Evidence for the evolution of eusociality in stem ants and a systematic revision of † Gerontoformica (Hymenoptera: Formicidae) Author Boudinot, Brendon E. Author Richter, Adrian Author Katzke, Julian Author Chaul, Júlio C. M. Author Keller, Roberto A. Author Economo, Evan P. Author Beutel, Rolf Georg Author Yamamoto, Shûhei text Zoological Journal of the Linnean Society 2022 2022-02-05 195 1355 1389 journal article 121763 10.1093/zoolinnean/zlab097 e4804bc4-8183-435b-8397-3be4c5537bb9 0024-4082 6994456 0BE2F196-846F-4AD2-8C4D-79639B7E409C GENUS † GERONTOFORMICA NEL & PERRAULT, 2004 = † Sphecomyrmodes Engel & Grimaldi, 2005 junior syn.: Barden & Grimaldi (2016) : 518. TAXONOMIC SYNOPSIS OF † GERONTOFORMICA I. Species group of cretacica (newly recognized) ( Charentese amber) [Note 1]: 1. † G. cretacica Nel & Perrault, 2004 nomen dubium (new status) [Note 2] – Type species of † Gerontoformica Nel & Perrault, 2004 2. † G. occidentalis ( Perrichot et al. , 2008 ) II. Species group of gracilis (newly recognized) ( Kachin amber): 3. † G. gracilis ( Barden & Grimaldi, 2014 ) 4. † G. robusta ( Barden & Grimaldi, 2014 ) 5. † G. spiralis ( Barden & Grimaldi, 2014 ) [Note 3] 6. † G. subcuspis ( Barden & Grimaldi, 2014 ) [Note 3] III. Species group of pilosa ( Kachin amber): 7. † G. sternorhabda sp. nov. 8. † G. contega ( Barden & Grimaldi, 2014 ) 9. † G. magna ( Barden & Grimaldi, 2014 ) 10. † G. pilosa ( Barden & Grimaldi, 2014 ) IV. Incertae sedis to species group within genus ( Kachin amber): (11.) † G. orientalis ( Engel & Grimaldi, 2005 ) nomen dubium (new status) [Note 4] – Type species of † Sphecomyrmodes Engel & Grimaldi, 2005 (12.) † G. rugosa ( Barden & Grimaldi, 2014 ) nomen dubium (new status) [Note 5] (13.) † G. tendir ( Barden & Grimaldi, 2014 ) nomen dubium (new status) [Notes 5, 6] Notes on classification Note 1: Under the label of ‘ orientalis group’, Boudinot et al. (2020b) previously united the cretacica and gracilis groups with the species † G. orientalis . The cretacica and gracilis groups are here divided based on the distinct forms of their petioles. The species † G. orientalis is of insufficient preservation for specieslevel identification (see Note 4 below). Note 2: The holotype of † Gerontoformica cretacica is poorly preserved, as noted by Barden & Grimaldi (2016) , with obvious distortions and elongation of the scapes and flagella, and little detail of the body visible. Due to this unfortunate circumstance, we newly consider † G. cretacica to be a nomen dubium . It remains in the cretacica species group of † Gerontoformica , because it is the type species of the genus. However, it will be highly desirable to clarify the identity of this species through examination and description of more Charentese ants. At present, the only distinction between † G. cretacica and the co-eval † G. occidentalis that is not apparently affected by preservation is body size, with the former having a longer mesosoma than the latter ( 2.1 mm vs. 1.4 mm , respectively). Body size is, of course, highly variable among nestmates of crown ants, thus is a weak diagnostic trait when used in isolation and without the quantification of variation across conspecific individuals. Note 3: † Gerontoformica spiralis and † G. subcuspis were difficult to separate in the present study based on the available anatomical evidence and may be conspecific. Specifically, we observe that, in addition to conditions outlined in the key (see that section below), the two species are highly similar in the following conditions, which we only roughly characterize here: (1) proportions and fine details of the head, including frontal carina shape; (2) the degree of mesonotal, metanotal and propodeal convexity; (3) the width of separation between the meso- and metanota plus metanotum and propodeum; (4) the shape and proportions of the petiolar node; and (5) the form of the third abdominal (first ‘gastral’) segment. It is possible that † G. spiralis and † G. subcuspis represent the smaller and larger ranges of body size of a single species, with the former reported to have a total body length of 4.22–5.22 mm and the latter 5.35–5.76 mm ( Barden & Grimaldi, 2014 ). Our focal uncertainties relate to the shapes and setational patterns of the tarsi of † G. subcuspis , and the form of the subpetiolar process and prora of † G. spiralis . A potential feature separating the two species is the distance of the toruli from the posterior clypeal margin, which appears to be narrower in † G. spiralis relative to † G. subcuspis , but this could be a visual artefact caused by the apparent light distortion in the holotype image of † G. spiralis . We recommend further re-evaluation of these two species, ideally using µ-CT and additional light photography to resolve the uncertainties of the tarsi, toruli and sternal processes of the metasoma. See also Note 1 on the diagnosis of † G. gracilis . Note 4: † Gerontoformica orientalis , the type species of † Sphecomyrmodes , is identifiable as † Gerontoformica relative to † Sphecomyrma Wilson & Brown, 1967 by the absence of the anteromedian clypeal process and presence of traction setae/ chaetae along the anterior clypeal margin, as recognized in the original description ( Engel & Grimaldi, 2005 ) and illustrated in Boudinot et al. (2020b) . However, in † Gerontoformica , the species † G. orientalis is unidentifiable due to poor preservation. No details of the head are clearly observable, except for the antenna, mandibles and anterior clypeal margin, while the mesosoma appears strongly distorted, the petiole is obscured, and the metasoma is mostly disarticulated. Boudinot et