Deltochilini Lacordaire, 1856

Tribe Deltochilini Lacordaire, 1856 View in CoL

A taxonomic history of ‘ Canthon sensu lato’

The taxonomic history of Sylvicanthon is intimately associated with that of Canthon and allied genera. Thereby, a full understanding of the context within which the genus was established by Halffter & Martínez (1977) is essential to any person interested in the taxonomy of Sylvicanthon , especially in the light of recent publications questioning the validity of the genus ( Ratcliffe 2002; Solís & Kohlmann 2012). As the last revision of this history was published over 50 years ago ( Halffter 1961; Halffter & Martínez 1968) and great taxonomic activity has occurred since then, we present here a brief discussion on the taxonomic history of Canthon and allied groups (hereafter referred as ‘ Canthon sensu lato’ 10) with the dual goal of presenting a historical background to the description of Sylvicanthon by Halffter & Martínez (1977), and providing some basic information that shall help future taxonomic works in Deltochilini .

In his 1817 revision of the insects described by the French entomologist Pierre André Latreille (1762– 1833) ( Fig. 1B View Fig ) in the work of Humboldt & Bonpland (1805 –1810), the German naturalist Johann Centurius von Hoffmannsegg (1766–1849) ( Fig. 1A View Fig ) proposed the genus Canthon for Ateuchus septemmaculatus Latreille, 1807 and 14 other species not cited by Latreille (1807). The type species of Canthon , Scarabaeus pilularius Linnaeus, 1758 , would be designated only 120 years later by Paulian (1939: 22). Probably unaware of the description of Hoffmannsegg (1817), Latreille (1829) established the new genus Coprobius to accomodate several species that had already been transferred to Canthon by Hoffmannsegg. Even so, Latreille’s new nominal genus was adopted by contemporary French entomologists such as Brullé (1838), Reiche (1841) and Dejean (1833 –1836, 1836–1837). The German author Jacob Sturm (1843), in the second edition of the catalogue of his beetle collection, was the first to recognize the synonymy between Canthon and Coprobius , but, interestingly enough, he used the second name as valid and indicated the first edition of his own catalogue (‘St. Cat.’; Sturm, 1826) as the original publication of Canthon . Nonetheless, by consulting Sturm (1826), we see that in that work the authorship of Canthon (there considered a valid name) was attributed to the German entomologist Johann Karl Wilhelm Illiger (‘Illig.’) and not considered a new genus. As far as we could find, Illiger never cited the name Canthon in his own works, neither before nor after Hoffmannsegg’s description. Finally, Castelnau (1840) also recognized the synonymy between Canthon and Coprobius , but, contrary to Sturm (1843), he was the first to consider Canthon as the valid name in this synonymy, a decision that, with the notorious exception of Burmeister (1874), has been followed until the present day. A second name that is currently considered a junior synonym of Canthon , ‘ Coeloscelis ’, was proposed by Reiche (1841) to accommodate only ‘ C. coriaceus Dej. ’. Erichson (1847) synonymized Canthon and Coeloscelis , but Lacordaire (1856) returned using Coeloscelis as valid and transferred some further species to this genus. The definitive synonymy would only come with Harold (1868a), in the first great comprehensive revision of the genus Canthon .

One of the greatest specialists of Scarabaeinae in the 19 th century, the German entomologist Edgar von Harold (1830–1886) ( Fig. 2 View Fig ) was responsible for some landmark revisions of genera with a large number of species and a very complex taxonomy which are still fundamental to our understanding of the diversity of New World dung beetles. Among those works are Harold’s revisons of Canthidium Erichson, 1847 ( Harold 1867) , Ateuchus Weber, 1801 (cited as Choeridium Lepeletier de Saint-Fargeau & Audinet-Serville, 1828; Harold 1868b) and Canthon ( Harold 1868a) . In this latter work, Harold recognized Canthon as the senior synonym of Coprobius , Coeloscelis , and Tetraechma Blanchard, 1846 , and being composed of 97 species – 46 of them new – distributed throughout the American continent. Twenty-three other nominal species were listed as of unknown assignment, the majority of them described by French entomologists such as Blanchard, Lucas, Castelnau and Guérin-Méneville, and the American LeConte, and whose type material Harold (1868a) did not have access to. Harold (1868a) also redescribed Canthon , discussed in fine detail the morphological variation observed in the genus, compared it to other taxa considered as close relatives such as Deltochilum , Anachalcos Hope, 1837 and Megathopa Eschscholtz, 1822 , and, perhaps his most important contribution, presented the first identification key to the species of Canthon . The following year, after examining some type material deposited in the MNHN, Harold (1869c) presented new information about some of the species considered as unknown to him in 1868.

