Onthophagus chevrolati, Harold, 1869

Halffter, Gonzalo, Zunino, Mario, Moctezuma, Victor & Sánchez-Huerta, José L., 2019, The integration processes of the distributional patterns in the Mexican Transition Zone: Phyletic, paleogeographic and ecological factors of a case study, Zootaxa 4586 (1), pp. 1-34 : 13-14

The Onthophagus chevrolati species group

Venturing into a new kind of biogeographical-phyletic discussion, in which the processes of speciation are properly compared with the geological history of the mountains, requires an adequate knowledge of the taxonomy of the group to be studied. Zunino & Halffter (1988a), based on the morphology of copulation apparatuses and external morphological characters, established the O. chevrolati species group, which they consider to be one of the best taxonomically studied of the American fauna of Onthophagus . These authors included in the group 39 species and subspecies, and later discoveries increased its number of species.

Based on the monograph by Zunino and Halffter (1988a), whose approaches continue to be valid, we have updated some details of this group’s taxonomy using the results of recent studies ( Delgado & Capistan 1996; Delgado 1999; Sánchez-Huerta et al. 2015, 2018; Arriaga et al. 2016; Moctezuma et al. 2016; Gasca-Álvarez et al. 2018; Joaqui et al. in press). We have also studied the morphology of the genitalia in depth, carrying out a detailed, comparative analysis of the characters of all the species of the O. hippopotamus species complex, except for O. padrinoi of which we have not yet been able to examine the anatomy.

With respect to Zunino & Halffter (1988a) we consider the following changes. All formerly recognized subspecies are elevated to species status. Onthophagus mesoamericanus Zunino & Halffter, 1988 ( Fig. 9 ) is provisionally considered a synonym of O. cyanellus ( Fig. 8 ). We consider O. cyanellus to be a species line, with the same rank as the O. chevrolati species line in which it was previously included. Within Onthophagus , the O. chevrolati species group is the model example of the Paleoamerican Mountain distributional sub-pattern, with the exception of O. cyanellus and related species that conform very well to the Paleoamerican Mesoamerican subpattern. The graphic representation of our taxonomic ideas (manually drawn in an intuitive manner, based on morphological similarity and supposed evolution) combined with the distribution of the lines is shown in Fig. 11 .

To distinguish and delimit species, in this work we are following the phylogenetic species concept as defined by Wheeler & Platnick (2000): “The smallest aggregation of (sexual) populations or (asexual) lines diagnosable by a unique set of character states”. Consequently, all of the taxa considered as subspecies in the revision by Zunino & Halffter (1988a) are raised to the rank of species, since each has a set of diagnostic characters that allow it to be distinguished from the others, potentially representing different evolutionary histories.

Figures

FIGURE 8. Onthophagus mesoamericanus, male holotype dorsal view, currently a synonym of O. cyanellus.

FIGURE 9. Main orographic formations of Mexico. Original map prepared by Violeta Halffter and used on many occasions in the biogeographic studies of the Mexican Transition Zone. I, Sierra Madre Occidental; II, Sierra Madre Oriental; III, Mexican Plateau; IV, Trans-Mexican Volcanic Belt; V, Balsas Basin; VI, Sierra Madre del Sur; VII, Sierra Madre de Chiapas; VIII, Chiapas Highlands; IX, Guatemalan Plateau; X, Central American Nucleous.

FIGURE 11. Graphic hypothesis of the phyletic diversification and relationships of the O. chevrolati species group in the Mexican Transition Zone. Tip names are the species complexes. The figure elucidates our ideas of the taxonomical relationships and evolution of this species group based on the male and female genitalia, external morphology and geographic distribution: 1. The ancestral line of the O. chevrolati species group must have arrived in what is now Mexico between the Eocene and the Miocene, as part of the most ancient fauna of northern origin. The ancestor of O. vespertilio early separated from the ancestral trunk (plesiomorphic genitalia characters). 2. The ancestral species of the O. hippopotamus species line and of the O. hippopotamus species complex dispersed throughout the Trans-Mexican Volcanic Belt prior to its current conformation (Miocene to Pliocene). The geomorphological evolution of this mountain system drived the processes of speciation in this complex. Its descendants adapted to the burrows of Geomyidae in the mountains. 3. Separation of the O. hippopotamus and O. brevifrons species complexes. The first evolved with the TMVB, with some expansion southwards and northwards (O. coproides reaching New Mexico and Arizona). The brevifrons species complex is found in the southeastern United States of America and north-northeastern Mexico. Both complexes are found in the mountains and associated with the protected environments of rodent nests: Geomyidae in the case of the O. hippopotamus and Neotoma species complexes, plus three species in caves in the case of the O. brevifrons species complex.