Rediscovery of the Bizarre Cretaceous AntHaidomyrmexDlussky (Hymenoptera: Formicidae), with Two New Species

Trait-Based Paleontological Niche Prediction Recovers Extinct Ecological Breadth of the Earliest Specialized Ant Predators

AbstractPaleoecological estimation is fundamental to the reconstruction of evolutionary and environmental histories. The ant fossil record preserves a range of species in three-dimensional fidelity and chronicles faunal turnover across the Cretaceous and Cenozoic; taxonomically rich and ecologically diverse, ants are an exemplar system to test new methods of paleoecological estimation in evaluating hypotheses. We apply a broad extant ecomorphological dataset to evaluate random forest machine learning classification in predicting the total ecological breadth of extinct and enigmatic hell ants. In contrast to previous hypotheses of extinction-prone arboreality, we find that hell ants were primarily leaf litter or ground-nesting and foraging predators, and by comparing ecospace occupations of hell ants and their extant analogs, we recover a signature of ecomorphological turnover across temporally and phylogenetically distinct lineages on opposing sides of the Cretaceous-Paleogene boundary. This paleoecological predictive framework is applicable across lineages and may provide new avenues for testing hypotheses over deep time.

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.

Non-destructive comparative evaluation of fossil amber using terahertz time-domain spectroscopy

Fossilized plant resins, or ambers, offer a unique paleontological window into the history of life. A natural polymer, amber can preserve aspects of ancient environments, including whole organisms, for tens or even hundreds of millions of years. While most amber research involves imaging with visual light, other spectra are increasingly used to characterize both organismal inclusions as well as amber matrix. Terahertz (THz) radiation, which occupies the electromagnetic band between microwave and infrared light wavelengths, is non-ionizing and frequently used in polymer spectroscopy. Here, we evaluate the utility of amber terahertz spectroscopy in a comparative setting for the first time by analyzing the terahertz optical properties of samples from 10 fossil deposits ranging in age from the Miocene to the Early Cretaceous. We recover no clear relationships between amber age or botanical source and terahertz permittivity; however, we do find apparent deposit-specific permittivity among transparent amber samples. By comparing the suitability of multiple permittivity models across sample data we find that models with a distribution of dielectric relaxation times best describe the spectral permittivity of amber. We also demonstrate a process for imaging amber inclusions using terahertz transmission and find that terahertz spectroscopy can be used to identify some synthetic amber forgeries.

On the value of Burmese amber for understanding insect evolution: Insights from †Heterobathmilla - an exceptional stem group genus of Strepsiptera (Insecta)

Burmese amber and amber from other periods and regions became a rich source of new extinct insect species and yielded important insights in insect evolution in the dimension of time. Amber fossils have contributed to the understanding of the phylogeny, biology, and biogeography of insects and other groups, and have also gained great importance for dating molecular trees. Another major potential is the documentation of faunal, floral and climatic shifts. Evolutionary transitions can be well-documented in amber fossils and can reveal anatomical transformations and the age of appearance of structural features. Here, using a new stem group species of Strepsiptera from Burmite, we evaluate this potential of amber insect fossils to assess the current phylogeny of Strepsiptera, with the main emphasis on the early splitting events in the stem group. Amber fossils have greatly contributed to the understanding of the evolution of Strepsiptera in the late Mesozoic and the Cenozoic. †Heterobathmilla kakopoios Pohl and Beutel gen. et sp. n. described here is placed in the stem group of the order, in a clade with †Kinzelbachilla (†Kinzelbachillidae) and †Phthanoxenos (†Phthanoxenidae). †Phthanoxenidae has priority over †Kinzelbachillidae, and the latter is synonymised. The superb details available from this new fossil allowed us to explore unique features of the antennae, mouthparts, and male copulatory apparatus, and to provide a phylogenetic hypothesis for the order. The younger †Protoxenos from Eocene Baltic amber was confirmed as sister to all remaining extinct and extant groups of Strepsiptera, whereas the position of the Cretaceous †Cretostylops in the stem group remains ambivalent. While the value of Burmite and amber from other periods has a recognized impact on our knowledge of the evolution in various lineages, this new fossil does not fundamentally change our picture of the phylogeny and evolution of early Strepsiptera. However, it offers new insights into the morphological diversity in the early evolution of the group.

