Two new species of the genus Ateuchus Weber are described from the region of Los Chimalapas, Oaxaca, Mexico: A. benitojuarezi sp. n. and A. colossus sp. n. A diagnosis for distinguishing these new species from the other species of this genus in North America is included. This paper is illustrated with pictures of the dorsal habitus and the male genitalia of the new species. The evolutionary relationships of the species are discussed, as well as their distribution and ecology. It is considered that the species of the genus Ateuchus present in North and Central America correspond to the Typical Neotropical and Mountain Mesoamerican distribution patterns.

Describimos dos especies nuevas del género Ateuchus Weber, de la región de Los Chimalapas, Oaxaca, México: A. benitojuarezi sp. n. y A. colossus sp. n. Incluimos diagnosis para distinguir a estas especies nuevas del resto de especies del género presentes en Norteamérica. Presentamos ilustraciones del hábito dorsal y la genitalia masculina de las nuevas especies. Discutimos las relaciones evolutivas de las especies, así como también su distribución y ecología. Consideramos que las especies del género Ateuchus corresponden con los patrones de distribución Neotropical Típico y Mesoamericano de Montaña.

Ateuchini , distribution patterns, dung beetles, Los Chimalapas, Mexican Transition Zone, Mountain Mesoamerican

Ateuchini , escarabajos del estiércol, Los Chimalapas, Mesoamericano de Montaña, patrones de distribución, Zona de Transición Mexicana

Ateuchus Weber is a dung beetle genus found in the New World. Ateuchus includes species of small to moderate size, oval and very convex in shape. These species normally have a bidentate clypeus, concealed scutellum, elytra with eight striae, mesosternon largely exposed, and protibia possessing three or four teeth. Most of the species present a certain degree of sexual dimorphism, since the males can present an expanded protibial spur and differences in the clypeal shape (Kohlmann 2003).

The greatest diversity of Ateuchus is concentrated in South America, where more than one hundred species are estimated to occur. However, the taxonomic status of several species that conform this genus is confusing and a review is required. Mexico is one of the countries where the species of the genus Ateuchus are best known: 14 are recorded from the country, A. candezei (Harold), A. carolinae Kohlmann, A. chrysopyge (Bates), A. gershensoni Kohlmann, A. guatemalensis (Bates), A. halffteri Kohlmann, A. hornai (Balthasar), A. illaesum (Harold), A. klugi (Harold), A. laetitiae Kohlmann, A. perezvelai Kohlmann, A. rodriguezi (Predhomme de Borre), A. texanus Robinson and A. tuza Kohlmann and Vaz-de-Mello (Kohlmann and Vaz-de-Mello in press). The objective of this article is to describe two new species of Ateuchus from Mexico and to discuss their possible evolutionary relationships, distributions, and ecology.

Abbreviations for the collections used in this work are as follows:

CEMT Seção de Entomologia da Coleção Zoológica, Departamento de Biologia e Zoologia, Universidade Federal de Mato Grosso, Cuiabá, Brazil

CMNC Canadian Museum of Nature, Gatineau, Quebec, Canada

CNIN Colección Nacional de Insectos, Instituto de Biología, Universidad Nacional Autónoma de México

FSCA Florida State Collection of Arthropods, Gainesville, FL, USA

IEXA Sección G Halffter, Colección Entomológica Miguel Ángel Morón Ríos, Instituto de Ecología, A. C., Xalapa, Veracruz, Mexico

JLSHC JL Sánchez-Huerta Collection, Xalapa, Veracruz, Mexico

TAMU Texas A&M University Insect Collection, TX, USA

VMC V Moctezuma Collection, Xalapa, Mexico

For this study, the phylogenetic species concept was used. The phylogenetic species concept is classified in two distinct versions according to Zachos (2016): the monophyly version and the diagnosability version. We follow the diagnosability version (Wheeler and Platnick 2000), which defines species as the smallest aggregation of populations or lineages that is diagnosable by a unique combination of character states. All type specimens bear determination labels printed on red acid-free paper. The internal sacs and aedeagi were prepared as outlined by Moctezuma et al. (2016) and we followed the sclerite nomenclature proposed by Marchisio and Zunino (2012). Male genitalia were stored in microvials with glycerol. Measurements and pictures were taken using a Leica Z16APOA stereomicroscope and the software of the manufacturer (z-stack image capture method).

