Cretaceous ants shed new light on the origins of worker polymorphism

A revision of the Cretaceous ant genus Zigrasimecia Barden & Grimaldi, 2013 (Hymenoptera: Formicidae: Zigrasimeciinae)

The extinct Cretaceous ant genus Zigrasimecia Barden & Grimaldi, the iron maiden ants from Myanmar, is revised, and five new species are described: Z. boudinoti sp. nov., Z. caohuijiae sp. nov.,Z. chuyangsui sp. nov., Z. perrichoti sp. nov., and Z. thate sp. nov. Zigrasimecia hoelldobleri paratype (CNU-HYM-MA2019054) is removed from the type series. New diagnoses for all species are provided and species boundaries are discussed. Studied specimens that are not ideally preserved are presented and discussed, some of them are putative new species. Two identification keys for the genus are provided, a traditional, dichotomous key and an interactive, multi-entry key hosted online at the website www.Xper3.fr. I briefly discuss the unlikeliness of the genus Boltonimecia to belong to the subfamily Zigrasimeciinae, and also the taxonomic problem caused by the description of species based on alates and poorly preserved fossils.

Unparalleled details of soft tissues in a Cretaceous ant

For social insects such as ants, the internal organs are likely important in understanding their eusocial behavior and evolution. Such organs, however, are rarely preserved on fossils. In each of the few cases reporting exceptionally fossilized soft tissues in arthropods, the nervous, muscular and cardiovascular systems have been described individually, but never in combination. Here, we report a female specimen (gyne) of the extinct ant group-^†Zigrasimecia-included in a Cretaceous amber piece from Kachin, Myanmar, with an almost complete system formed by various internal organs. These include the brain, the main exocrine system, part of the digestive tract, and several muscle clusters. This research expands our knowledge of internal anatomy in stem group ants. As the gyne bears a morphologically unique labrum, our specimen's internal and external features support the notion that the early ant may have special ecological habits during the Cretaceous period.

Morphological Phylogeny of New Cretaceous Fossils Elucidates the Early History of Soil Dwelling Among Bugs

Burrowing bugs are distinctive, beetle-like insects of the pentatomoid family Cydnidae, noteworthy for their morphological specializations for digging and a hemiedaphic life history. However, less is known about their biological significance and the early origin of soil dwelling. Direct fossil evidence illuminating the evolutionary history of soil dwelling in cydnids is extremely rare. In this study, we report four new species of the burrowing bug subfamily Amnestinae from mid-Cretaceous Burmese amber, including two exhibiting specialized bulldozing and digging morphological traits on the anterior of the head and forelegs. Associated morphological features and phylogenetic placement indicate that Acanthamnestus represents the earliest unequivocal soil-dwelling cydnids and pushes back the geological record of hemiedaphic true bugs to 99 Ma. Environmental evidence, the distribution of host plants, and the fossils provide a new window for understanding the early origin of soil habits in Amnestinae and may be linked to the appearance of Moraceae.

Evidence for the evolution of eusociality in stem ants and a systematic revision of †Gerontoformica (Hymenoptera: Formicidae)

It is generally assumed that Cretaceous stem ants were obligately eusocial, because of the presence of wingless adult females, yet the available evidence is ambiguous. Here, we report the syninclusion of a pupa and adult of a stem ant species from Mid-Cretaceous amber. As brood are immobile, the pupa was likely to have been transported by an adult. Therefore, the fossil substantiates the hypothesis that wingless females were cooperators, thus these were true ‘workers’. Re-examination of all described Cretaceous ant species reveals that winged–wingless diphenism – hence a variable dispersal capacity – may have been ancestral to the total clade of the ants, and that highly specialized worker-specific phenotypes evolved in parallel between the stem and crown groups. The soft-tissue preservation of the fossil is exceptional, demonstrating the possibility of analysing the development of the internal anatomy in stem ants. Based on the highest-resolution µ-CT scans of stem ants to date, we describe †Gerontoformica sternorhabda sp. nov., redescribe †G. gracilis, redefine the species group classification of †Gerontoformica, and provide a key to the species of the genus. Our work clarifies the species boundaries of †Gerontoformica and renders fossils relevant to the discussion of eusocial evolution in a way that has heretofore been intractable.

The first queen-worker association for Cretaceous Formicidae: the winged caste of Haidomyrmex cerberus

Two queen ant specimens, one alate and one dealate, from mid-Cretaceous (Late Albian–Early Cenomanian) Burmese amber are herein reported as belonging Haidomyrmexcerberus Dlussky, 1996. This is the first discovery and documentation of an alate queen in Haidomyrmex. Compared with workers of Haidomyrmexcerberus, alate and dealate queens are larger in body size, have smaller compound eyes, a longer antennal scape, more complex mandibles, and a relatively large-sized metasoma. It is hypothesized that these differences are due to caste differences.

Vespids from the mid-Cretaceous with club-shaped antennae provide new evidence about the intrafamiliar relationships of Vespidae

Three new species of wasps (Hymenoptera: Vespidae) from mid-Cretaceous Myanmar (Burmese) amber, with club-shaped antennae, are assigned to Archaeovespa gen. nov. and provide morphological information for new phylogenetic analyses of Vespidae. Phylogenetic results suggest that Archaeovespa has more affinities with Masarinae than with Protovespinae and, along with Masarinae, composes the sister clade to Polistinae, Eumeninae, Vespinae and Stenogastrinae. New findings indicate that the antennal morphologies of Vespidae diversified significantly during or before the mid-Cretaceous.