Skip to main content

Unparalleled details of soft tissues in a Cretaceous ant

Abstract

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.

Peer Review reports

Background

With a species number of more than 14,000, ants represent one of the dominant groups of eusocial insects and survive in almost all terrestrial ecosystems of this planet [1, 2]. The evolution of ants and the origin of their eusocial behavior are intriguing not only for entomologists but also the general public. To address these topics, biologists have carried out morphological [1], ethological [1, 2], and molecular [3,4,5,6] studies on living ants. Phylogenetic analyses based on large-scale molecular data sets have suggested that ants evolved from wasp-like ancestors at least 115–135 million years ago [4,5,6]. Ants have been considered as the first group of the ground-dwelling predatory eusocial insects [7]. The sociality of ants is generally thought to be empowered by a complex nervous system [8, 9] and chemical pheromones [7, 10, 11]. On the one hand, the social behaviour of ants has been reported from Cretaceous amber pieces of Kachin [12]. On the other hand, neural and other soft tissues that enable ants’ sociality remains poorly known from the fossils. The few examples include a gland reported by Brady and colleagues based on their microscopic observations [3], and the nerve and muscle tissues discovered by Boudinot and his colleagues in a Cretaceous amber piece [13, 14].

In general, internal organs are rarely preserved on fossils due to decay and compaction processes during fossilization [15]. Compared to other types of fossil preservation, ambers provide a better chance for soft tissues to be preserved and observed [16]. Examples of soft tissue preservation in Kachin ambers include muscles of a wasp [17] and a beetle [18], giant sperms of an ostracod [19], and the digestive duct of a shrimp [20]. An extreme case is the full-body 3D reconstruction including the brain, muscles, and sperm pump of the extinct group—†Mengea (stem group of Strepsiptera)—from a Baltic amber [21].

Here, we report our findings in a female specimen (gyne) of the extinct ant group—Zigrasimecia—in a Cretaceous amber piece from Kachin, with exceptionally preserved and almost complete internal organs such as the brain, the main exocrine system, the digestive tract, and several muscle clusters. Zigrasimecia belongs to the subfamily Zigrasimeciinae which was considered as one of the oldest ant subfamilies [22,23,24,25,26,27]. Our findings, therefore, not only expand the knowledge of fossil ants to their internal anatomy but also shed new lights on understanding the early evolution of ants and their early sociality.

Materials and methods

Studied material

Amber samples. Specimen No. YKLP-AMB-002. A gyne trapped in a piece of nearly transparent and yellow amber (Fig. 1a), which was collected by Wanglin Zheng in Noije Bum Village, Danai Town, Kachin Province, Myanmar. The dimensions of the piece are 24.7 mm (length), 18.8 mm (width) and 4.3 mm (height). The weight of the piece is 1.75 g. The U–Pb isotope dating age of this Kachin amber is 98.79 ± 0.62 Ma [28]. Yu and Wang have reaffirmed this dating result by trapped ammonite and ostracods in the amber [19, 29]. The amber (YKLP-AMB-002) is deposited at the Yunnan Key Laboratory for Palaeobiology (YKLP), Institute of Palaeontology, Yunnan University, Kunming, China. Structure termilogy refer from Bolton, 1994 [30].

Fig. 1
figure 1

Specimen (YKLP-AMB-002) and 3D reconstructions of †Zigrasimecia sp. a Specimen in amber. b Head in anterior view. c Head in posterior view. d Left compound eye. e Close-up of mouth part. f Close-up of labrum with labial setae each bearing a groove. g, h Mesosoma in lateral and dorsal views. i, j Gaster in lateral and bottom views. ab antennal base; an anepisternum; cd clypeal denticles; kn katepisternum; lbr labrum; lbs labial setae; lp labial palp; lsg labial setae groove; md mandible; mds mandibular setae; mn mesonotum; mp maxillary palp; mpgo metapleural gland opening; op oral opening; pe petiole; pn pronotum; pp postpetiole; ppd propodeum; sl scutellum; st sting; t tooth; wb wing base (III–VII, third through seventh abdominal segments)

Micro-CT scanning

The specimen was scanned at the micro-CT laboratory of YKLP with an X-ray microscope (3D-XRM), Zeiss Xradia 520 versa, and divided into three parts. Scanning parameters are as the following: YKLP-AMB-002 (head): Beam strength: 60 kV/5w, Filter: no, Resolution: 0.97 µm, Exposure time: 5 s, Number of TIFF images: 986; YKLP-AMB-002 (mesosoma): Scanning parameters are as the following: Beam strength: 60 kV/5w, Filter: no, Resolution: 0.94 µm, Exposure time: 5.5 s Number of TIFF images: 1617; YKLP-AMB-002 (gaster): Scanning parameters are as the following: Beam strength: 60 kV/5w, Filter: no, Resolution: 1.15 µm, Exposure time: 4.5 s, Number of TIFF images: 992. Volume rendering and 3D reconstruction were performed using the open-source software Drishti 2.4 [31]. For shooting and super depth of field synthesis of the surface characteristics of the whole amber, a Keyence VHX-6000 3D microscope was used. Figures were processed in Photoshop 2019 and Illustrator 2020 (Adobe Systems Incorporated, San José, USA).

