Boreocanthon Halffter 1958

Genus Boreocanthon Halffter, 1958, restored generic status Boreocanthon Halffter 1958: 208 (as genus) Canthon (Boreocanthon Halffter) (downgraded to subgenus of Canthon Hoffmannsegg per Halffter and Martínez 1977: 81) Type species. Ateuchus ebenus Say, 1823, by original designation (as Canthon ebenus [Say]). Summary diagnosis. North American dung beetles, ball-rolling species attracted to a wide range of animal feces; usually dark or black in color, small-sized (length usually 4–7 mm); head margin conspicuously quadri- or sexdentate; parameres compressed laterally, truncate apically, lacking elongate projection of lower apical angle; hind femora lacking fine carina along anterior edge; metatibia with single apical spur. Diagnosis. Dorsum (head + pronotum + elytra + pygidium) shagreened (alutaceous), with either punctures (Fig. 38, 48, 75, 105) or raised granules (Fig. 8. 19), punctures often accompanied by field of faintly lustrous microspotting (Fig. 61, 88); pterothoracic and abdominal sclerites shagreened, usually with accompanying flattened granules or punctures. Head: Clypeus quadridentate (Fig. 20, 66, 79, 112) (sometimes virtually bidentate in B. coahuilensis, Fig. 47); paraocular areas often also angulate such that head appears sexdentate (Fig. 66, 79). Clypeal teeth reflexed, surface behind concave, finely rugose and often somewhat shinier than rest of head surface, which is usually punctate on shagreened background. Clypeal process a low, transverse ridge. Labiogular fimbria always setose, often V-shaped, usually with associated field of coarse punctures at midline (Fig. 60, 69). Prothorax: Subapical tubercle beneath anterior angle of pronotum usually absent. Hypomeral carina usually absent (Fig. 24), sometimes partially developed but never long, extending at most only one-fifth to one-quarter of distance to edge of prothorax (Fig. 40, 108, arrow). Legs: Protibia strongly tridentate, usually serrate along lateral margin and between large teeth, inner margin smoothly curved (Fig. 104) or offset at level of basal tooth (Fig. 51). Anterior tibial spurs sexually dimorphic in most species: acute apically in female (Fig. 116a), unequally bifurcate apically in male (Fig. 116b); apex acute in both sexes in B. ebenus and B. depressipennis (Fig. 16). Posterior femora not margined anteriorly. Posterior tibiae with single apical spur. Elytra: Nine elytral striae; striae superficial, not impressed, lateral margins sometimes sharpened by fine carinulae (Fig. 61, 71); 9 th (epipleural) stria, if distinct, effaced anteriorly; 8 th (subhumeral) sometimes carinulate (Fig. 88, arrow); 7 th and 8 th sometimes almost effaced anteriorly (Fig. 99). Interstriae always shagreened, either microspotted or granulate, sometimes presenting distinct humeral puncturing; anterior ends of first elytral interstriae depressed below level of second interstriae; anterior ends of 2 nd and 3 rd interstriae sometimes swollen (Fig. 67, 111), otherwise discal interstriae flattened. Genital capsule: Distal portion of parameres laterally compressed, truncate, posterior edges straight, appressed, profile varies from abbreviated (Fig. 113) to elongate triangle (Fig. 85), lacking any curved elongation of the lower apical angle (as in Fig. 43a). General: Small, length rarely over 7.5 mm, usually 4.5–6 mm. North America from Trans-Mexican Volcanic Belt in southern Mexico, through the United States into far south-central Canada (Fig. 1). Comments. Boreocanthon is here restored to its original status of genus because, like its close relative, Melanocanthon, it exhibits a distinguishing suite of morphological features (see Diagnosis) and a plainly North American distribution. I have not discovered any unique feature (synapomorphy) that both unites its 13 species and separates them from other “ Canthon,” sensu lato, and, therefore, I cannot argue with a high degree of confidence that the group is monophyletic. Boreocanthon and Melanocanthon, along with the pilularius species-group of Canthon (sensu Halffter 1961) constitute the endemic United States contingent of the North American fauna of deltochiline Scarabaeinae. All other deltochilines occurring in temperate North America, north of the Rio Grande, are recently arrived representatives of southern groups; these include Deltochilum gibbosum (Fabricius), Pseudocanthon perplexus (LeConte), Canthon (Glyphrocanthon) viridis (Palisot de Beauvois), C. (Canthon) humectus (Say), C. (C.) indigaceus LeConte and C. (C.) cyanellus LeConte. Boreocanthon is closely related to Melanocanthon, and the two are sometimes confused in collections. The latter group, however, is distinguished by having two apical spurs on the hind tibia (rather than one) as well as, among other features, subtle differences in dorsal sculpturing and the shape of the head and sides of the prothorax and elytra, and a distribution largely confined to the southeastern quadrant of the United States, including the eastern one-half of Texas. There are presently