During the second half of the 19 th century and the early 20 th century, the number of species in Canthon continued to grow. In the catalogue of Harold (1869b), 123 species were included in the genus, while that number rose to 144 in the catalogue of Gillet (1911). In the interwar period, Adolf Schmidt (1856–1923) ( Fig. 3A View Fig ), a German specialist in Aphodiinae , devoted himself to the elaboration of a second revision of Canthon , the only scarabaeine genus on which he has ever published (Vaz-de-Mello & Cupello in press). In his first work, Schmidt (1920) described 26 new species-group taxa in Canthon . Shortly after, Schmidt (1922) proposed 27 additional new species and presented an updated identification key to 143 species of Canthon (not including 20 names he did not know how to apply to species taxa).

With the description of 16 additional new species, the Czech entomologist Vladimír Balthasar (1897– 1978) ( Fig. 3B View Fig ) updated Schmidt’s key in 1939 expanding it to encompass 162 species; 23 other names were not included by being of unkown application to species taxa.As Balthasar (1939) himself recognized several times throughout his text and even in its title (“ Eine Vorstudie zur Monographie der Gattung Canthon ”, or “ A Preliminary Sudy Towards a Monograph of the Genus Canthon ”), his modifications were only a first draft towards a future revision of Canthon , a genus that, in his opinion, was extremely heterogeneous and ill defined. In the future, Balthasar (1939) believed, with a deeper analysis of the relationships between its species, groups of species and their morphological diversity, Canthon would inevitably be divided into several other genera and subgenera (e.g., on page 236, where he wrote “Die ganze Gattung [ Canthon ] scheint sehr heterogen zu sein und ich bin mir dessen gut bewusst, dass beim näheren Studium sich als notwendig zeigen wird, mindestens einige Arten als selbständige Gattungen abzugliedern”).

In fact, this process of dividing Canthon into small, peripheral genera had already been started in the 19 th century. As examples, we can cite the description of Tetraechma Blanchard, 1846 , Pseudocanthon Bates, 1887 , Agamopus Bates, 1889 and, later on in the early 20 th century, Zonocopris Arrow, 1932 , Plesiocanthon Gillet, 1933 (currently a junior synonym of Zonocopris ) and Canthochilum Chapin, 1934 . Furthermore, new species that, in the past, would probably have been positioned in Canthon , were being described in new monotypic or oligodiverse genera such as Sinapisoma Boucomont, 1928 , Canthonella Chapin, 1930 , Canthonidia Paulian, 1939 , Canthotrypes Paulian, 1939 , Paracanthon Balthasar, 1938 and Cryptocanthon Balthasar, 1942 . Even so, the situation continued to be one of extreme chaos. On the one hand, there were disagreements over the validity of some of those names ( Pseudocanthon and Tetraechma , for instance, were considered junior synonyms of Canthon by most authors almost until the second half of the 20 th century). On the other hand, the newly-proposed divisions helped little to reduce the great heterogeneity and artificiality of Canthon , since this genus continued to contain almost 200 species, while the other small genera were composed of at most three species.

The 1940s–1970s represented, however, a period of intense changes in that panorama thanks to the joint effort of three Latin American entomologists: the Brazilian priest Father Francisco Silvério Pereira (1913–1991) ( Fig. 4A View Fig ), the Argentinian Antonio Martínez (1922–1993) ( Fig. 4B View Fig ) and the Spanish-born Mexican Gonzalo Halffter (b. 1932) ( Fig. 4C View Fig ). In a series of taxonomic works (among them, we highlight: Pereira 1946, 1949, 1953; Martínez 1948 a, 1948b, 1950, 1952, 1953; Martínez & Pereira 1956; Halffter, 1958, 1961; Pereira & Martínez 1956, 1959, 1960; Martínez et al. 1964; Vulcano & Pereira 1964, 1966; Martínez & Halffter 1972), culminating in the four-part major revision of the New World ‘Canthonina’ by Halffter & Martínez (1966, 1967, 1968, 1977), new species were described and the genus Canthon was divided into a large number of taxa (sometimes categorized as its subgenera, sometimes as full genera). Halffter & Martínez (1977), in the final part of their series of revisions, recognized 28 valid genera, with the genus Canthon itself divided into nine subgenera and its nominotypical subgenus into 22 groups or lineages (‘ líneas ’) of species.

Therefore, as discussed so far, the historical tendency in the taxonomy of ‘ Canthon sensu lato’, especially in the second half of the 20 th century, has been the division of the former concept of the genus Canthon into several small, peripherical subgenera and genera, a classification that, in principle, attempts to reflect the phylogeny of the group. Nevertheless, as argued by Solís & Kohlmann (2002, 2012) and Medina et al. (2003), that goal, in reality, has not been fully met, something recognized even by both Martínez (1987), who considered Canthon an “omnibus genus”, and Halffter (2003). As shown by Solís & Kohlmann (2002, 2012) and Medina et al. (2003), several of the characteristics used to define supraspecific taxa do not have an adequate phylogenetic signal as to indicate old relationships. These features (e.g., shape of meso- and metatarsi, eyes and clypeus) have apparently been subject to strong action from directional natural and sexual selection and, hence, are in constant change, passing through convergences, parallelisms and reversions throughout the evolutionary history of the group. Having been based on such plastic characteristics, the classification proposed by Halffter & Martínez (1977) is largely artificial 12.