Results

Systematic palaeontology

Subfamily Zigrasimeciinae Borysenko et al., 2017.

Genus Zigrasimecia Barden et al., 2013.

Species Zigrasimecia sp.

Material YKLP-AMB-002, gyne (Fig. 1a).

Type locality and stratigraphy Hukawng Valley, Kachin Province, northern Myanmar. Upper Albian-lower Cenomanian (ca. 98.79 ± 0.62 Ma).

External morphology

Head (Fig. 1b–f): wide and flattened in face view (Fig. 1b), oval-shape in lateral view. Oval-shaped small compound eyes (Fig. 1b, d). Three ocelli small (Fig. 1b). Antenna 12 flagellomeres (Fig. 1a). Lateromedially broad clypeus with concave anterior margin, lateral edges curved downward and forming convex lobes covering the mandibular bases; anterior clypeal margin with 36 clypeal denticles, which closely arranged, shortening from middle to sides. Labrum large, with 98 labral setae (Fig. 1e). Upper and middle labral setae are longer than lateral and lower setae, each with distinct groove above. The deepest depth of labral setae’s groove about one-third of setae's diameter (Fig. 1f). The lowermost part of labrum densely covered with short, soft setae. Hypostomal carina deeply V-shaped in ventral view, hypostomal cavity thus rather open, giving space for the labiomaxillary complex to move. Mandibles barely overlap each other, with two teeth on chewing side, apex of subapical tooth aligning with apical tooth; inner edge covered with hard and sharp needle-like setae, each mandibular seta can insert into a root of labral setae groove. Maxillary palp 3-merous. Labial palp 4-merous.

Mesosoma (Fig. 1g, h): Developed. Neck short, almost completely covered by pronotum. In lateral and dorsal views, junctions between each notum well developed. Pronotum well developed and wider than mesonotum. Anepisternum and katepisternum well differentiated from each other by grooves. Scutellum thick in dorsal view. Propodeum concaved in the middle and rear side wider than front side, developing posteriorly towards the body. Petiole one segment. Dorsal surface of the petiole is slightly concave.

Gaster (Fig. 1i, j): with five segments and distinctly broader than head and mesosoma, about one-half of body length. The first gastral segment large in lateral view, measuring about one-third of gastral length. The second gastral segment distinctly longer than other segments in dorsal view, about one-half of gastral length. Third gastral segment longer than the fourth and comparable in length to the fifth. Exposed part of sting short in length, but the sting inner part about two thirds of gaster length.

Measurements (in μm): head (including eyes) length 492.6, width 765.2; ocelli length 33.3, width 22.1; compound eyes length 212.8, width 169.7; clypeal denticles length 12.0–26.0; labrum length 135.9, width 317.9; labral setae longest length 30.3, shortest length 15.7; mandible length 309.2; oral opening length 199.8, width 285.7. Mesosoma length 829.1, width 432.2 and height 640.8; petiole length 292.0, height 368.7. Gaster length 1084.2, width 621.6, height 640.5.

Nervous system

In the head of YKLP-AMB-002, a sub-reniform-shaped organ is preserved. It is located at the center of the head and bears connecting structures to the compound eyes and antenna (Fig. 2a, b). Based on its shape and position, it can be determined as the nerve center, or brain, of YKLP-AMB-002 [32,33,34]. Although the preservation of the fossil may be subjected to deformation and shrinkage, the brain is well developed (width: 557 μm, height: 341 μm, maximum thickness: 241 μm, i.e., 49% of the head length). The brain is connected to the subesophageal complex to form a compact unit, connected by neurons on both sides of the oesophagus. The optic nerve is thick in frontal view, connecting the brain to the compound eyes. The antennal nerve developing from brain, extending towards antennal base. The suboesophageal complex is clearly narrower than the brain in frontal view, but in lateral view its maximum length is approximately twice the maximum length of the brain. But no ganglia in the ventral nerve cord are observed.

