Fallomyrma

†Fallomyrma; Temporal range: Priabonian, Late Eocene K Pa Eo Ol Mi Pl Rovno amber, Ukraine
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Formicidae
Subfamily:	Myrmicinae
Genus:	†Fallomyrma; Dlussky & Radchenko, 2006
Type species
	Fallomyrma transversa;
Diversity
	4 fossil species; (Species Checklist)

Identification

Radchenko & Dlussky (2018) - Workers monomorphic, body 2.5– 4 mm long. Antennae 12-segmented, with distinct large three-segmented club longer than remaining funicular segments taken together; antennal scape relatively short, slightly curved at base, not reaching occipital margin. Eyes well developed, located somewhat lower (anteriorly) than midlength of lateral head margins. Ocelli absent. Frontal lobes widely spaced, so that central part of clypeus wedging in between these lobes wider than each lobe. Median portion of clypeus longitudinally concave, delineated laterally by longitudinal carinae; anterior clypeal margin widely and weakly medially concave, with two long setae. Masticatory margin of mandibles with six or seven teeth. Promesonotum distinctly narrowing posteriorly, subtrapezoidal, delineated markedly anteriorly and laterally by sharp ridge; its anterior margin convex (in dorsal view); dorsal and lateral surfaces of promesonotum positioned at right or even acute angle. Petiole with distinct cylindrical anterior part; its node high, with rounded dorsum. Middle and hind tibiae with one simple spur. Whole body smooth, without punctation or granulated sculpture; rugosity sometimes developed on various parts of head and mesosoma.

Based on the complex of morphological characters listed above, Fallomyrma differs from all known fossil myrmicine genera from the Late Eocene European ambers. Fallomyrma species are superficially similar to the extant Neotropic genus Oxyepoecus in the shape of head, mesosoma, and petiole, but the antennae of workers of the latter genus are 11-segmented, the masticatory margin of the mandibles has only three teeth, and the frontal lobes are positioned closely, so that the central part of the clypeus, which wedges in between the frontal lobes is narrower than each lobe. We do not assume that these genera are related, most probably this similarity results from convergence.

Distribution

This taxon is known from Bitterfeld amber (Bartonian, Middle to Late Eocene), Danish-Scandinavian amber (Bartonian, Middle to Late Eocene), Rovno amber (Priabonian, Late Eocene).

Notes

Radchenko & Dlussky (2018) - It is intriguing that representatives of the genus Fallomyrma have been found in coeval (Late Eocene) Rovno, Bitterfeld, and Scandinavian ambers, but have not in the Baltic amber. Taking into account the fact that the number of studied ant inclusions in the Baltic amber is at least five times greater than that of the three above ambers taken together (Wheeler, 1915; Dlussky and Rasnitsyn, 2009; our unpublished data), it is reasonably safe to suggest that the species of Fallomyrma did not inhabit the habitats where the Baltic amber was formed. At the same time, only one species of the studied genus, Fallomyrma transversa, was found in all of three (Rovno, Bitterfeld, and Scandinavian) ambers, while three newly described species are only known in the Rovno amber. However, this is possibly accounted for by relative rarity compared to F. transversa.

It is noteworthy that Fallomyrma does not belong to the Myrmicinae genera particularly rich in species from Late Eocene European ambers. Taking into account species that are already present in collections, but have not yet been described, there are at least 15 species of Temnothorax, eight of Myrmica, seven of Aphaenogaster, five of Monomorium, five of Eocenomyrma, four of Carebara, and four of Fallomyrma. That is, seven listed genera comprise more than half of all known species (78) of amber myrmicine ants and remaining 20 genera comprise from one to three species each. At the same time, Fallomyrma is among the genera of this subfamily represented by the greatest number of specimens. The richest in this parameter are the genera Temnothorax and Monomorium each represented by 100 or even more specimens; the next richest is Fallomyrma with 44 specimens; the genera Aphaenogaster and Myrmica comprise about 35 and 20 specimens, respectively.

It is plausible that small (2.5–4 mm long) Fallomyrma species, like amber Temnothorax and Monomorium of approximately the same size, were rather common representatives of the arboreal myrmecofauna, which built nests and fed in the tree crown. This probably explains the fact that myrmicine assemblages are distinctly dominated by representatives of three above mentioned genera; they are almost twice as abundant as remaining 24 genera (more than 260 specimens versus about 140). Adding the genera Aphaenogaster, Myrmica, and Carebara, which are rather diverse and abundant, this ratio becomes even more evident: more than 330 specimens in the six genera versus 70 in remaining 21 genera.

Finally, the place of origin of various Late Eocene European ambers, in particular, Rovno and Baltic ambers, remains an important question. It was previously believed that the Rovno amber is redeposited Baltic amber (Katinas, 1971), but to date, it has become generally accepted by geologists that Rovno and Baltic ambers were formed independently at the same geological time, but in different territories (Maidanovich and Makarenko, 1988; Kosmowska- Ceranowicz, 2012; Karlovich and Prokopets, 2014). The last statement is corroborated by the composition of the myrmecofauna and some other insect groups (Perkovsky et al., 2003, 2010; Dlussky and Rasnitsyn, 2009; Perkovsky, 2013; Ivanov et al., 2016). In our opinion, the distributional pattern, abundance of representatives of Fallomyrma in Rovno, Scandinavian, and Bitterfeld ambers, and complete absence of this genus in the Baltic amber are evidence of independent origin of Rovno and Baltic ambers.

Castes

Nomenclature

The following information is derived from Barry Bolton's Online Catalogue of the Ants of the World.

†FALLOMYRMA [Myrmicinae]
- †Fallomyrma Dlussky & Radchenko, 2006a: 154. Type-species: †Fallomyrma transversa, by original designation.

References