In an exploratory cladistic analysis of ‘ Canthon sensu lato’ based on 39 species and 83 characters, Medina et al. (2003) found that Canthon , as defined by Halffter & Martínez (1977), is highly artificial, with several other genera such as Sylvicanthon , Scybalocanthon Martínez, 1948 , Anisocanthon Martínez & Pereira, 1956 and Melanocanthon Halffter, 1958 , nested within it. Based on those results, Solís & Kohlmann (2002, 2012) proposed the rejection, at least temporarily, of a large part of the generic and subgeneric names and that their species be referred to Canthon . This latter name would then represent only a miscellany of not-particularly-closely-related species waiting for a revision that could form natural groups (i.e., monophyletic sensu Hennig, 1966) based on more robust and stable suite of characters having stronger phylogenetic information.

Although we certainly agree with the diagnosis presented by Solís & Kohlmann (2002, 2012), we disagree on the temporary solution put forward by them. At the moment, the most urgent concern about ‘ Canthon sensu lato’ is, in our view, the alpha taxonomy, since it is crystal clear that we still have a very limited knowledge of its species diversity. Three revealing examples are the revision of the Mexican Glaphyrocanthon by Rivera-Cervantes & Halffter (1999), which described 10 new species and hence doubled the number of known representatives of this group in that country, the revision of Hansreia Halffter & Martínez, 1977 by Valois et al. (2015), which added five species to a genus considered monotypic for almost 40 years, and the present work, which triples the number of species in Sylvicanthon (including the description of six new species).

If we gather again under the same name ‘ Canthon ’ the cohesive and easily identifiable groups of species (i.e., genera and subgnera) delimited during the second half of the 20 th century (such as Sylvicanthon ), we would have once more an overly inflated and heterogeneous genus. Naturally, this situation would inhibit the start of any taxonomic revision with the group, as it had already occurred during the 19 th and the early 20 th centuries and, in fact, still occurs with Canthon s. str. Moreover, contrary to what was written by Solís & Kohlmann (2002), the results obtained by Medina et al. (2003) do not tell us that the genera peripheral to Canthon (e.g., Sylvicanthon ) are artificial (i.e., whether they are polyphyletic or paraphyletic). In fact, for the most part, this would be impossible to ascertain, since only a single species of each group was included in the analysis ( Medina et al. (2003: 59) indeed recognized that: “[...] only a few species from the other genera were included in the analysis [...], so we cannot speculate as to whether genera such as Anisocanthon , Melanocanthon , or Sylvicanthon are natural species groups”). On the other hand, except for the genus Canthon as a whole and Glaphyrocanthon in particular, the other two groups of genus/subgenus rank that had more than one species included in the analysis, Francmonrosia Pereira & Martínez, 1959 and Scybalocanthon , appeared monophyletic.

The major problem, thus, seems to be the delimitation of Canthon (especially Canthon s. str. and Glaphyrocanthon ) and not necessarily that of its closely related taxa. Therefore, the synonymy of Sylvicanthon and other genera and subgenera with Canthon as proposed by Solís & Kohlmann (2012) and Ratcliffe (2002) seems to be overly conservative and would only bring more uncertainty to our understanding of the diversity of “ Canthon sensu lato ”. Future phylogenetic analyses based on both morphological and molecular data will lead us closer to a natural classification, but this will only be possible with a deep understanding of the species and morphological diversity of “ Canthon sensu lato ”. For now, we should follow a bottom-up approach; i.e., from the delimitation of closely related species to the recognition of their slightly more distant phylogenetic affinities (something already suggested by Solís & Kohlmann, 2002: 3). Hence, the formation of easily identifiable sets of species with similar morphology is of great utility and practicality, even if these groups are not monophyletic. The next step, at last, will be of unravelling the deeper and older relationships between those species. When we finally have a sound understanding of their phylogeny, it will be time to propose a new classification reflecting it, one that will leave no room for non-monophyletic groups.

Accordingly, here we follow Halffter & Martínez (1977) and consider valid Sylvicanthon and all the other names categorized by them as genera and subgenera. Despite that, we do believe that the taxa considered by them as subgenera of Canthon will eventually turn out to be independent genera, since there is no synapomorphy nor clear diagnostic characteristic bringing them together, and there are evident affinities between some of the subgenera of Canthon with other genera of “ Canthon sensu lato ” (e.g., between Sylvicanthon and at least some groups of Glaphyrocanthon Martínez, 1948 ). See Table 1 View Table 1 for information on the genus-group names proposed throughout the taxonomic history of ‘ Canthon sensu lato’.