Fig. 2
figure 2

Anatomy of †Zigrasimecia sp. (YKLP-AMB-002). a, b Head anatomy in anterior and lateral views. c Longitudinal section showing mesosoma anatomy in lateral view. d Cross section of mesosoma showing mesosoma anatomy in dorsal view. e Cross section of mesosoma showing metapleural gland and its opening. f Longitudinal section showing gaster anatomy in lateral view. g Horizontal section showing gaster anatomy in top view. br brain; bu bubble; c cloacal chamber; cd clypeal denticles; dp main duct of poison sac; fm flying muscle; ibp infrabuccal pocket; lbr labrum; lg labial gland; md mandible; mdg mandibular gland; mpedl M. pharyngoepipharyngalis lateral dorsal portion; mpel M. pharyngoepipharyngalis lateral portion; mpg metapleural gland; mpgo metapleural gland opening; nan antennal nerve; omd1 M. craniomandibularis internus; omd3 M. craniomandibularis externus; oan M. tentorioscapalis; olb2 M. frontoepipharyngalis; on optical nerve; ph pharynx; phg pharyngeal gland; st sting; sog suboesophageal ganglion; tes thoracic esophagus; vs venom gland sac; wr wing remain

Muscular system

A striated soft tissue structure is preserved on the sides and posterior edge of the interior of the head of the specimen. This structure starts at the root of the mandible and labrum, extending towards the posterior edge of the head and lower margin of pharyngeal gland (Fig. 2a, b). Based on its shape and position, it can be determined as the musculature of the head [32,33,34,35]. Some muscle tissues can be identified by comparison with the head muscles of living ants. M. crani-omandibularis internus (omd1) is the largest cephalic muscle, developing from the root of the mandibles, extending to the very back of the head, with clear signs of shrinkage; M. craniomandibularis externus (omd3) is lightly flattened triangular muscle developing at the inner base of the head, shrinkage; M. tentorioscapalis (ona) is the muscle attached to the root of the antanna; M. frontoepipharyngalis (olb2) is rather thick muscle, developing from the frontal area of the antennal base and synchronous attaching the base of labrum; M. pharyngoepipharyngealis (mpe) is very strongly developed longitudinal muscle, connecting the dorsal prepharyngeal wall and labral base. In addition, on both sides of the thoracic oesophagus, there are also distinctive remnants of flight muscles, distinctively shrinkage (Fig. 2d).

Digestive system

In the head and mesosoma of YKLP-AMB-002, there is a duct-like tissue that connects to the mouthparts, passes through the brain and runs from the neck to the thorax of the specimen. Based on the shape and position, this tissue can be identified as the pharynx and oesophagus [32,33,34] (Fig. 2a, b). When viewed from the top, the pharynx is in a trapezoid shape–similar to its occurrence in living ants (maximum width: 231 μm, height: 104 μm). Beneath the pharynx, a plump spherical infrabuccal pocket is present (length: 134 μm, width: 174 μm, height: 91 μm). The pharynx is followed by an inflated oesophagus (maximum width: 271 μm, about 35% of the maximum head width, maximum height: 115 μm) passing through the middle of the brain and the head muscles, and eventually joins the thorax at the neck. In the mesosoma, the thoracic oesophagus is distinctively enlarged (maximum length: 657 μm, maximum height: 481 μm, i.e., nearly 75% of the height of thorax) (Fig. 2c, d). Due to the limitations of fossil preservation, there is no remnants of the abdominal oesophagus and the connection.

In the gaster, a sharp and prominent sting is present (maximum width: 84 μm, total length: 1529 μm) (Fig. 2f), developing in the end of gaster. Inside the sting, the lancet, lancet valves and venom canal are well-preserved (Fig. 3). The external of the sting and the top posterior part of the abdomen cannot be shown in the CT data, but can be observed in optical photographs (Fig. 1a).

Fig. 3
figure 3

Sting anatomy. a Cross section of sting showing anatomy in dorsal view. b Longitudinal section of sting showing anatomy in lateral view. ln lancet; lv lancet valves; ss sting shaft; vc venom canal

Exocrine system

YKLP-AMB-002 also preserves the main exocrine glands, including mandibular glands, pharyngeal gland, labial glands, metapleural glands and venom gland. A pair of developed mandibular glands (width: 209 μm, length: 338 μm, the length means 69% of head length) connected with both mandible in the head. Though the left one is a bit residue, they are symmetrically arranged like living ants. Behind mandibular gland, a complete pharyngeal gland (width: 267 μm, length: 165 μm, thickness: 67 μm) connects with upper pharynx. In the head view, the pharyngeal gland shows a heart-shaped sac (Fig. 2a, b).

