Proleptothorax

The diagnostic characters of Proleptothorax are obvious plesiomorphies, and there are no apomorphic features compared with genera of the Leptothorax genus group, i.e., Leptothorax, Harpagoxenus, and nonergatoid Formicoxenus. Moreover, the time when Proleptothorax existed is much older than the estimated time of separation of the genera Leptothorax, Harpagoxenus, and Formicoxenus but younger than the divergence of the Leptothorax genus group and Temnothorax. Consequently, we assume that Proleptothorax might be considered a putative ancestor of the genera of the Leptothorax genus-group but not of Temnothorax.

Identification
Males of Proleptothorax have 13-segmented antennae with a filiform funiculus without an apical club, a very short antennal scape, short and narrow, bidentate mandibles, and, which is very important, a closed cell 3r on the forewing. Interestingly, the second funicular segment in Proleptothorax is not the longest and is subequal to the third one (although distinctly longer than the scape), while the longest segments are 5th, 6th and 7th. The differences in structure of the basal funicular segments between Leptothorax genus group, i.e., Leptothorax, Harpagoxenus, and nonergatoid Formicoxenus, and Proleptothorax can be explained by the fusion of the 2nd and 3rd segments in the genera of Leptothorax genus group, which makes the 2nd segment the longest. A similar fusion of the second and third funicular segments was noted in ants from different subfamilies, for example in males of some species of Leptomyrmex (Dolichoderinae) and in males of Tetramorium and Strongylognathus (Myrmicinae) (Dlussky & al. 2014, Radchenko 2016). On the other hand, Proleptothorax males have 5-segmented maxillary palps and 3-segmented labial palps. These features as well as the general shape and structure of the head, mesosoma and waist, and the presence of the notauli fully match the diagnostic characters of males of the Leptothorax genus group.

It should be noted that the short and narrow bidentate mandibles in all castes are a plesiomorphy in the family Formicidae (Wilson & al. 1967, Dlussky 1983, Dlussky & Rasnitsyn 2007), and a closed cell 3r on the forewing is a plesiomorphy in the subfamily Myrmicinae (Dlussky & Radchenko 2009, Radchenko & Dlussky 2013).

Distribution
This taxon is known from.

Nomenclature

 * † PROLEPTOTHORAX [Myrmicinae: Crematogastrini]
 * †Proleptothorax Radchenko et al., 2018: 113. Type-species: †Proleptothorax primitivus, by original designation.

Diagnosis:
 * antennae 13-segmented
 * antennal scape very short (SL / HL 0.16, SL / HW 0.18)
 * antennal funiculus filiform, without apical club
 * length of the second funicular segment subequal to the third one, while distinctly longer than scape, the longest segments are 5th, 6th and 7th
 * mandibles short and narrow, bidentate
 * forewing with cell 3r closed by vein 5RS
 * maxillary palps 5-segmented, labial palps 3-segmented
 * scutum with deeply impressed and crenulated notauli

Etymology
From Greek “pro” – before, prior to, and the ant genus Leptothorax.