Opetia americana Amorim and Greenwalt, 2022

Greenwalt, Dale E., Amorim, Dalton De Souza, Hauser, Martin, Kerr, Peter H., Fitzgerald, Scott J., Winterton, Shaun L., Cumming, Jeffrey M., Evenhuis, Neal L. & Sinclair, Bradley J., 2022, Diptera of the Middle Eocene Kishenehn Formation II, Palaeontologia Electronica (a 22) 25 (2), pp. 1-52 : 37-42

Opetia americana Amorim and Greenwalt sp. n.

Figures 22-25

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Type species. Opetia nigra Meigen, 1830: 357 (=? Opetia aberrans Shatalkin, 1985 = Opetia lonchopteroides Curtis, 1834 ), by subsequent monotypy.

Holotype. Female, USNM 621520 , compression fossil, deposited in the Department of Paleobiology , National Museum of Natural History ( NMNH), Smithsonian Institution, Washington, District of Columbia, USA.

Locality and horizon. Park site, Middle Fork of the Flathead River (Pinnacle, Montana, USA). Middle Eocene Coal Creek Member, Kishenehn Formation.

Etymology. The specific epithet refers to the species being the first record of the genus for North America.

Differential diagnosis. First flagellomere slender, not elongate oval. Sc reaching C at basal third of wing, tip of R 1 reaching C well beyond base of medial fork, at distal third of wing. R 1 curved towards anterior margin along basal third, distal end gradually approaching C. M 1+2 beyond origin of r-m three fourths the length of medial fork. Female abdominal tergite 5 slightly more slender than tergite 4, tergites 6, 7 and terminalia slender.

Description. Female. Head blackish, dichoptic. Ocellar triangle not visible. Frons apparently with four well-developed fronto-orbital setae evenly spaced on each side; ocellars and postocellars not visible. Eyes apparently strongly holoptic, anterior end closely approximated, frons triangular from dorsal view ( Figure 22B ). Antenna inserted at tip of frons, proportion between scape and pedicel not clear, both much shorter than flagellomere 1; first flagellomere slender, not widening midway to apex, with scattered small setae on basal half, arista with two slender articles of about same length, texture similar to first flagellomere, covered with microtrichia, lacking setation ( Figures 22B , 23A ). Thorax with scutum dark brown, dorsocentrals not visible; setae along lateral margin of scutum present, but not distinguishable. Scutellum concolorous with scutum, apparently marginal scutellars present. Legs delicate, fore tibia and tarsi, mid femur and tibia, and hind femur, tibia and tarsi visible. Apparently tibial spurs absent on all legs, tarsomeres gradually shorter ( Figure 23A ). Wing length, 2.5 mm. Membrane light fumose brown, slightly darker along anterior margin, no maculae. Hu present, transverse, Sc complete, reaching C at basal third of wing; R 1 long, gradually approaching C, reaching margin at distal third of wing; R 2+3 long, reaching C close to wing tip, apparently devoid of dorsal macrotrichia, R 5 ending at wing tip. Circumambient costal vein not verifiable. Transverse vein r-m discrete, at basal fourth of wing; M 1+2 (including sector basal to r-m) about as long as medial fork; M 4 thicker than other posterior veins. Cells bm and br small, closed, m-cu present, cell cua present, closed, CuA+CuP reaching wing margin ( Figures 23B , 24A ). Abdominal tergites and sternites brown, segments 1–3 well developed, segment 4 slightly more slender at distal end, segment 5 slender. Segments 6–8 strongly modified to constitute well sclerotized, elongated ovipositor, single sclerotized line dorsally on terminalia ( Figure 24B ).

Male. Unknown.

Synimpressions. One Hymenoptera (Ichneumonoidea) .

Remarks. The Opetiidae are a small family of Platypezoidea that includes Opetia Meigen, 1830 —known from the northern hemisphere, with one species from Europe and two species from east Asia (see Chandler, 2001)—and Puyehuemyia Amorim, Silva and Brown, 2018 , known in the southern hemisphere only from southern Chile ( Amorim, Silva and Brown, 2018).

