Pheidole comprises approximately 1,000 extant species distributed worldwide, being particularly diverse in the New World. In addition to its high diversity and ecological prevalence, the genus is also characterized by the predominantly intraspecific dimorphism, with major and minor workers. Currently, five fossil species are known, all of which are represented only by minor workers. A new species, †Pheidole anticua sp. nov., is described from Dominican amber, based on a major worker. Additionally, the identity of the currently known fossil species in Pheidole is discussed and †P. cordata from Baltic amber is considered as incertae sedis, resulting in no Pheidole species currently recognized for Baltic amber

Miocene, morphological diversity, new status, taxonomy

Pheidole Westwood 1839 is the largest myrmicine ant genus with 1,047 species worldwide (Bolton 2019). Species in this genus are generally characterized by conspicuous dimorphism, with major and minor workers. Currently, five fossil species of Pheidole are known: †Pheidole primigenia Baroni Urbani, 1995 and †Pheidole tethepa Wilson, 1985 from Dominican amber (Early Miocene) dated from 16–19 mya (Seyfullah et al. 2018); †Pheidole tertiaria Carpenter, 1930 based on compression fossils from the Florissant in Colorado (Late Eocene) dating from 34.07 ± 10 mya (Evanoff et al. 2001); †Pheidole rasnitsyni Dubovikoff, 2011 originally described as a Baltic amber fossil, but now recognized as a copal inclusion (Perkovsky 2016); and †Pheidole cordata (Holl 1829) described from the Baltic amber (Late Eocene) dating from 34–48 mya (Seyfullah et al. 2018). All fossil records mentioned for Pheidole so far are exclusively represented by minor workers. Here we describe a new species of Pheidole for the Dominican amber based on a major worker. We also propose changes to the status of the other fossil species in the genus.

The studied inclusion was originally immersed in a 26 × 14 × 14 mm, orange, oval Dominican amber piece with a fragmentary specimen of Psocoptera as a syninclusion, which was lost after treatment of the stone. This piece was faceted and polished for better visualization using increasingly finer sandpapers and, lastly, liquid silver polishing on a soft, clean, and dry cloth. The specimen was bought from the eBay store “ambergalleryboutique1” in July 2017. The seller confirmed that the specimen was mined in “La Toca” site. The specimen had the morphospecies code “Pheidole ufv-65” from 2017 to 2019 on Antweb.

The holotype is deposited at the Padre Jesus Santiago Moure Entomological Collection of the Universidade Federal do Paraná, Curitiba, Brazil (DZUP). Observations were made at 80× magnification with a Zeiss SteREO Discovery.V8 dissecting microscope. Measurements were made with a dual-axis micrometer stage with output in increments of 0.001 mm. All measurements are given in mm. The high-resolution images were made with an Axiocam 305 color camera coupled to a Zeiss SteREO Discovery.V20. Extended depth focus was made with Zen Blue v.2.3 and subsequently treated to correct for brightness and contrast. Digital vectorization was based on original photographs.

We adopted morphological terminology and measurements proposed by Longino (2009) and sculpture terminology by Harris (1979).

†Pheidole anticua is the first fossil species of the genus for which the major worker is described. In Wilson’s (2003) monograph 22 undescribed Pheidole fossil specimens are cited, including majors and minors, most of which are deposited in private collections. In the same work is an image of a major worker from the Dominican amber owned by Elizabeth J. Romans and photographed by Frank M. Carpenter (his fig. 6 on p. 11). However, it is impossible to confirm if this specimen is the same as †P. anticua, due to low image resolution and not direct comparison.

Castes and subcastes pose a greater challenge to palaeomyrmecology than they do to the alpha-taxonomy of modern taxa. While discussing the taxonomy of extant groups one should refrain from describing a species based on a particular caste. The same procedure is advised for the description of fossil taxa, with specimens sometimes only tentatively associated to a given species (e.g., Baroni Urbani and de Andrade 2003). However, this is not always possible due to the rarity of the material and the difficulty of finding conspecific specimens in the same deposit (e.g., Pachycondyla oligocenica Dlussky et al. 2015, described from only a male specimen). Pheidole species yet to be discovered in the Dominican amber and from other relatively young New World deposits will most likely suffer from limitations in caste association. We encourage descriptions of majors and minors of Pheidole, but we do not recommend descriptions based on males and queens, which will potentially be conspecifics of species already described. Informal descriptions (e.g., “Pheidole sp.” of Baroni Urbani 1995, p.12) are highly recommended, as these will add to the knowledge of fossil fauna diversity without artificially inflating the genus taxonomically.