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.

The origin of wing polyphenism in ants: An eco-evo-devo perspective

The evolution of eusociality, where solitary individuals integrate into a single colony, is a major transition in individuality. In ants, the origin of eusociality coincided with the origin of a wing polyphenism approximately 160 million years ago, giving rise to colonies with winged queens and wingless workers. As a consequence, both eusociality and wing polyphenism are nearly universal features of all ants. Here, we synthesize fossil, ecological, developmental, and evolutionary data in an attempt to understand the factors that contributed to the origin of wing polyphenism in ants. We propose multiple models and hypotheses to explain how wing polyphenism is orchestrated at multiple levels, from environmental cues to gene networks. Furthermore, we argue that the origin of wing polyphenism enabled the subsequent evolution of morphological diversity across the ants. We finally conclude by outlining several outstanding questions for future work.

† Camelosphecia gen. nov., lost ant-wasp intermediates from the mid-Cretaceous (Hymenoptera, Formicoidea)

Fossils provide primary material evidence for the pattern and timing of evolution. The newly discovered "beast ants" from mid-Cretaceous Burmite, †Camelosphecia gen. nov., display an exceptional combination of plesiomorphies, including absence of the metapleural gland, and a series of unique apomorphies. Females and males, represented by †C. fossor sp. nov. and †C. venator sp. nov., differ in a number of features which suggest distinct sexual biologies. Combined-evidence phylogenetic analysis recovers †Camelosphecia and †Camelomecia as a clade which forms the extinct sister group of the Formicidae. Notably, these genera are only known from alate males and females; workers, if present, have yet to be recovered. Based on ongoing study of the total Aculeata informed by the beast ant genera, we provide a brief diagnosis of the Formicoidea. We also provide the first comprehensive key to the major groupings of Mesozoic Formicoidea, alongside a synoptic classification in which †Zigrasimeciinaestat. nov. and †Myanmyrma maraudera comb. nov. are recognized. Finally, a brief diagnosis of the Formicoidea is outlined.

Specialized Predation Drives Aberrant Morphological Integration and Diversity in the Earliest Ants

Extinct haidomyrmecine "hell ants" are among the earliest ants known [1, 2]. These eusocial Cretaceous taxa diverged from extant lineages prior to the most recent common ancestor of all living ants [3] and possessed bizarre scythe-like mouthparts along with a striking array of horn-like cephalic projections [4-6]. Despite the morphological breadth of the fifteen thousand known extant ant species, phenotypic syndromes found in the Cretaceous are without parallel and the evolutionary drivers of extinct diversity are unknown. Here, we provide a mechanistic explanation for aberrant hell ant morphology through phylogenetic reconstruction and comparative methods, as well as a newly reported specimen. We report a remarkable instance of fossilized predation that provides direct evidence for the function of dorsoventrally expanded mandibles and elaborate horns. Our findings confirm the hypothesis that hell ants captured other arthropods between mandible and horn in a manner that could only be achieved by articulating their mouthparts in an axial plane perpendicular to that of modern ants. We demonstrate that the head capsule and mandibles of haidomyrmecines are uniquely integrated as a consequence of this predatory mode and covary across species while finding no evidence of such modular integration in extant ant groups. We suggest that hell ant cephalic integration-analogous to the vertebrate skull-triggered a pathway for an ancient adaptive radiation and expansion into morphospace unoccupied by any living taxon.

Distal leg structures of the Aculeata (Hymenoptera): A comparative evolutionary study of Sceliphron (Sphecidae) and Formica (Formicidae)

