FORMICIDAE (ANTS)

Figures 9 A−C

AMNH WH-1: A single worker ant specimen, approximately 1.7 mm total body length (excluding antennae), attributable to the subfamily Formicinae. The acidopore, a primary diagnostic feature of formicine ants, appears faintly visible as a circular opening at the terminus of the abdomen. The petiole—a waistlike segment separating the trunk and gaster—is conspicuously scale shaped, with its dorsal margin reaching the same approximate height as the propodeum, a syndrome found in multiple extant and fossil formicine genera. Specimen WH-1 is heavily desiccated and partially disarticulated, making precise placement difficult. Nevertheless, the specimen clearly does not fit into any currently described genus of formicine, in particular due to its widely spaced mandibular dentition and shortened antennal scape (figs. 9B, C). The isolated antenna of another ant specimen, in piece GC-A4 (fig. 14E), has different segmental proportions than the formicine above, indicating the existence of another ant species in this amber. Very little can be determined taxonomically on the basis of an antenna.

The subfamily Formicinae is presently distributed worldwide, and fossils are similarly cosmopolitan. In total, 196 fossil formicine species are described, comprising 43 genera from 44 localities across North America, Europe, Asia, and New Zealand (Barden, 2017). Potentially a result of high sampling bias (nearly 13,000 ant inclusions were utilized in a recent analysis of ant species richness [Penney and Preziosi, 2014]), nearly 40 formicine species are described from Baltic amber, the greatest of any deposit. The Chickaloon amber species has not yet been formally described, however, it may represent a stem relative of the primarily Palearctic and Nearctic tribe Formicini or the cosmopolitan tribe Lasiini, based on current understanding of relationships (e.g., Lapolla et al., 2010; Ward et al., 2016)—both represented in the fossil record, including in Baltic amber. This preliminary diagnosis is based on the large circular propodeal spiracle, gradually sloping mesonotum, 5:4 palpomere formula, and mandibular shape. Interestingly, this new specimen is contemporaneous with formicine ants described from Fushun amber (Hong, 2002), making it, along with its counterparts in Asia, the oldest formicines known following Kyromyrma neffi in Turonian-aged New Jersey amber (Grimaldi and Agosti, 2000). This is a valuable window into ant evolution as the Fushun amber holotypes have since been lost. Slightly younger ants (Dolichoderinae, Myrmecinae, Formicinae, and Myrmeciinae) are reported in mid to Late Eocene shales and Hat Creek amber from British Columbia (Archibald et al., 2018). It should be noted that the identification of Technomyrmex (Dolichoderinae) in Poinar et al. (1999) was changed to Formicinae incertae sedis in Archibald et al. (2018).