al. (2020b) placed † G. orientalis in the orientalis species group along with the type species of † Gerontoformica based on the absence of the cinctus of abdominal segment IV. As a more refined placement is not possible, we newly consider this species to be a nomen dubium . Note 5: † Gerontoformica rugosa and † G. tendir are newly considered as nomina dubia due to their poor preservation, being strongly desiccated and thus distorted. Both species appear to have some degree of abdominal segment III petiolation, as observed in the three confirmed members of the pilosa group, but it cannot be determined whether this is natural or exaggerated due to preservation. It is possible, but not yet determinable, that † G. rugosa is conspecific with † G. gracilis . That † G. rugosa or † G. tendir do have sculptured integument remains possible but requires substantiation via additional material of these species. We note that little surface texture variation has been explicitly documented among stem ants thus far. Note 6: † Gerontoformica tendir was defined by Barden & Grimaldi (2014) as having a medial clypeal lobe. This anteromedian lobe not only bears traction setae/chaetae, as previously observed, but is also lateromedially broader and proximodistally shorter than that of † Sphecomyrma . Given the poor preservation, it is possible that the apparent lobate form may be due to distortion of the amber matrix, as the lobe consists of the entire medial portion of the clypeus, which is distinct from the lateral clypeal lobes. Based on direct examination of the holotype , it appears that there is a transverse mesonotal carina in † G. tendir , but this also requires re-evaluation. Without additional specimens having an exaggerated and broadly, medially lobate clypeus, we remain uncertain about the identity of the species. A state of potential value for confirming the identity of † G. tendir from additional material is the absence of teeth on the pretarsal claws, as illustrated by Barden & Grimaldi (2014) . Remarks The genus † Gerontoformica currently consists of 13 species, including the newly described † G. sternorhabda . The holotypes of nine of these species are sufficiently preserved for species-level identification, with eight of these being sufficiently defined given the potential synonymy of two (see Note 3 above). At the generic level, † Gerontoformica differs from † Sphecomyrma by presence of traction setae/chaetae along the anterior clypeal margin and absence of a narrow anteromedian clypeal lobe. These two conditions were used by Engel & Grimaldi (2005) to establish the genus † Sphecomyrmodes , which was synonymized under † Gerontoformica by Barden & Grimaldi (2016) after examination of the holotype of the type species of the latter taxon. Collectively, † Gerontoformica and † Sphecomyrmodes have been revised piecemeal after the former’s establishment by Nel et al. (2004) . Specifically, Barden & Grimaldi (2014) added nine species of † Sphecomyrmodes , Barden & Grimaldi (2016) transferred all species of † Sphecomyrmodes to † Gerontoformica , and Boudinot et al. (2020b) moved one species to † Myanmyrma and recognized two morphologically defined groups of species in the genus. To understand the shifting boundaries of † Gerontoformica in the light of the present µ-CT-driven study, we detail the species-level history of the genus. Species attributed to the genus are distributed in Kachin amber (11 total) and Charentese amber (two total). The Charentese species, † G. cretacica and † G. occidentalis , were described four years apart by Nel et al. (2004) and Perrichot et al. (2008) , respectively. Perrichot et al . described the smaller-bodied † G. occidentalis as † Sphecomyrmodes in comparison to the type species of that genus – † G. orientalis from Kachin amber – without comparison to † G. cretacica . Unfortunately, given the current status of morphological knowledge, the holotypes of both † G. orientalis and † G. cretacica are too poorly preserved to allow for confident specieslevel identification. However, both Charentese species uniquely share a distinct, nearly squamiform petiolar shape, with the node being anteroposteriorly narrow and dorsoventrally tall; this indicates that the Charentese species are closely related to one another, relative to the Kachin species. For this reason, we group † G. cretacica and † G. occidentalis together in the cretacica species group. It is possible that † G. cretacica and † G. occidentalis are conspecific, but any taxonomic action should wait for the accumulation and processing of more material from the Charentese formation. The identifiable Kachin species of † Gerontoformica , i.e. excluding † G. orientalis , † G. rugosa and † G. tendir , are evenly distributed in the G. gracilis and G. pilosa groups, with four species each. All species of the pilosa group are highly distinct in body form and setation, with † G. sternorhabda being an outlier, having the smallest body size and least pronounced constriction of the fourth abdominal segment, in addition to other defining features (see the species diagnosis below). In contrast, species of the newly recognized gracilis group are all largely similar to one another, without easily recognizable diagnostic structures. The most distinct Kachin species of the gracilis group is † G. robusta , which has a boxy mesosoma, with the meso- and metanota forming a nearly linear dorsal margin in lateral view. The other species of the gracilis group have a multi-humped profile due to the bulgelike development of the meso- and metanota and are otherwise similar to one another (see Note 1 on the diagnosis of † G. gracilis ).