The fossil species †P. primigenia and †P. tethepa are unique among New World Pheidole for having pronotal humeral spines (Wilson 1985; Baroni Urbani 1995), a trait never found in the extant members of the genus in this region. While Wilson (1985) suggested convergent evolution of pronotal spinescence in the Neotropics (and questioned the placement of †P. tethepa within Pheidole), Baroni Urbani (1995) concluded a relationship between †P. primigenia and †P. tethepa to Old World spinescent Pheidole lineages was more likely. Recent molecular phylogenies have shown that four extant Old-World lineages (aristotelis, quadricuspis, quadrispinosa, and bifurcata clades) have independently evolved pronotal spinescence (Sarnat et al. 2017). This suggests that this trait, although uncommon in the genus as a whole, has arisen repeatedly a sufficient number of times to justify Wilson’s hypothesis of convergent evolution of spinescence in the Neotropics.

Among the extinct species of Pheidole, the most dubious fossil is †P. cordata. Its first record in the literature is Schweigger (1819). In this work, the author listed fossils from Baltic amber and described informally and illustrated an ant with a remarkably large head, showing triangular projections on the propodeum. These projections can be interpreted as propodeal spines or teeth. However, Schweigger did not name this specimen, and some years later, Holl (1829: 140) named it as †Formica cordata, using the same characters as Schweigger.

Mayr (1868) transferred it to Pheidole (Mayr 1868), even though he believed that Schweigger’s sketch was not clear and Holl’s description was somewhat crude. We conclude that Holl’s decision to describe this species and Mayr’s placement in Pheidole may have been hasty. The specimen studied by Schweigger is presumably lost, which precludes its proper placement using current genus concepts in Formicidae (Mayr 1868; Antweb 2019). Dlussky (2008) suggested treating †Formica cordata as Formicidae incertae sedis, and we concur that there is no strong reason to assume it belongs to Pheidole, though it is certainly a myrmicine ant. Thus, we consider †P. cordata as incertae sedis in Myrmicinae.

Among the fossil ant genera known from Baltic amber, at least two can be associated with †P. cordata: †Stiphromyrmex Wheeler and Aphaenogaster Mayr. Both genera are morphologically very close to Pheidole and are characterized by an enlarged head, 12-segmented antenna with a club of three segments, presence of propodeal spines, and a two-segmented waist (Mackay and Mackay 2002; Radchenko and Dlussky 2017).

Pheidole was inferred to have originated in the Neotropics at 58 mya with a single colonization in the Old World around 20 mya (Moreau 2008; Economo et al. 2015). Therefore, the presence of a Pheidole species in Baltic amber would imply an unexpectedly early dispersal of the genus to the Old World, in the Eocene or earlier. It is also important to highlight that, except for †P. cordata, no Pheidole species are currently recognized for the Baltic amber, which corroborates the current hypothesis of a New World diversification of the genus with a single event of colonization in the Old Word. In this scenario, we consider †P. cordata cannot be safely assigned to any known genus, mostly due to the poor knowledge about its morphology. All definitive fossil records for Pheidole are restricted to the New World (Table 1).

A question remains regarding the identity of the fossil species Pheidole rasnitsyni. Dubovikoff (2011) proposed Pheidole rasnitsyni from pieces assumed to be truly Baltic amber, but he latter informed Perkovsky (2016: 117) that they were actually copal. Considering the putative young age of this inclusion (< 1 mya), it is possible that P. rasnitsyni is a junior synonym of a modern species. To accurately ensure the identity of this species, direct comparison with extant species would be necessary. However, in addition to the traditional limitations in observing details of morphology in resin inclusions, a second problem is that P. rasnitsyni is known only from minor workers. This makes it extremely difficult to determine the relationship between P. rasnitsyni and other extant species, since the morphology of minor workers in Pheidole is extremely conserved, especially in the Palaearctic species. In this scenario, although we think that P. rasnitsyni clearly belongs to Pheidole, we encourage a careful analysis of its identity in the future.

Thanks to Georg Fischer (OIST) and Julian Katzke (OIST) for their help to translate the main information in Schweigger (1819) and Mayr (1868). We are deeply indebted to Gabriel Melo, John Lattke, Eli Sarnat, and Jack Longino for the critical reading of a previous version of this manuscript. We also acknowledge the Brazilian Council of Research and Scientific Development (CNPq) for the support provided to ACF (grant 140260/2016-1), JCMC (grant 130779/2016-4), and RMF (grant 302462/2016-3).