Boreocanthon coahuilensis ( Howden, 1966 )

Edmonds, W. D., 2022, Taxonomic review of the North American dung beetle genus Boreocanthon Halffter, 1958 (Coleoptera: Scarabaeidae: Scarabaeinae: Deltochilini), Insecta Mundi 2022 (952), pp. 1-65 : 31-33

Boreocanthon coahuilensis ( Howden, 1966) , new generic combination

Fig. 46–53 , 100

Canthon ( Boreocanthon ?) coahuilensis Howden 1966: 732 .

Canthon (Boreocanthon) coahuilensis Howden (subgeneric confirmation per Halffter and Martínez 1977: 82). Boreocanthon coahuilensis (Howden) , new generic combination.

Type material. Holotype male, Canadian National Collection, Ottawa ( CNC312445 = CNC Type No. 9143). Examined by photograph (https://www.cnc.agr.gc.ca/taxonomy/ Specimen. php?id=3684).

Type locality. 21 mi SE Saltillo, Coahuila, Mexico, 6000 ft.

Diagnosis. Head: Apical one-third of clypeus very finely rugose, slightly shinier than rest of head; posterior portion of clypeus and frons with dense shagreen and widely spaced, small punctures ( Fig. 47 ). Clypeus usually distinctly quadridentate, if only weakly so; middle teeth moderately developed, rounded apically, separated by broad, rounded emargination; lateral teeth small, weakly angular, sometimes obsolete such that head appears bidentate ( Fig. 46–47 ). Paraocular notch virtually absent, lateral margin of head only very slightly curved inward. Labio-gular fimbria V-shaped, apex extending about one-third length of gula. Prothorax: Pronotum convex, elevated above elytral surface (viewed laterally, Fig. 53b ), not depressed posteromedially; surface weakly lustrous, silky, with dense shagreen and widely spaced, inconspicuous punctures and numerous weak microspots ( Fig. 46, 48 ); lateral margin very weakly upturned apically. Underside of lateral margin bearing a small rounded tubercle near anterior end, which can be very small or obsolete. Hypomeral carina absent. Pterothorax: Mesoventrite minutely punctured laterally. Metaventrite evenly, widely punctate on shagreen background. Elytra: Interstriae sculptured like pronotum, 2 nd and 3 rd not swollen apically, 2 nd, 3 rd and 5 th usually bearing minute tubercle on anterior end (sometimes obscured by pronotal margin). Discal striae (1 st –6 th) weak, lacking well defined margins, lateral striae (7 th –9 th) weaker, effaced anteriorly, subhumeral (8 th) never carinulate, humeral area with scattered, fine punctures. Legs: Inner margin of protibia abruptly widened (offset) at level of basal tooth ( Fig. 51 ); protibial spur sexually dimorphic, apically acute in female, bifurcate in male. Hind femur lacking conspicuous anterior row of long setae. Abdomen: Pygidium sculptured like pronotum. Last (6 th) ventrite punctate, more strongly so than pygidium; other ventrites shagreened, with few lateral punctures. Genital capsule: Compressed apical portion of parameres narrowly triangular ( Fig. 49 ); lower apical angles in form of elongate, rounded knobs ( Fig. 50 ). General: Black, pronotum and venter weakly shiny. Length: 5.5–10.0 mm. Geographic distribution ( Fig. 100 ): Sierra Madre Oriental in extreme SE Coahuila and neighboring Nuevo Leon, Mexico. Ecogeographic environment ( Fig. 2 ): Desert Shrublands biome (Meseta Central matorral ecoregion). Specimens examined: 40.

Collection localities.

MEXICO — COAHUILA: Mpio. Saltillo ● 21 mi SE Saltillo, Coahuila , 6000 ft [Jul] ● Chapinque Mesa , 20 km SE Saltillo, 2050 m [Jul] . NUEVO LEON: Mpio. Galeana ● 4 mi E San Roberto [Jul] ; Mpio. Montemorelos ● Las Raíces [Aug].

Comments. In his original description, Howden (1966), with some doubt, assigned this species to Canthon using the heading “ Canthon ( Boreocanthon ?) coahuilensis n. sp. ”. That his hesitancy applied to subgeneric rather than generic placement is certain as he expressly (p. 727) treated Boreocanthon as a subgenus: “I have treated all [generic and subgeneric] divisions [of Canthon ] as subgenera, and my assignments of some species considered below are tentative.” I gather that his hesitancy to assign B. coahuilensis to Boreocanthon derived especially from the variation in clypeal dentition that departed from the usual form in other Boreocanthon . Nevertheless, he was struck by obvious similarities to B. simplex . I agree with Howden’s assessment. Indeed, its closest relative appears to be B. simplex , with which it shares several common features: moderately to weakly developed clypeal teeth; small paraocular notch; basally effaced 7 th and 8 th elytral striae, and truncated profile of the parameres. The abruptly offset profile of the inner protibial margin is shared only with B. depressipennis and B. ebenus . The pronotum of B. coahuilensis is rather strongly humped ( Fig. 53b ), a feature rarely seen in other Boreocanthon .

This species is so far known from but four nearby localities in far southern Coahuila and across the state border into the southwest corner of Nuevo Leon. My guess is that it extends farther to the south along the flanks of the Sierra Madre Oriental , at least as far south as southern San Luis Potosí, that is, along the length of the Meseta Central matorral ecoregion ( Fig. 2 ). Its geographical isolation and sharp morphological distinctness bespeaks an early separation from B. simplex . Given that it is an arid land species, which regularly exhibit population explosions during sort-lived favorable conditions, I rather doubt that it is rare, or even uncommon, in spite of what appears might be a very localized distribution. Howden’s description was based on a type series of 35 specimens collected near Saltillo, Coahuila. For this study, I was fortunate to examine two other series totaling 37 specimens collected from horse and cattle dung by Paul Lago from Chapinque Mesa , Coahuila, and San Roberto, Nuevo Leon, as well as three specimens from the Halffter collection from Las Raíces.

Figures

Figures 46–53. Boreocanthon coahuilensis (Howden). 46) Habitus, dorsal view (inset magnifies elytral sculpturing). 47) Frontal view of head and pronotal margin. 48) Dorsal view of portion of pronotum. 49) Lateral view of genital capsule. 50) Apical view of parameres. 51) Anterior view of protibia. 52) Dorsal view of head. 53) Comparison of body shape in profile. a) Boreocanthon simplex. b) Boreocanthon coahuilensis.

Figures 100–101. Boreocanthon distribution.100) Approximate distributions ofBoreocanthon simplex (LeConte), B. lecontei (Harold) and B. coahuilensis (Howden).101) Solid red diamonds mark California collection sites for B. simplex with red elytral humeri in one or more specimens; shaded area marks general limits of the Central Valley.

Figures 1–2. 1) Approximate corporate distribution of the genus Boreocanthon. 2) Generalized depiction of vegetation biomes of the United States (Biome key in box; See Materials and Methods for explanation).