four recognized species of Melanocanthon, but my informal impression is that there may be but three. A future, more rigorous look at Melanocanthon is very much in order. Halffter et al. (2022) recently created the subgenus Bajacanthon to accommodate the unique Baja California endemic species, Canthon obliquus Horn. In their phylogenetic analysis (Melanocanthon was not included) the new genus emerged as sister group to Boreocanthon, with which it shares several features, especially the form of the parameres (Fig. 44a,b). Boreocanthon is a Nearctic taxon with deep Neotropical roots. The center of diversity of the genus is clearly the United States and adjacent northern Mexico, where it undoubtedly originated. It, as well as Melanocanthon and the pilularius species group of Canthon, are the only representatives of the Neotropical tribe Deltochilini that originated and diversified in the Nearctic. The historical biogeographic perspective proposed by Kohlmann and Halffter (1990) seems reasonable in the context of the results of this study: “The ancestor of Melanocanthon - Boreocanthon would have been distributed in northern Mexico and south-central USA, while the area was covered by tropical deciduous forest. By the end of Miocene the forest started to be replaced by grasslands and piñon-oak woodland, a process that was advanced in the Pliocene by the spread of pine forest and very recently the appearance of deserts. Most Boreocanthon species … live in this area, which leads us to think that this was the main evolutionary arena for this group. Nowadays, many of the species live in grasslands of the Great Plains … or in arid zones …” (p. 13). In the context of Gonzalo Halffter’s theory of the Mexican Transition Zone (Morrone 2015; Halffter 2017; Halffter and Morrone 2017 and numerous references therein), the modern Nearctic taxa, including Boreocanthon, are the descendants of deltochilines among a wave of Neotropical immigrants that arrived in North America during early Cenozoic, where they originated and began to radiate during Miocene-Pliocene times. This early wave of taxa from the south produced the founding cenocron of Halffter’s Mexican Plateau Dispersal Pattern, one of five distribution patterns comprising the biogeographical panorama of the Mexican Transition Zone (Halffter and Morrone 2017). This cenocron included many other scarabaeines and other insects, including Phanaeus, whose biogeographical history in part parallels that of the Boreocanthon and other Nearctic deltochilines (Kohlmann and Halffter 1990; Halffter and Morrone 2017) as well as other groups (Edmonds 1994). The distribution of the genus covers much of North America (Fig. 1). Of interest is the fact that the distributions of the member species of putative species pairs (B. ebenus-B. depressipennis, Fig. 28; B. melanus-B. forreri, Fig. 45; B. puncticollis-B. integricollis, Fig. 63; B. probus-B. halffteri, Fig. 82; and B. simplex-B. coahuilensis, Fig. 100) are allopatric or nearly so, which suggests a long history of vicariant/dispersal events punctuating the history of the group (see, for example, Gámez et al. 2016). The ecogeographic distributions of Boreocanthon species conform rather closely to the vegetation biomes depicted in Figure 2: B. depressipennis and most B. probus, to eastern coniferous open-canopy forests; B. praticola and B. ebenus, to mid-latitude steppe and prairie; B. lecontei and B. integricollis to arid south Texas shrubland; and B. simplex to the complex highly variable mosaic of biomes present west of the Rocky Mountains. Western populations of B. probus, as well as B. halffteri, have colonized west Texas / New Mexico steppe and desert shrub habitats. All the Mexican species (B. melanus, B. forreri, B. ateuchiceps, B. coahuilensis, and B. puncticollis) are associated with desert and desert shrub areas. That the diversity hotspot for Boreocanthon lies in the southwestern portion of the United States is unsurprising in that it is the focus of diversity for numerous vertebrate and invertebrate groups (see Parmenter et al. 1995). In this complex assemblage of arid and semi-arid range and basin habitats characteristic of western Texas, and southern New Mexico and Arizona, seven species come together: B. puncticollis, B. melanus, B. ebenus, B. halffteri, B. simplex, B. praticola and B. probus. Only two, however, are endemics: B. halffteri and B. melanus. For comparison, in the United States west of the Rocky Mountains north of about 34° latitude, an area much larger than the U.S. Southwest, is home to a single, albeit highly variable species, B. simplex. All other species, B. lecontei, B. integricollis, B. depressipennis, B. coahuilensis, B. ateuchiceps and B. forreri can be considered peripheral, relatively localized relatives of these seven. What this says about the biogeographical history of the genus is ripe for analysis. Available data bespeak a group that is fundamentally coprophagous, with but very f

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