Most of the Mesozoic fossils originally included in the Opetiidae — Palaeopetia Zhang, 2018 , Pseudopetia Zhang et al., 2018 , Mesopetia Zhang et al., 2018 , Lithopetia Zhang et al., 2018 and Sinolesta Hong and Wang, 1974 —do not fit into the family ( Chandler, 2001). The Cretaceous New Jersey amber fossil Electrosania Grimaldi and Cumming, 1999 was assigned by Grimaldi and Cumming (1999) to the Platypezidae , a position with which Chandler (2001) agreed, mentioning that it “has a thoracic chaetotaxy and aristal structure as in Opetiidae ”. Coram et al. (2000) described Opetiala from Early Cretaceous Purbeck Limestone, but the fossil does not belong either to the crown Opetiidae or to its stem ( Amorim et al., 2018). Grimaldi and Cumming (1999) discussed the similarities between the Neocomian early Cretaceous Lebanon amber Lonchopterites prisca Grimaldi and Cumming, 1999 and Opetia . Some features of the fossil suggest, according to Amorim et al. (2018), that it could be a stem Opetiidae . Grimaldi (2018) has provided an extensive review of platypezoid fossils, with clear stem and crown Platypezidae , stem Ironomyiidae , stem Lonchopteridae , and stem and crown Phoridae .

Information on Cenozoic fossil crown opetiids is extremely limited. Meunier (1893) described Oppenhemiella baltica Meunier, 1893 and carefully compared his Baltic amber fossil with Calomyia Röser, 1840 and Opetia , but illustrated only the tip of the hind tibia and hind tarsus. Hennig (1964) mentioned that the type is lost and questioned Meunier’s assumption of proximity of Oppenhemiella Meunier, 1893 to the platypezids. Meunier’s comments on the antennal structure of Oppenhemiella apply either to a platypezid or to an opetiid. With the type lost, it will not be possible to solve this riddle.

Opetia atra Statz, 1940 from the Oligocene Rott locality, was largely overlooked in the literature although it was listed by Evenhuis (1994) in Platypezidae . We hardly have any question that his illustrated specimen is a male Opetia , but the holotype should be examined for a careful understanding of the morphology. The fossil is not particularly well preserved and the antennae cannot be observed. The hind tarsi do not suggest a platypezid. There is a rather long medial fork and no dm-cu crossvein connecting M 1+2 to M 4. R 2+3 is shorter and curved towards the anterior margin, differing from both extant genera of Opetiidae , but the wing cell cua is shorter than in most platypezids (although slightly longer than in O. nigra and Puyehuemyia ) and the crossveins have position and size similar to that of opetiids ( Statz,1940, figure 26). There is an additional reference to an Oligocene fossil of Opetia in the Rott Statz Collection from Germany (http://paleobiodb.org).

In this paper, we describe a fly from the middle Eocene Kishenehn Formation of Montana, that belongs to the extant genus Opetia , the first record of the genus for North America and the first definite Cenozoic fossil record of the genus. We here formally describe and illustrate this species and discuss its implications for the understanding of the evolution of the family. The fossil is very well preserved, with the exception of the unexpectedly triangular shape of the head, the antennae placed at its tip. The most reasonable explanation for this unique head shape seems to be that the eyes collapsed and were not preserved, only the frons being seen, giving a general triangular shape to the head.

There are quite clear differences between Opetia americana and O. nigra , in such a way that a diagnosis can be made, especially in the wing ( Figure 25 A-C). In the Eocene Kishenehn species, Sc and R 1 are longer than in O. nigra , with the tip of these veins reaching C more distally. The ratio M 1+2 /medial fork, on the other hand, is smaller in O. americana than in O. nigra . Interestingly, the conditions of these features in O. americana is more similar to the condition seen in Puyehuemyia . Apparently R 2+3 does not bear a row of dorsal setae. The basal third of the sclerotized ovipositor in O. nigra has a constriction, but this feature cannot be verified in the holotype of O. americana , since the anterior half of the ovipositor is inside the distal segments of the abdomen.