In the mesosoma, the membranous reservoir of labial gland (Fig. 2c, d) is preserved. It divides into left and right parts on the upper side of the thorax. The duct to the opening site was lost. Another one is the metapleural gland (Fig. 2c, e). A portion of soft tissue of the metapleural gland is recognized by the connection with its opening orifice (body surface). The gland itself was shown up in a symmetrical pair of irregularly spherical glands (width: 81 μm, height: 119 μm, thickness: 71 μm).

In the gaster, only an extremely developed venom gland (Fig. 2e, f) was preserved. The reservoir was distinctly inflated (Fig. 2f, g). The venom gland sac and the sting was connected by the main duct of poison sac (maximum height: 604 μm, i.e., 88% of the abdomen height, maximum width: 511 μm, i.e., 75% of abdomen width). Paired Cloacal chamber like structure (Hölldobler and Wilson, 1990) preserved in the end of abdomen.

Based on the findings of the internal structure of the specimen, the internal structure pattern has been mapped for this ancient ant, Zigrasimecia (Fig. 4).

Fig. 4
figure 4

Diagram of internal organs of †Zigrasimecia sp. br brain; ibp infrabuccal pocket; lg labial gland; mb mushroom bodies; mdg mandibular gland; mpg metapleural gland; oe oesophagus; on optical nerve; ph pharynx; phg pharyngeal gland; st sting; sog suboesophageal ganglion; tes thoracic esophagus; vs venom gland sac

Discussion

Fossils with soft-tissue preservation are rare, and often with only a certain part of the internal structures such as the muscular [17, 18, 21, 36,37,38,39,40], the neural [13, 14, 41,42,43,44,45,46], the cardiovascular [47], the glandular [13, 14, 21], the reproductive [19, 21, 48], or digestive [13, 14, 20] systems. Although the oldest fossil record of soft-tissue preservation can be traced back to the early Cambrian [41,42,43, 47], knowledge of the fossilization process of soft tissue in generally still remains rather limited [49], sometimes even controversial [50, 51]. Recently, the mineralization process of insects preserved in Kachin amber has been investigated and hypothesized—the resin prevented the degradation of the insects until it was cracked when buried in marine sediment [52]. Pore water with reactive chemical species of diagenesis then infiltrated the amber pieces through the cracks, which resulted in the calification or silification of the insect bodies [52]. On the one hand, this hypothesis explains why a great proportion of known amber inclusions are preserved in a solid state filled with minerals [16, 53]. On the other hand, it indicates that only a small number of amber pieces—most likely those least affected by chemical species of diagenesis—can preserve internal soft tissue. We summarized all the studies we know on the preservation of soft tissues within amber pieces, and noticed that almost every kind of internal organs of an arthropod can possibly carry have been reported (Additional file 1: Table S1). In a few cases, destructive methods such as sectioning combined with Transmission Electron Microscopy (TEM) even helped reveal cellular structures of the soft tissue [16, 38, 40, 53,54,55]. Notably, none of the previous studies on soft-tissue preservation in amber caught the complete internal organs of the animals. In the present study, we employed microscopic and computed tomographic techniques to reconstruct the exoskeleton and the almost complete internal organs of an ant—a gyne of Zigrasimecia—preserved in an amber piece (YKLP-AMB-002) from Kachin. We recovered the intact nervous, digestive, muscular and exocrine systems of the ant without destructing the sample (Figs. 14; Additional file 2: Video S1, Additional file 3: Video S2, Additional file 4: Video S3, Additional file 5: Video S4, Additional file 6: Video S5, Additional file 7: Video S6, Additional file 8: Video S7, Additional file 9: Video S8, Additional file 10: Video S9).

Nervous system

Brain is an important organ for arthropods to control their activities including feeding, locomotion, and mating [56]. For social insects such as ants, their relatively large brain plays a key role in various actions such as communicating with other individuals from the same and/or different colonies [7, 57,58,59]. A brain and a sub esophageal ganglion are recovered from our specimen (Fig. 2a, b), suggesting that the gyne of Zigrasimecia bears a well-developed nervous system possibly for reproductive and brooding purposes [7].

Similar findings of a brain and a suboesophageal ganglion have been reported from a strepsipteran insect preserved in a Baltic amber piece [21]. Such anatomical characters are used to improve the understandings of the ground pattern situation of the entire Strepsiptera, and to further confirm the systematic position of †Mengea as the sister group of Strepsiptera [21]. Various population sizes, degrees of sociality and environments can lead to marked differences in the brain development and evolution of different ant groups [60]. In the present study, the proportion of the brain in the head of the gyne of Zigrasimecia is somewhere between that of the omnipotent ants [33, 35] and the more social groups such as Formica rufa [33]. The optic neuropils of this specimen look comparatively large, it may be related to the reproductive behavior of the gyne. However, further discussion of the size of ant brain at the species level is not possible at this stage, as the situation in other castes and worker specializations must be taken into account [57]. Future studies on the worker’s brain of Zigrasimecia should be explored when the material is available.

