Sylvicola silibrarius Greenwalt, 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 : 14-16

Sylvicola silibrarius Greenwalt , sp. nov.

Figures 9-11

zoobank.org/ A1401C87-3CD1-40F7-8995-A63FBD5C4034 Etymology. The specific epithet is a combination of the abbreviation SI (for the Smithsonian Institution) and the Latin term librarius (pertaining to books) and is in appreciation for the essential services that the Smithsonian libraries perform.

Holotype. USNM 626077 , 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. Deep Ford site, Middle Fork of the Flathead River (Pinnacle, Montana, USA).

Differential diagnosis. This species of Sylvicola is distinguished by the basal separation of M 1 -M 2 1/6 of that between M 2 and M 3; pterostigma extended along cell r 1.

Description

Female (Figure 9.1), head and thorax dark brown/black, abdomen brown. Body length 5.3 mm, wing length 4.4 mm.

Head. Antenna setose; with, probably, 14 flagellomeres, basally wider than long, becoming thinner, longer than wide apically; apical flagellomere 2.5 x long as wide. Scape with ring of fine setulae in a single row. Terminal segment of palpus visible, protruding well beyond oral margin. Three occipital bristles present (Figure 10.1).

Thorax. 1.25 mm long. Scutum setose. Femur distinctly shorter than tibia; tarsomere 1 twice the length of tarsomere 2 which, in turn, is about twice the length of tarsomere 3, tarsomeres 4 and 5 short; legs setose.

Wing. Slightly dusky, membrane covered with macrotrichia. Wing shorter than body, 1.82 mm wide. C ending at R 4+5; pterostigma extended along cell r 1; radial veins much thicker than posterior veins, with fine setulae. M 1, M 2 and M 3 connected directly to discal cell, no forks. Distance between M 1 and M 2 at base about 1/6 of that between M 2 and M 3. CuP faint, anal lobe present (Figure 9.2-3).

Abdomen and genitalia. Abdomen, 3.4 mm long, 1.2 mm wide; uniformly brown, evenly covered with black setulae. A single sclerotized spermatheca present, 82 μm in diameter. Cerci indistinct, poorly preserved (Figure 10.2).

Allotype. Male unknown.

Syncompressions. None.

Paratype. A second specimen of Sylvicola silibrarius ( USNM621508 ) is designated as a paratype ( Figure 11 ). The specimen more clearly shows the small spherical shape of the head, the setose flagellomeres, a short arched scutum and additional legs. The specimen was collected at the Dakin site which is 0.6 km from the Deep Ford site, a possible indication of the of prevalence of the species .

Remarks

The family Anisopodidae consists of 15 extant genera and more than 200 species and is widely distributed ( Kania et al., 2019). The fossil record, with 49 described species ( EOL, 2017; PBDB, 2018), is rich, ancient and controversial. Michelsen (1999), in reference to the unsettled Mesozoic record of anisopod stem groups, stated “all fossil family-group names ... may by necessity be referred incertae sedis to the lineage Anisopodidae .” New Mesozoic species continue to be described, however, and the genera Mesorhyphus Handlirsch, 1920 , and Megarhyphus Kovalev, 1990 , date back to the Lower Jurassic ( Ewa et al., 2010). The family has been proposed as sister to the Bibionomorpha ( Wiegmann et al, 2011). Twelve of the fossil species date from the Eocene epoch. The genus Sylvicola contains 78 extant species and nine fossil species, the oldest of which, Sylvicola prisca Brodie, 1845 , is from the Early Cretaceous of the Middle Purbeck; seven described species are from the Eocene ( Wojtoń et al., 2018; PBDB, 2018). Pratt and Pratt (1980) proposed division of the genus Sylvicola into two subspecies, Anisopus Meigen, 1803 , and Sylvicola Harris, 1776 s. str., based, in part, on the distance between M 1 and M 2 at base of cell m1 ≤ 1/4 ( Anisopus ) or ≥ 2/3 ( Sylvicola ) the length of the vein separating M 2 and M 3 ( Krivosheina and Menzel, 1998). This proposal was rejected by Amorim and Tonzoni (1994) and Michelsen (1999).

