Dissimulodorylus perseus Sosiak, Borowiec, & Barden, 2022

Sosiak, Christine E., Borowiec, Marek L. & Barden, Phillip, 2022, An Eocene army ant, Biology Letters (20220398) 18 (11), pp. 1-5 : 2-3

Dissimulodorylus perseus Sosiak, Borowiec, & Barden sp. nov.

Diagnosis: as for genus.

Description: Head: Antennae 11-segmented. Scape 0.48 mm; pedicel roughly twice the length of following segment. Apical antennal segment (as far as it is visible) looks broadly rounded, moderately enlarged relative to others but not a full club. Length of funiculus 1.01 mm. Head capsule rectangular, longer than wide (length of capsule 0.83 mm); breadth of capsule broadly tapers towards oral opening. Ventrolateral margins of head without ridge or lamella. Eyes absent; ocelli absent. Lacking antennal scrobes or frontal carinae; antennal insertions set closely together. Frontal lobes fused and projecting into a triangular structure overhanging the clypeus; clypeus not visible. Lateroclypeal teeth and parafrontal ridges absent or not discernible. Mandibles falcate: elongate and narrow, lacking teeth and tapering midway suddenly towards sharp apex. Cuticle shining with sparsely punctate sculpturing; scattered short and broad setae over dorsal surface; some setae appear apically spatulate. Cuticular colour medium brown, though may be affected by preservation.

Mesosoma: Weber’ s length 1.21 mm. Pronotal flange not separated from collar by distinct ridge. Pronotomesopleural suture complete, continuous with promesonotal suture. Enlarged propleuron in lateral view. Mesonotum is reduced to small flanged sclerite angled upwards anteriorly; metanotal groove absent. Mesopleuron elongate, lacking transverse groove or impression. Mesosoma dorsolaterally immarginate. Propodeal cuticle imbricate; fine-short dark setae scattered dorsally but not as thickly as on dorsal pronotal surface. Propodeal spiracle large and circular, positioned high and anteriorly on sclerite. Propodeal declivity rounded; posterior surface shorter than dorsal surface; propodeal lobe present and upturned slightly. Metapleural gland bulla visible through cuticle. Propleural cuticle appearing imbricate, though may be a function of desiccation during preservation. Dorsally pronotum covered in short-fine dark setae; pronotal cuticle also imbricate; promesonotal suture well-defined and unfused. Cuticular colour as in head. Coxae large and boxy; procoxa 0.39 mm; mesocoxa 0.33 mm; metacoxa 0.34 mm; coxa cuticular texture imbricate and covered with fine pale setae. Profemur cuticle smooth and shining; profemur length 0.76 mm; single pectinate protibial spur; protarsus mostly missing. Mesofemur cuticle smooth and shining, scattered fine light setae; mesofemur length 0.84 mm; distal half of mesotibia missing. Metafemur cuticle smooth and shining; metafemur length 1.06 mm; metatibia bearing single large simple spur; metatarsus long, first segment roughly as long as next four segments combined, with segments steadily shrinking in length distally; metatarsus covered in fine-long setae; metatarsal claws large, simple and sharply curved.

Metasoma: Petiole immarginate, dorsally broadly triangular tapering anteriorly, laterally broadly rounded with sloping anterior face and overhanging posterior face. Attachment to propodeum elongate and constricted; ventral surface with spine positioned anteriorly and pointing anteriorly. Helcium positioned ventrally. Abdominal segment III smaller relative to segment IV, girdling constriction present between segments, though constriction not especially deep. Pygidium and hypopygium not clearly visible in specimen or scan; no cuticular or conspicuous peg-like teeth on the pygidium. Sting present, mostly retracted. Cuticle mostly smooth and shining with sparsely punctate sculpturing. Cuticular colour as in head and mesosoma, slightly darker.

Type material: Holotype PALE-8463 deposited within the Museum of Comparative Zoology ( MCZ) at Harvard University.

Etymology: In reference to the mythological Greek hero Perseus, a warrior who defeated Medusa without the use of direct sight ( figure 1 ).

(b) Phylogenetic reconstruction and morphospace analysis

We recover D. perseus as nested within ‘true’ EH army ants across optimality criteria and datasets ( figure 2 a ). Our morphological equal and implied weights parsimony reconstruction recovers EH and WH as monophyletic, reflecting the strong phenotypic convergence of these lineages (electronic supplementary material, figures S1 and S 2 ). Army ant monophyly is also recovered in our 109 loci (44 kb) UCE + morphology Bayesian Inference search, a result that is notably sensitive to search parameters and input data [ 4] (electronic supplementary material, figure S3). Our ‘slow-evolving’ 100 supergene loci (178 kb) + morphology Bayesian Inference topology is in-line with previous hypotheses generated from this dataset [ 4]; D. perseus is recovered among a polytomy with Aenictogiton and Dorylus with modest support, but monophyletic with all EH army ants with strong support ( figure 2 , electronic supplementary material, figure S4). Distance-based morphospace analysis highlights the affinities among Dorylus , Aenictogiton and D. perseus , to the exclusion of most other doryline taxa and WH army ants ( figure 2 b ).

Figures

Figure 1. Specimen PALE-8463 from Eocene age Baltic amber. (a) Photomicrograph of lateral profile view of D. perseus. (Bottom) Micro-CT scan reconstruction of D. perseus, in (b) lateral and (c) frontal view. Scanning primarily resolved internal ‘void space’ air trapped within the insect cuticle, apparent holes in specimen reflect imperfect infilling of gas. Scale bars 0.5 mm. Specimen images are © President and Fellows of Harvard College, Museum of Comparative Zoology, Harvard University.

Figure 2. Phylogenetic and phenotypic position of Dissimulodorylus perseus relative to extant dorylines. (a) Consensus tree resulting from combined analysis of 178 662 (100 supergene loci obtained using weighted statistical binnings [4,8]) molecular and 62 morphological characters under Bayesian Inference. Nodes with posterior probability support <0.95 noted with numerical values. Node ages reflect mean divergence time estimates from Borowiec [4]; nodes denoted with an asterisk were not recovered in divergence dating estimation of Borowiec [4] and so are arbitrary. Triangles denote estimated crown ages from Borowiec [4]. Fossils belonging to extant genera are indicated with the dagger (†) symbol. (b) Principal coordinates morphospace derived from Euclidean distance matrix of binary morphological characters included in phylogenetic reconstruction.