Muscular system

Muscles seem to be the most commonly preserved soft tissue found in arthropod ambers (Additional file 1: Table S1). The best muscular preservation in ambers known so far probably comes from a dance fly in a Dominican amber pieces, with fine preservation of identifiable cellular structures such as myofibrils and densely packed mitochondria [38]. Muscle tissue has also been reported from Kachin ambers in the recent years. These include the dorsoventral flight muscles of a stinging wasp [17], the muscular bundles in the metathorax of a beetle [18], and those connecting the thorax and the abdomen of a stem ant [13, 14]. By contrast, muscles in the head of the animals are barely recovered.

The pattern of musculature in the head of extant ants is well studied [33,34,35, 61]. Those muscles play important roles in controlling the mouthparts during feeding and attacking processes. Our data allows us to carry out fine-scale identification of the head muscles in Cretaceous arthropods (Fig. 2). Similar to living ants, our specimen bears strong mandibular muscles that empowers predatory and carriage behaviors [34]. Both M. frontoepipharyngalis and M. pharyngoepipharyngealis of this fossil specimen are more likely robust than those of the Leptanillinae—a basal group of extant ants that catch preys [35]. Combined with the stout setae on the labrum (Fig. 1e, f), these powerful muscles may allow the animal to trap and feed on its prey with help from the armed labrum.

Digestive system

Our specimen serves as an ideal model showing the feeding system of early ants—externally, it preserves the well-armed mouthparts (Fig. 1b, e, f) and an infrabuccal pocket sitting in the oral cavity whose width occupies one third of the head (Fig. 2b); internally, an inflated oesophagus runs through the head and extends the entire length of the body (The effect of deformation during petrification on the entire digestive tract cannot be excluded). Considering the relatively intact state of preservation (insignificant structural deformation in the vicinity of the esophagus), we can hypothesized reasonably that with such a unique feeding system, the Cretaceous Zigrasimecia could hunt on preys, absorb larger solid food debris than most living ants of similar body size [62, 63], and inject digestive enzymes from the labial gland, which usually opens at the base of the labium.

The crop, also called ‘social stomach’, is an important organ for the trophallaxis behaviour—food and/or information substance exchanging in the society of ants and many other eusocial animals [7]. The presence of a crop may be helpful in evaluating the society evolution level of the Cretaceous ants. However, the crop is not observable/or undeveloped in the gaster of our specimen (can not exclude fossilization artifact) (Fig. 2c, d), it may indicating that an undeveloped crop situation for the gyne of the Cretaceous Zigrasimecia, as seen in the queen of the modern ant Monomorium pharaonis [64]. The inflated oesophagus forms a crop-like structure in the thorax of our specimen, likely similar to the ‘thoracic crop’ [64,65,66] seen in two modern ants (Pheidole aberrans and Pheidole deima) [67] but bearing little evolutionary indication, in our view.

Exocrine system

The exocrine system of ants plays important roles in many aspects such as alarm defense [11, 34] and pheromone secretion to attract mates during nuptial seasons [68]. Within the exocrine system, the metapleural gland is a diagnostic feature of ants and secrets volatiles as territory and nest-entrance marking, antisepsis, hygiene and chemical defense [69,70,71]. However, on fossil specimens, this gland is often only identified by its small opening on the exoskeleton of the posterior body [22, 72]. For instance, the openings of metapleural gland on the exoskeleton had been documented in both worker and reproductive castes of several fossil ants including Zigrasimecia [22,23,24, 26]. For the first time, we recover the inner structure of the metapleural gland in the fossil ants as a whole.

Our specimen also preserves a fossilized pharyngeal gland. The pharyngeal gland is present in almost all studied ant species [7] and is one of the main sources of colony odor by which ants can discriminate workers and queens, foragers and nurses, nestmates and enemies [73, 74]. Furthermore, the CT data shows a difference of this gland body with the most part of living ants, whose pharyngeal gland is usually glove-shaped [33, 75] with the fingers are extended inside the ants’ head, even cover most of the brain, but the fossil’s one is formed by a heart-shaped sac. In another word, the ant preserved in YKLP-AMB-002 already have the basic pharyngeal gland structure which may making effect to the social behavior of this ant group.

Availability of data and materials

The datasets generated and/or analysed during the current study are available in the Zenodo repository (https://doi.org/10.5281/zenodo.7193141). Additional information supporting the results are provided as Additional file 2: Videos S1–Additional file 10: Videos S9. All specimens are kept at YKLP at Yunnan University. Photographic material of the studied material is available from the corresponding authors on request.