Sylvicola silibrarius differs from the four Eocene species of Sylvicola as follows: S. cadaver

USNM

Scudder, 1890, from the Green River Formation, is slightly smaller (wing length, 3.5 mm vs. 4.4 for S. silibrarius ), its bm is “about half as long as the wing”, and its br terminates apically “scarcely beyond the tip of” Sc. In addition, M 1 -M 2 is 2/3 (left wing) or nearly equal to (right wing) the length of M 2 -M 3, as determined from the Scudder’s figure 17. Sylvicola hooleyi Cockerell, 1921 ( Cockerell 1921b) , from the Isle of Wight, is an isolated wing. The distance between the bases of M 1 and M 2, as determined from Cockerell’s original figure, is 62% of that between M 2 and M 3 (vs. 17% for S. silibrarius ). In addition, in S. silibrarius , the wing length is slightly shorter (4.4 mm vs 5.2 mm), lacks apical pigmentation, has the r-m crossvein contacting the discal cell at its apical half rather than its basal half and has a much wider cell m 3 relative to the discal cell (1.5 x vs. 0.8 x) measured at or as a continuation of the vein separating M 2 and M 3. The description of S. splendida Meunier, 1907 , mentions only the large pulvilli and claws of this species. S. silibrarius , which is slightly smaller (5.3 mm vs. 5.75 mm in length) differs from S. splendida , a male from Baltic amber, in having smaller tarsal claws (<the width of the base of the tarsomere). Sylvicola silibrarius differs from the male and female specimens of S. thiriona Meunier, 1904 , also from Baltic amber, in that S. thiriona has a filiform antenna, pedicel <twice the width of F12 (pedicel more stout,> twice the width of the terminal flagellomere in S. silibrarius ), R 1 reaching C closer to Sc than to the end of R 2+3, M 1 -M 2 ≥ 2/3 of M 2 -M 3 and terminal tarsomeres with small claws (plate 17, figure 14 in Meunier, 1904a).

Sylvicola baltica , S. hoffeinsorum and S.punctata were recently described by Wojtoń et al. (2018); S. baltica (male) and S. punctata (female) are both based on single specimens from Baltic amber whereas eight specimens of S. hoffeinsorum (both male and female) were studied, seven from Baltic amber and one from Bitterfield amber. Sylvicola silibrarius differs from all of these specimens in having m’-m’ much shorter than m-m. In addition, S. baltica has a terminal flagellomere six times as long as wide and all flagellomeres longer than wide; the terminal flagellomere of S. librarius is three times as long as wide. Sylvicola punctata has an undulating M 4 and the ratio of the distances between Sc and R 1 and R 1 and R 2+3 is 3 whereas it is about 2 in S. librarius .; this ratio is 1.5 in S. hoffeinsorum . In all three of the species described by Wojtoń et al. (2018), r-m terminates in the middle of dm whereas the cross vein intersects with dm at the distal third of the cell in S. librarius .

Notes on some other fossil Sylvicola: Lewis (1987) figured the wing of an anisopodid from the Oligocene Ruby River Basin in southwestern Montana. The entire insect was said to have been preserved. The specimen was identified as resembling the type species of Sylvicola , S. brevis Harris, 1780 [ Tipula fenestralis Scopoli, 1763 ]. The specimen was not described, and a repository was not designated. Although the specimen was collected by H. Becker, of the New York Botanical Gardens, sometime between 1950 and 1970, an inquiry made to the New York Botanical Gardens established that the specimen was not housed at the Gardens. Inquiries to St. Cloud State University, where Lewis’ collection is reported to be housed, have gone unanswered.

Evenhuis (1994) treated the name Rhyphus lugubris Heer, 1849 , under Sylvicola . However, Heer (1849) neither described nor mentioned Rhyphus lugubris . Evidently, Evenhuis had made a typographical error for Plecia lugubris Heer, 1849 , which is in the text immediately after the description of Sylvicola maculata Heer, 1849 (N.L. Evenhuis., personal commun., 2017).

Figures

FIGURE 9. Sylvicola silibrarius sp. nov., USNM 626077. 1, Habitus; 2, Right wing. Inset: Left wing macrotrichia from area proximal of Sc terminus; 3, Line drawing of right wing. Scale bars equal 3.0 mm (1), 1.0 mm (2).

FIGURE 10. Sylvicola silibrarius sp. nov., USNM 626077. 1, Head and antennae. Arrow denotes terminal palpomere. Arrowhead points to occipital bristles; 2, Terminalia, female. Arrow denotes the spermatheca. Scale bars equal 1.0 mm (1) and 0.25 mm (2).

FIGURE 11. Sylvicola silibrarius sp. nov., 621508. Paratype.Scale bar equals 2.0 mm.