The most recent key for identification of Ateuchus species of North America was presented by Kohlmann (1984). We decided not to include a key for identification, since a new key for identification is currently in press (Kohlmann and Vaz-de-Mello in press).

We hypothesized that A. benitojuarezi is closely related to A. guatemalensis and that they represent an evolutionary lineage, since both species present similarities in the general morphology of the sclerites of the internal sac: two hook-shaped and one filiform copulatrix lamellae, and a large canoe-shaped accessory lamella. This combination of shared characters is unique among the known species of North and Central America. Other shared characters of the external morphology are dorsal green sheen, midline impressed on posterior half of pronotum and anterior pronotal margin incomplete. Despite these similarities, A. benitojuarezi can be easily distinguished from its sister species by body size (smaller in guatemalensis, 5.2–6.7 mm), pronotum punctures, clypeal margin and differences in the sclerites of the internal sac of the aedeagus (Kohlmann 2000).

It has not been possible for us to hypothesize about the evolutionary relationships of A. colossus by studying its external morphology. The species of this genus present great morphological homogeneity (Kohlmann 1984). However, the sclerites of the internal sac of the aedeagus of A. colossus present similarities to those of A. illaesum and therefore these two species could be closely related and form part of an evolutionary lineage. Ateuchus colossus possesses notable diagnostic characters that make its identification relatively easy, since it is the largest species of North America and has very acute and apically projected anterior pronotal angles.

In order to determine the evolutionary relationships of the Ateuchus species present in North and Central America, it is necessary to conduct new phylogenetic studies. The most recent study that presents a hypothesis of evolutionary relationships is based on a cladistic analysis using morphological characters (Kohlmann 1984, Kohlmann and Halffter 1988). However, the number of species described in the region has since increased. As a consequence of the morphological homogeneity presented by the genus Ateuchus, future implementation of molecular phylogenetic analysis would be of great interest.

The genus Ateuchus is Neotropical in origin, and is thought to have extended northwards from South America, following the low tropical areas, at the time at which the Panama land bridge was reestablished (Kohlmann and Halffter 1988, Morrone and Halffter 2017). However, the exact period during which the closing of Central America permitted the migration of Ateuchus (Pliocene vs. Middle Miocene), as well as other groups of organisms, is still the subject of debate (Montes et al. 2015, O’Dea et al. 2016). On entering the Mexican Transition Zone, Ateuchus derived in at least four evolutionary lines that coincide with distinct geographic zones: The Pacific slope (halffteri-rodriguezi), Mesoamerica (candezei-laetitiae), the mountains of central Mexico (carolinae-illaesum-klugi) and the Gulf of Mexico coastal plain (histeroides-lecontei-texanus; Kohlmann 1984, Kohlmann and Halffter 1988).

Ateuchus guatemalensis was not included in the cladistic analysis of the Ateuchus species of Mexico (Kohlmann 1984). Kohlmann (1984) states that this species corresponds to the phyletic line of the Pacific. Due to the fact that the lineage of the Pacific lacks the putative synapomorphies of benitojuarezi-guatemalensis (a filiform copulatrix lamellae and a large canoe-shaped accessory lamellae), we hypothesized that they correspond to distinct but closely related lineages.

Ateuchus benitojuarezi (Los Chimalapas, cloud forest and subtropical rainforest) and A. guatemalensis (Sierra Madre de Chiapas and Central American Mountain Nucleus, cloud forest) fit with the Mountain Mesoamerican distribution pattern (Kohlmann 2000, Halffter and Morrone 2017). This distribution pattern corresponds to typical taxa of the mountain forests of Central America and the south of Mexico (cloud and pine-oak forests), which evolved in the Central American Mountain Nucleus and have ancient South American affinities (Halffter and Morrone 2017). Taking its geographic distribution and ecological affinities as a reference, A. colossus also fits with the Mountain Mesoamerican distribution pattern.