References

  1. Wheeler W. Ants: their structure, development and behavior. Am Nat. 1910;91:5927–41.

    Google Scholar 

  2. Brady L, Parr C, Abbott K. Ant ecology. Oxford: Oxford University Press; 2010.

    Google Scholar 

  3. Brady S, Schultz T, Fisher B, Ward P. Evaluating alternative hypotheses for the early evolution and diversification of ants. Proc Natl Acad Sci USA. 2006;103:18172–7.

    Article  CAS  Google Scholar 

  4. Moreau C, Bell C, Vila R, Archibald S, Pierce N. Phylogeny of the ants: diversification in the age of Angiosperms. Science. 2006;312:101–4.

    Article  CAS  Google Scholar 

  5. Ward S. The phylogeny and evolution of ants. Annu Rev Ecol Evol S. 2014;45(1):23–43.

    Article  Google Scholar 

  6. Romiguier J, Borowiec M, Weyna A, Helleu Q, Loire E, Mendola C, Rabeling C, Fisher B, Ward P, Keller L. Ant phylogenomics reveals a natural selection hotspot preceding the origin of complex eusociality. Curr Biol. 2022;32(13):2942–7.

    Article  CAS  Google Scholar 

  7. Hölldobler B, Wilson E. The ants. Berlin: Springer; 1990. p. 732.

    Book  Google Scholar 

  8. Kamhi J, Traniello J. Biogenic amines and collective organization in a superorganism: neuromodulation of social behavior in ants. Brain Behav Evolut. 2013;82:220–36.

    Article  Google Scholar 

  9. Farris S. Insect societies and the social brain. Curr Opin Insect Sci. 2016. https://doi.org/10.1016/j.cois.2016.01.010.

    Article  Google Scholar 

  10. Dahbi A, Lenoir A. Queen and colony odour in the multiple nest ant species, Cataglyphis iberica (Hymenoptera, Formicidae). Insect Soc. 1998;45:301–13.

    Article  Google Scholar 

  11. Schultner E, Oettler J, Helanterä H. The role of brood in eusocial Hymenoptera. Q Rev Biol. 2017;92:39–78.

    Article  Google Scholar 

  12. Barden P, Grimaldi D. Adaptive radiation in socially advanced stem-group ants from the Cretaceous. Curr Biol. 2016;26:515–21.

    Article  CAS  Google Scholar 

  13. Boudinot B, Richter A, Katzke J, Chaul J, Keller R, Economo E, Beutel R, Yamamoto S. Evidence for the evolution of eusociality in stem ants anda systematic revision of †Gerontoformica (Hymenoptera: Formicidae). Zool J Linn Soc Lond. 2022;4:1355–89.

    Article  Google Scholar 

  14. Richter A, Boudinot B, Yamamoto S, Katzke J, Beutel R. The first reconstruction of the head anatomy of a Cretaceous insect, † Gerontoformica gracilis (Hymenoptera: Formicidae), and the early evolution of ants. Insect Syst Diver. 2022;6(5):1–80.

    Google Scholar 

  15. Sansom R, Gabbott S, Purnell M. Decay of vertebrate characters in hagfish and lamprey (Cyclostomata) and the implications for the vertebrate fossil record. Proc Biol Sci. 2010;278:1150–7.

    Google Scholar 

  16. Grimaldi D, Bonwich E, Delannoy M, Doberstein S. Electron microscopic studies of mummified tissues in amber fossils. Am Mus Novit. 1994;3097.

  17. Grimaldi D, Peñalver E, Barrón E, Herhold H, Engel M. Direct evidence for eudicot pollen-feeding in a Cretaceous stinging wasp (Angiospermae; Hymenoptera, Aculeata) preserved in Burmese amber. Commun Biol. 2019;2:408.

    Article  Google Scholar 

  18. Li Y, Yamamoto S, Huang D, Cai C. New species of Paraodontomma from mid-Cretaceous Burmese amber with muscle tissue preservation (Coleoptera: Archostemata: Ommatidae). Pap Avulsos Zool. 2021;61:20216153.

    Article  Google Scholar 

  19. Wang H, Matzke-Karasz R, Horne D, Zhao X, Cao M, Zhang H. Exceptional preservation of reproductive organs and giant sperm in Cretaceous ostracods. Proc Biol Sci. 2020;287:20201661.

    Google Scholar 

  20. Xing L, Liu Y, McKellar R, Luque J, Li G, Wang Y, Yi Q, Sun R, Wang E, Audo D. The first shrimp preserved in mid-Cretaceous Kachin amber: systematics, palaeoecology, and taphonomy. Sci Bull. 2021;66:1723–6.