The extensive study of Chandler (2001) on Opetia and Amorim et al.’s (2018) discussion on the southern Neotropical representative member of the family helps in the analysis of the fossil specimen of Opetia americana . The specimen is a female and shares the absence of a well-developed anal lobe and the shape of the ovipositor. One striking difference between Opetia and Puyehuemyia is the number of articles in the arista. Opetia has typically two segments, while Puyehuemyia has three, a plesiomorphic condition in the Cyclorrhapha. The fossil has two segments and is assigned to the extant species of Opetia .

Opetia has only three known extant species, including Opetia nigra from Europe, and two species from Japan and Taiwan. Opetia americana obviously differs from O. nigra in details of the wing venation. Indeed, there seems to be a gradual displacement of the tip of veins Sc, R 1 and R 2+3 to a more basal position along the anterior margin, from the condition seen in Puyehuemyia to the condition seen in Opetia americana and O. nigra .

Amorim et al. (2018) discussed the question of the age of the Opetiidae . The estimated divergence age for the Cyclorrhapha (Wiegmann et al., 2011), about 150 mya, is compatible with the Neocomian age of the Lebanon amber fossil Lonchopterites ( Grimaldi and Cumming, 1999) and a disjunction with the southern temperate Chilean opetiid genus Puyehuemyia . This suggests a possible Jurassic origin for the stem Opetia , and the Opetia americana Kishenehn fossil would be a much later divergence in the genus. The presence of an extinct Eocene Nearctic species of an extant genus also known from the Palaearctic Region is not new. A fossil species of the canthyloscelid genus Synneuron Lundström, 1910 that also has Palaearctic representatives, was recently described, also from the Eocene Kishenehn ( Amorim and Greenwalt, 2020). Another example is the genus Reissa , extant species of which were originally described from the Canary Islands by Greathead and Evenhuis (2001). The Eocene Reissa kohlsi was described from the Green River Formation in western North America ( Evenhuis, 2019). The second half of the Cenozoic may have witnessed the loss of an important number of Holarctic elements in North America.

Figures

FIGURE 22. Opetia americana Amorim and Greenwalt sp. n., female, USNM 621520. A, Habitus; B, Head and anterior end of scutum. Abbreviations: ar1–2, aristomeres 1 and 2; flg1, flagellomere 1; orb, orbital setae; presut sa, presutural supra-alars. Scale bars equal 1.0 (A) and 0.1 mm (B).

FIGURE 23. Opetia americana Amorim and Greenwalt sp. n., female, USNM 621520. A, Head, anterior end of scutum, and distal end of anterior leg. Abbreviations: pe, antennal pedicel; sc, antennal scape; flg1, flagellomere 1; tcl, tarsal claw; ti1, front tibia; tm1–5, tarsomeres 1 to 5; B, Right wing. Scale bars equal 0.2 mm (A) and 0.5 mm (B).

FIGURE 24. Opetia americana Amorim and Greenwalt sp. n., female, USNM 621520. A, Left wing base; B, Tip of abdomen and terminalia. Abbreviations: ovi, ovipositor; scl, ovipositor sclerotized lines; T3–6, tergite. Scale bars equal 0.2 mm.

FIGURE 25. Comparison of wing venation of three opetiids. A, Puyehuemyia chandleri Amorim, Silva and Brown; B, Opetia americana Amorim and Greenwalt sp. n. Comparison of landmarks on wing of O. americana, sp. n.: 1. anterior end of Hu at C; 2. tip of Sc at C; 3. tip of R1 at C; 4. tip of R2+3 at C; 5. origin of Radial Sector at bR; 6. anterior end of r-m; 7. fork of M1 and M2; 8. distal end of M1; 9. tip of CuA+CuP. C. Opetia nigra Meigen (modified from Chandler, 2001).