The distal parts of the legs of Sceliphron caementarium (Sphecidae) and Formica rufa (Formicidae) are documented and discussed with respect to phylogenetic and functional aspects. The prolegs of Hymenoptera offer an array of evolutionary novelties, mainly linked with two functional syndromes, walking efficiently on different substrates and cleaning the body surface. The protibial-probasitarsomeral cleaning device is almost always well-developed. A complex evolutionary innovation is a triple set of tarsal and pretarsal attachment devices, including tarsal plantulae, probasitarsomeral spatulate setae, and an arolium with an internal spring-like arcus, a dorsal manubrium, and a ventral planta. The probasitarsal adhesive sole and a complex arolium are almost always preserved, whereas the plantulae are often missing. Sceliphron has retained most hymenopteran ground plan features of the legs, and also Formica, even though the adhesive apparatus of Formicidae shows some modifications, likely linked to ground-oriented habits of most ants. Plantulae are always absent in extant ants, and the arolium is often reduced in size, and sometimes vestigial. The arolium contains resilin in both examined species. Additionally, resilin enriched regions are also present in the antenna cleaners of both species, although they differ in which of the involved structures is more flexible, the calcar in Sceliphron and the basitarsal comb in Formica. Functionally, the hymenopteran distal leg combines (a) interlocking mechanisms (claws, spine-like setae) and (b) adhesion mechanisms (plantulae, arolium). On rough substrate, claws and spine-like setae interlock with asperities and secure a firm grip, whereas the unfolding arolium generates adhesive contact on smooth surfaces. Differences of the folded arolium of Sceliphron and Formica probably correlate with differences in the mechanism of folding/unfolding.

An unusual 100-million-year old holometabolan larva with a piercing mouth cone

Holometabola is a hyperdiverse group characterised by a strong morphological differentiation between early post-embryonic stages (= larvae) and adults. Adult forms of Holometabola, such as wasps, bees, beetles, butterflies, mosquitoes or flies, are strongly differentiated concerning their mouth parts. The larvae most often seem to retain rather plesiomorphic-appearing cutting-grinding mouth parts. Here we report a new unusual larva preserved in Burmese amber. Its mouth parts appear beak-like, forming a distinct piercing mouth cone. Such a morphology is extremely rare among larval forms, restricted to those of some beetles and lacewings. The mouth parts of the new fossil are forward oriented (prognathous). Additionally, the larva has distinct subdivisions of tergites and sternites into several sclerites. Also, the abdomen segments bear prominent protrusions. We discuss this unusual combination of characters in comparison to the many different types of holometabolan larvae. The here reported larva is a new addition to the 'unusual zoo' of the Cretaceous fauna including numerous, very unusual appearing forms that have gone extinct at the Cretaceous-Palaeogene boundary.

A Cretaceous peak in family-level insect diversity estimated with mark-recapture methodology

The history of insects' taxonomic diversity is poorly understood. The two most common methods for estimating taxonomic diversity in deep time yield conflicting results: the 'range through' method suggests a steady, nearly monotonic increase in family-level diversity, whereas 'shareholder quorum subsampling' suggests a highly volatile taxonomic history with family-level mass extinctions occurring repeatedly, even at the midpoints of geological periods. The only feature shared by these two diversity curves is a steep increase in standing diversity during the Early Cretaceous. This apparent diversification event occurs primarily during the Aptian, the pre-Cenozoic interval with the most described insect occurrences, raising the possibility that this feature of the diversity curves reflects preservation and sampling biases rather than insect evolution and extinction. Here, the capture-mark-recapture (CMR) approach is used to estimate insects' family-level diversity. This method accounts for the incompleteness of the insect fossil record as well as uneven sampling among time intervals. The CMR diversity curve shows extinctions at the Permian/Triassic and Cretaceous/Palaeogene boundaries but does not contain any mass extinctions within geological periods. This curve also includes a steep increase in diversity during the Aptian, which appears not to be an artefact of sampling or preservation bias because this increase still appears when time bins are standardized by the number of occurrences they contain rather than by the amount of time that they span. The Early Cretaceous increase in family-level diversity predates the rise of angiosperms by many millions of years and can be better attributed to the diversification of parasitic and especially parasitoid insect lineages.

Systematics of the ant genus Proceratium Roger (Hymenoptera, Formicidae, Proceratiinae) in China - with descriptions of three new species based on micro-CT enhanced next-generation-morphology