    Article  Google Scholar 

  21. Pohl H, Wipfler B, Grimaldi D, Beckmann F, Beutel R. Reconstructing the anatomy of the 42-million-year-old fossil Mengea tertiaria (Insecta, Strepsiptera). Sci Nat. 2010;97:855–9.

    Article  CAS  Google Scholar 

  22. Barden P, Grimaldi D. A new genus of highly specialized ants in Cretaceous Burmese amber (Hymenoptera: Formicidae). Zootaxa. 2013;3681:405–12.

    Article  Google Scholar 

  23. Perrichot V. A new species of the Cretaceous ant Zigrasimecia based on the worker caste reveals placement of the genus in the Sphecomyrminae (Hymenoptera: Formicidae). Myrmecol News. 2014;19:165–9.

    Google Scholar 

  24. Cao H, Boudinot B, Wang Z, Miao X, Shih C, Ren D, Gao T. Two new iron maiden ants from Burmese amber (Hymenoptera: Formicidae: Zigrasimeciini). Myrmecol News. 2020;30:161–73.

    Google Scholar 

  25. Cao H, Boudinot B, Shih C, Dong R, Gao T. Cretaceous ants shed new light on the origins of worker polymorphism. Sci China Life Sci. 2020;63:1085–8.

    Article  Google Scholar 

  26. Zhuang Y, Ran H, Li X, Feng C, Liu Y. A new species of the stem-group ants based on an alate female from Burmese amber (Hymenoptera: Formicidae: †Zigrasimecia). Cretac Res. 2022;130: 105056.

    Article  Google Scholar 

  27. Boudinot B, Perrichot V, Chaul J. †Camelosphecia gen. nov., lost ant-wasp intermediates from the mid-Cretaceous (Hymenoptera, Formicoidea). ZooKeys. 2020;1005:21–55.

    Article  Google Scholar 

  28. Shi G, Grimaldi D, Harlow G, Wang J, Wang J, Yang M, Lei W, Li Q, Li X. Age constraint on Burmese amber based on U-Pb dating of zircons. Cretac Res. 2012;37:155–63.

    Article  Google Scholar 

  29. Yu T, Kelly R, Lin M, Ross A, Kennedy J, Broly P, Xia F, Zhang H, Dilcher D. An ammonite trapped in Burmese amber. Proc Natl Acad Sci USA. 2019;116:11345–50.

    Article  CAS  Google Scholar 

  30. Bolton B. Identification guide to the ant genera of the world. Cambridge: Harvard University Press; 1994.

    Google Scholar 

  31. Limaye A, Stock S. Drishti: a volume exploration and presentation tool. Proc SPIE. 2012;8506:85060X.

    Article  Google Scholar 

  32. Richter A, Keller R, Baumgarten Rosumek F, Economo E, Hita Garcia F, Beutel R. The cephalic anatomy of workers of the ant species Wasmannia affinis (Formicidae, Hymenoptera, Insecta) and its evolutionary implications. Arthropod Struct Dev. 2019;49:26–49.

    Article  Google Scholar 

  33. Richter A, Hita Garcia F, Keller R, Billen J, Economo E, Beutel R. Comparative analysis of worker head anatomy of Formica and Brachyponera (Hymenoptera: Formicidae). Arthropod Syst Phylo. 2020;78:133–70.

    Google Scholar 

  34. Richter A, Schoeters E, Billen J. Morphology and closing mechanism of the mandibular gland orifice in ants (Hymenoptera: Formicidae). J Morphol. 2021;282:1127–40.

    Article  Google Scholar 

  35. Richter A, Hita Garcia F, Keller R, Billen J, Katzke J, Boudinot B, Economo E, Beutel R, Unit B. The head anatomy of Protanilla lini (Hymenoptera: Formicidae: Leptanillinae), with a hypothesis of their mandibular movement. Myrmecol News. 2021;31:85–114.

    Google Scholar 

  36. Kornilowitsch N. Has the structure ofstriated muscle been retained in amber fossils? Naturf Gesell Zu Dorpat. 1903;13:198–206.

    Google Scholar 

  37. Petrunkevitch A. Striated muscles of an amber insect. Nature. 1935;135:760–1.

    Article  Google Scholar 

  38. Henwood A. Exceptional preservation of dipteran flight muscle and the taphonomy of insects in amber. Palaios. 1992;7:203–12.

    Article  Google Scholar 

  39. Mierzejewski P. On application of scanning electron microscope to the study of organic inclusion from baltic amber. Ann Soc Geol Pol. 1976;46:291–5.

