in recent species), slightly separated from R 1; R 1 short, extending to about one-third of visible wing length; R 2+3 convex, slightly converging towards wing apex; R 4+5 extended to wing apex, reaching C just before wing tip; costal setae ending at R 4+5; r-m originating at R 4+5 just beyond origin of R 2+3, strongly curved basally; CuA+CuP joining M 4 before wing margin (as in modern females); fused vein CuA+CuP about as long as base of medial fork; wing cells br, bm, and cua (this latter hard to delimit) small, displaced to the base of the wing. Setulae visible on veins R 1, M 1+2, M 2, M 4, and base of CuA+CuP.

Legs. Legs apparently slightly shorter than in recent species.

Abdomen and genitalia. Abdomen shorter than recent species, tergites and sternites apparently well sclerotized. Terminalia short, only a short cercus visible.

Allotype. Male unknown.

Syncompressions. None.

Remarks

Lonchopteridae is a small family of extant flies that is found nearly worldwide. The modern fauna is represented by a single genus, Lonchoptera Meigen , for which about 50 species have been described. The Nearctic species and much of the information about the genus were reviewed by Klymko and Marshall (2008). The common name for this group, “spear-winged flies”, is based on the narrowed wing with pointed apex. Other distinctive characters are the setulose wing veins, the shortened wing veins R 1 and Sc, and the sexually dimorphic wing venation, with CuA+CuP joining M 4 in females, but extended to the wing margin in males.

Fossils assigned to this family are few, basically two species of uncertain relationships to modern lonchopterids ( Grimaldi and Cumming 1999). They both are, however, considerably different relative to the modern genus Lonchoptera and, if they belong at all to the clade, they would clearly belong to the stem group of the family. Indeed, Amorim et al. (2018) suggested that Lonchopterites Grimaldi and Cumming could be a stem-group Opetiidae , not a Lonchopteridae .

Few details of the head, thorax, legs, abdomen, and terminalia are visible in the specimen ( Figure 32 ). Nevertheless, the wings are largely well preserved and visible, and there is scarcely any doubt that this species fits together with the recent species of Lonchoptera as a clade. Recent Lonchoptera species are considerably similar in the wing shape and the wing venation. Some modified wing venation features present in Lonchoptera are shared with Opetiidae and Phoridae within the Platypezoidea ( Amorim et al., 2018), particularly the very basal origin of R 1, the stronger R 4+5, the reduction in size of cells bm, br, and cua, the loss of the dm-m crossvein and the shape of the long medial fork (some of these features secondarily modified in the phorids). A number of apomorphic conditions are seen in the wing of the recent species of Lonchoptera . These include the typical elongate shape of the wing, with a pointed tip, the displacement of R 1 to a considerably basal position in the wing, the weakly sclerotized and short Sc, a convex R 2+3, the tip of R 2+3 displaced to close to the wing tip, R 4+5 reaching precisely the acute wing tip, the strong displacement of the posterior end of r-m towards the base of the wing, CuA distally fused to M 4 in females and a very short CuP actually coming out from the wing margin (actually, the circumambient C) to join CuA.

All of these features are present in the Eocene species described here, except the lack of the general elongate shape and pointed wing apex — while the CuP emerging from the wing margin cannot be verified ( Figure 33 ). Some of the features in L. eocenica are modified, but not to the same degree as in recent species: this includes the displacement R 1 to the base, of R 2+3 to the apex and R 4+5 ending at the very tip of the wing.

This set of features does not leave any doubt about the position of the species together with the recent Lonchoptera . A thin plate spline analysis was performed to show the direction of the changes from the fossil to modern wing veins (Figure 34.1-2; dots indicate modern venation, line bases the fossil equivalent). The analysis makes clear that the distance between R 1 and R 2+3 is expanded (orange/red) in the recent species, while R 2+3 is compressed towards R 4+5 (light/dark blue). Also, R 1 is in an even more basal position in the recent species. The process of narrowing the wing in the modern species takes place in both the anterior and posterior portions of the wing.

It is quite unfortunate that the some details of the fossil cannot be fully described, particularly the head, thorax, and legs. The very characteristic shape of the head of recent Lonchoptera and the quite elongate thorax cannot be properly checked in L. eocenica , but the fossil seems to be slightly stouter. The legs of L. eocenica seem to be also shorter. Based on the discussion above, L. eocenica appears to be the sister species to the recent species of Lonchoptera . In other words, the Eocene fossil described here is a stem Lonchoptera . Despite some differences in the wing shape and in the wing venation, we do not see any particular reason to have a separate monotypic taxon of generic rank to hold the fossil species. It is worth noting that Bouchenak-Khelladi et al. (2010) date the crown node of Poaceae (i.e., BEP+PACCMAD) at the early Eocene, 57 Ma. The larvae of recent species have quite varied habitats, but adults are associated with grassy habitats ( Klymko and Marshall, 2008), and the genus may have largely expanded its distribution with the diversification of grassy, open environments at the mid of the first half of the Cenozoic. This fossil will certainly bring important benefits to calibration in studies on age divergence in the Cyclorrhapha.

Figures

FIGURE 32. Lonchoptera eocenica sp. nov. USNM 625379. Scale bar equals 2.0 mm.

FIGURE 33. Lonchoptera eocenica sp. nov. USNM 625379. Close-up of wing.

FIGURE 34. A thin plate spline analysis demonstrates the direction of the changes from fossil to modern wing veination. 1, Wing of modern Lonchoptera sp. (from the Puyehue National Park, Chile), showing placement of landmarks; 2, Expansion (red) and compression (blue) of wing venation of a modern species of Lonchoptera relative to Lonchoptera eocenica. Basal, left; distal, right. Base of lines represent fossil landmark; dots at tips of lines represent modern landmark.