The genus Proceratium Roger, 1863 contains cryptic, subterranean ants that are seldom sampled and rare in natural history collections. Furthermore, most Proceratium specimens are extremely hairy and, due to their enlarged and curved gaster, often mounted suboptimally. As a consequence, the poorly observable physical characteristics of the material and its scarcity result in a rather challenging alpha taxonomy of this group. In this study, the taxonomy of the Chinese Proceratium fauna is reviewed and updated by combining examinations of traditional light microscopy with x-ray microtomography (micro-CT). Based on micro-CT scans of seven out of eight species, virtual 3D surface models were generated that permit in-depth comparative analyses of specimen morphology in order to overcome the difficulties to examine physical material of Proceratium. Eight Chinese species are recognized, of which three are newly described: Proceratium bruelheidei Staab, Xu & Hita Garcia, sp. n. and P. kepingmaisp. n. belong to the P. itoi clade and have been collected in the subtropical forests of southeast China, whereas P. shoheisp. n. belongs to the P. stictum clade and it is only known from a tropical forest of Yunnan Province. Proceratium nujiangense Xu, 2006 syn. n. is proposed as a junior synonym of P. zhaoi Xu, 2000. These taxonomic acts raise the number of known Chinese Proceratium species to eight. In order to integrate the new species into the existing taxonomic system and to facilitate identifications, an illustrated key to the worker caste of all Chinese species is provided, supplemented by species accounts with high-resolution montage images and still images of volume renderings of 3D models based on micro-CT. Moreover, cybertype datasets are provided for the new species, as well as digital datasets for the remaining species that include the raw micro-CT scan data, 3D surface models, 3D rotation videos, and all light photography and micro-CT still images. These datasets are available online (Dryad, Staab et al. 2018, http://dx.doi.org/10.5061/dryad.h6j0g4p).

Taxonomic overview of the hyperdiverse ant genus Tetramorium Mayr (Hymenoptera, Formicidae) in India with descriptions and X-ray microtomography of two new species from the Andaman Islands

With 600 described species, the ant genus Tetramorium represents one of the most species-rich ant radiations. However, much work remains to fully document the hyperdiversity of this remarkable group. Tetramorium, while globally distributed, is thought to have originated in the Afrotropics and is particularly diverse in the Old World. Here, we focus attention on the Tetramorium fauna of India, a region of high biodiversity value and interest for conservation. We overview Tetramorium diversity in India by providing a species list, accounts of all species groups present, an illustrated identification key to Indian Tetramorium species groups and notes on the Indian Tetramorium fauna. Further, we describe two new species, Tetramorium krishnani sp. n. and Tetramorium jarawa sp. n. from the Andaman Islands archipelago and embed them into currently recognized Tetramorium tonganum and Tetramorium inglebyi species groups. We also provide illustrated species level keys for these groups. Along with detailed species descriptions and high-resolution montage images of types, we provide 3D cybertypes of the new species derived from X-ray micro-computed tomography.

Next-generation morphological character discovery and evaluation: an X-ray micro-CT enhanced revision of the ant genus Zasphinctus Wheeler (Hymenoptera, Formicidae, Dorylinae) in the Afrotropics

New technologies for imaging and analysis of morphological characters offer opportunities to enhance revisionary taxonomy and better integrate it with the rest of biology. In this study, we revise the Afrotropical fauna of the ant genus Zasphinctus Wheeler, and use high-resolution X-ray microtomography (micro-CT) to analyse a number of morphological characters of taxonomic and biological interest. We recognise and describe three new species: Z. obamaisp. n., Z. sarowiwaisp. n., and Z. wilsonisp. n. The species delimitations are based on the morphological examination of all physical specimens in combination with 3D scans and volume reconstructions. Based on this approach, we present a new taxonomic discrimination system for the regional fauna that consists of a combination of easily observable morphological characters visible at magnifications of around 80-100 ×, less observable characters that require higher magnifications, as well as characters made visible through virtual dissections that would otherwise require destructive treatment. Zasphinctus are rarely collected ants and the material available to us is comparatively scarce. Consequently, we explore the use of micro-CT as a non-invasive tool for the virtual examination, manipulation, and dissection of such rare material. Furthermore, we delineate the treated species by providing a diagnostic character matrix illustrated by numerous images and supplement that with additional evidence in the form of stacked montage images, 3D PDFs and 3D rotation videos of scans of major body parts and full body (in total we provide 16 stacked montage photographs, 116 images of 3D reconstructions, 15 3D rotation videos, and 13 3D PDFs). In addition to the comparative morphology analyses used for species delimitations, we also apply micro-CT data to examine certain traits, such as mouthparts, cuticle thickness, and thoracic and abdominal muscles in order to assess their taxonomic usefulness or gain insights into the natural history of the genus. The complete datasets comprising the raw micro-CT data, 3D PDFs, 3D rotation videos, still images of 3D models, and coloured montage photos have been made available online as cybertypes (Dryad, http://dx.doi.org/10.5061/dryad.4s3v1).