Kohlmann and Halffter (1988) state that the distribution of the genus Ateuchus corresponds to the Typical Neotropical distribution pattern, although they recognize a similarity with the distribution of various species with the Mountain Mesoamerican distribution pattern. In that study, these authors argue that the penetration of Ateuchusinto the Central American Mountain Nucleus corresponds to the Pliocene-Pleistocene period. It is not possible to temporally calibrate the cladistic analysis on which they base their conclusions, for which reason this assumption has not been corroborated. It is therefore necessary to conduct molecular phylogenetic or phylogeographic analyses in order to estimate the geologic time in which the penetration and radiation of Ateuchus took place in the Mexican Transition Zone, and thus adequately determine the biogeographic affinities of the species of the genus.

In a preliminary manner, we propose the following species as members of the Mountain Mesoamerican distribution pattern (Kohlmann 1981, 1984, 1996–1997, 2000, 2003, Kohlmann and Halffter 1988): A. carolinae (Trans-Mexican Volcanic Belt and Sierra Madre del Sur; oak forest and tropical dry forest), A. fetteri Kohlmann (Costa Rican Pacific mountains slope, cloud forest and tropical rainforest), A. gershensoni (Sierra Madre de Chiapas, cloud forest), A. ginae Kohlmann (Central American mountains Pacific slope, cloud forest), A. hendrichsi Kohlmann (Costa Rican Pacific mountains, cloud forest and subtropical rainforest), A. illaesum (Sierra Madre Oriental and Gulf of Mexico slope; cloud forest, tropical forest, temperate pine-oak and oak forest), A. klugi (Trans-Mexican Volcanic Belt and Gulf of Mexico slope; tropical forest, temperate pine-oak and oak forest), and A. zoebischi (Costa Rican Atlantic Volcanic slopes, cloud forest and subtropical rainforest).

Little information is available regarding the geographic area of distribution of A. benitojuarezi and A. colossus. To date, these species have only been collected in the region of Los Chimalapas, in the east of the state of Oaxaca. However, given the habitat preferences of these two new species, they may also be distributed in the states of Chiapas and Veracruz. The two new species are found in sympatry with A. candezei, A. rodriguezi and an unidentified species of Ateuchus (unpublished data).

Little information is available regarding the natural history of Ateuchus. They are insects of apparently nocturnal habit, with a broad ecological spectrum. Adults of the genus feed mainly on different types of dung and, to a lesser extent, on carrion, fungi and decomposing plants. Some species present a degree of association with vertebrate burrows and detritus of ants of the genus Atta (Navarrete-Heredia 2001, Kohlmann 2003, Génier 2015). To date, we have only suggested the coprophagous habit of A. benitojuarezi and A. colossus, and a certain degree of attraction to light in A. benitojuarezi, since no collections have been made with carrion-baited traps in the region of origin of the two new species. It is unknown whether these species may also be associated with ant detritus. We have not recorded interactions between these species and the burrows of vertebrates; however, in the region of study, pocket gopher burrows are inhabited by Ateuchus sp.

Finally, we recommend the use of multifocal pictures in order to illustrate the genitalia and external morphology of the species in future descriptions or redescriptions. We consider it important to illustrate the internal sac of the aedeagus, presenting its components dissected and separated, since this allows greater clarity for comparisons and species identifications. As exemplified in this and previous studies (Moctezuma and Halffter 2017, Génier 2015, Génier and Moretto 2017), this technology allows detailed visualization of the structures of the internal sac of the aedeagus as well as the aedeagus itself, facilitating the identification of taxa of great morphological homogeneity, as is the case of Ateuchus.

CONACYT-Mexico awarded a grant for postgraduate studies to V. Moctezuma (No. 412700). We sincerely thank the General Directorate of the Instituto de Ecología, A.C. (No. 20035/30916), the Sectorial Research Fund for Education SEP-CONACYT Mexico (No. 257039) and the Organization for Tropical Studies (Hovore-Horn Fellowship No. 3116) for funding this work. This project was possible with the support of the residents of Los Chimalapas, particularly those of the communities of Benito Juárez, López Portillo, San Antonio and Santa María Chimalapa. We thank K. MacMillan for English language translation and editing. F. Escobar-Hernández (INECOL) helped us with the fieldwork.