The Onthophagus brevifrons species complex

Within the O. hippopotamus species line, the O. brevifrons species complex has five species whose distribution forms an arch (Fig. 4, 5, 14, 15) to the north of the Mexican High Plateau, from the Sierra Madre Occidental to the Sierra Madre Oriental. As mentioned, it is an equivalent line to the O. hippopotamus species complex, but it may have diversified in more recent geological times (see “The Onthophagus hippopotamus species line and species complex” and “Comparative analysis of the genitalia”).

There is a series of morphological and ecological differences that support the idea of conserving both species complexes. The supposedly plesiomorphic characters present in the O. brevifrons species complex include the midkeel of the lamella copulatrix lacking a sagittal suture and the orientation of the axis of the apexes of the parameres. Supposedly synapomorphic characters are the introflexion of the left lateral wall of the copulatory lamella, as well as the development of the caudal branches of the sclerotization of the vaginal support that is much larger than the cephalic branches. In this species complex, O. brevifrons Horn and O. cuevensis Howden show the supposedly plesiomorphic character states. The supposedly most apomorphic character states are present in O. cavernicollis Howden & Cartwright. Onthophagus moroni (Fig. 5) is supposed to be a closely related species to O. cavernicollis . The relationship of O. subtropicus Howden & Cartwright with the other species in the O. brevifrons complex is unclear, but this is the single species that occurs near sea level in tropical lowland habitats.