Extreme Morphogenesis and Ecological Specialization among Cretaceous Basal Ants

Ants comprise one lineage of the triumvirate of eusocial insects and experienced their early diversification within the Cretaceous [1-9]. Their ecological success is generally attributed to their remarkable social behavior. Not all ants cooperate in social hunting, however, and some of the most effective predatory ants are solitary hunters with powerful trap jaws [10]. Recent evolutionary studies predict that the early branching lineages of extant ants formed small colonies of ground-dwelling, solitary specialist predators [2, 5, 7, 11, 12], while some Cretaceous fossils suggest group recruitment and socially advanced behavior among stem-group ants [9]. We describe a trap-jaw ant from 99 million-year-old Burmese amber with head structures that presumably functioned as a highly specialized trap for large-bodied prey. These are a cephalic horn resulting from an extreme modification of the clypeus hitherto unseen among living and extinct ants and scythe-like mandibles that extend high above the head, both demonstrating the presence of exaggerated morphogenesis early among stem-group ants. The new ant belongs to the Haidomyrmecini, possibly the earliest ant lineage [9], and together these trap-jaw ants suggest that at least some of the earliest Formicidae were solitary specialist predators. With their peculiar adaptations, haidomyrmecines had a refined ecology shortly following the advent of ants.

A diverse ant fauna from the mid-cretaceous of Myanmar (Hymenoptera: Formicidae)

A new collection of 24 wingless ant specimens from mid-Cretaceous Burmese amber (Albian-Cenomanian, 99 Ma) comprises nine new species belonging to the genus Sphecomyrmodes Engel and Grimaldi. Described taxa vary considerably with regard to total size, head and body proportion, cuticular sculpturing, and petiole structure while all species are unified by a distinct shared character. The assemblage represents the largest known diversification of closely related Cretaceous ants with respect to species number. These stem-group ants exhibit some characteristics previously known only from their extant counterparts along with presumed plesiomorphic morphology. Consequently, their morphology may inform hypotheses relating to basal relationships and general patterns of ant evolution. These and other uncovered Cretaceous species indicate that stem-group ants are not simply wasp-like, transitional formicids, but rather a group of considerable adaptive diversity, exhibiting innovations analogous to what crown-group ants would echo 100 million years later.

Tracing the rise of ants - out of the ground

The evolution of ants (Hymenoptera: Formicidae) is increasingly well-understood due to recent phylogenetic analyses, along with estimates of divergence times and diversification rates. Yet, leading hypotheses regarding the ancestral habitat of ants conflict with new findings that early ant lineages are cryptic and subterranean. Where the ants evolved, in respect to habitat, and how habitat shifts took place over time have not been formally tested. Here, we reconstruct the habitat transitions of crown-group ants through time, focusing on where they nest and forage (in the canopy, litter, or soil). Based on ancestral character reconstructions, we show that in contrast to the current consensus based on verbal arguments that ants evolved in tropical leaf litter, the soil is supported as the ancestral stratum of all ants. We also find subsequent movements up into the litter and, in some cases, into the canopy. Given the global importance of ants, because of their diversity, ecological influence and status as the most successful eusocial lineage on Earth, understanding the early evolution of this lineage provides insight into the factors that made this group so successful today.

Ants and the fossil record

The dominance of ants in the terrestrial biosphere has few equals among animals today, but this was not always the case. The oldest ants appear in the fossil record 100 million years ago, but given the scarcity of their fossils, it is presumed they were relatively minor components of Mesozoic insect life. The ant fossil record consists of two primary types of fossils, each with inherent biases: as imprints in rock and as inclusions in fossilized resins (amber). New imaging technology allows ancient ant fossils to be examined in ways never before possible. This is particularly helpful because it can be difficult to distinguish true ants from non-ants in Mesozoic fossils. Fossil discoveries continue to inform our understanding of ancient ant morphological diversity, as well as provide insights into their paleobiology.