    Google Scholar 

  40. Poinar G. Life in Amber. Redwood City: Stanford University Press; 1992.

    Book  Google Scholar 

  41. Ma X, Hou X, Edgecombe G, Strausfeld N. Complex brain and optic lobes in an early Cambrian arthropod. Nature. 2012;490:258–61.

    Article  CAS  Google Scholar 

  42. Tanaka G, Hou X, Ma X, Edgecombe G, Strausfeld N. Chelicerate neural ground pattern in a Cambrian great appendage arthropod. Nature. 2013;502:364–7.

    Article  CAS  Google Scholar 

  43. Cong P, Ma X, Hou X, Edgecombe G, Strausfeld N. Brain structure resolves the segmental affinity of anomalocaridid appendages. Nature. 2014;513:538–42.

    Article  CAS  Google Scholar 

  44. Yang J, Ortega-Hernández J, Butterfield N, Liu Y, Boyan G, Hou J, Lan T, Zhang X. Fuxianhuiid ventral nerve cord and early nervous system evolution in Panarthropoda. Proc Natl Acad Sci USA. 2016;113:2988–93.

    Article  CAS  Google Scholar 

  45. Bicknell R, Ortega-Hernández J, Edgecombe G, Gaines R, Paterson J. Central nervous system of a 310-m.y.-old horseshoe crab: expanding the taphonomic window for nervous system preservation. Geology. 2021;49:1381–5.

    Article  CAS  Google Scholar 

  46. Ortega-Hernández J, Lerosey-Aubril R, Losso S, Weaver J. Neuroanatomy in a middle Cambrian mollisoniid and the ancestral nervous system organization of chelicerates. Nat Commun. 2022;13:410.

    Article  Google Scholar 

  47. Ma X, Cong P, Hou X, Edgecombe G, Strausfeld N. An exceptionally preserved arthropod cardiovascular system from the early Cambrian. Nat Commun. 2014;5:3560.

    Article  Google Scholar 

  48. Heethoff M, Helfen L, Norton R. Description of Neoliodes dominicus n. sp. (Acari, Oribatida) from dominican amber, aided by synchrotron x-ray microtomography. J Paleontol. 2009;83:153–9.

    Article  Google Scholar 

  49. Parry L, Smithwick F, Nordén K, Saitta E, Lozano-Fernandez J, Tanner A, Caron J, Edgecombe G, Briggs D, Vinther J. Soft-bodied fossils are not simply rotten carcasses—toward a holistic understanding of exceptional fossil preservation. BioEssays. 2018;40:1700167.

    Article  Google Scholar 

  50. Edgecombe G, Ma X, Strausfeld N. Unlocking the early fossil record of the arthropod central nervous systemPhil. Trans R Soc B. 2015;370:20150038.

    Article  Google Scholar 

  51. Liu J, Steiner M, Dunlop J, Shu D. Microbial decay analysis challenges interpretation of putative organ systems in Cambrian fuxianhuiids. Proc R Soc B. 2018;285:20180051.

    Article  Google Scholar 

  52. Jiang H, Tomaschek F, Muscente AD, Niu C, Nyunt TT, Fang Y, Schmidt U, Chen J, Lönartz M, Mähler B, Wappler T, Jarzembowski EA, Szwedo J, Zhang H, Rust J, Wang B. Widespread mineralization of soft-bodied insects in Cretaceous amber. Geobiology. 2022;00:1–14.

    Google Scholar 

  53. Henwood A. Soft-part preservation of beetles in Tertiary amber from the Dominican Republic. Palaeontology. 1992;35:901–12.

    Google Scholar 

  54. Poinar G, Hess R. Ultrastructure of 40-million-year-old insect tissue. Science. 1982;215:1241–2.

    Article  Google Scholar 

  55. Poinar G, Hess R. Preservative qualities of recent and fossil resins: Electron micrograph studies on tissue preserved in Baltic amber. J Baltic Stud. 1985;16:222–30.

    Article  Google Scholar 

  56. Strausfeld N. Aarthropod brains. Evolution, functional elegance, and historical significance. Cambridge: Harvard University Press; 2012. p. 830.

    Book  Google Scholar 

  57. Gronenberg W. Structure and function of ant (hymenoptera: formicidae) brains: strength in numbers. Myrmecol News. 2008;11:25.

    Google Scholar 

  58. Strausfeld N. Brain organization and the origin of insects: an assessment. Proc R Soc B. 2009;276:1929–37.

    Article  Google Scholar 

  59. Makoto M. Alarm pheromone processing in the ant brain: an evolutionary perspective. Front Behav Neurosci. 2010;4:28.

    Google Scholar 

  60. Odonnell S. Brain development and brain evolution. In: Starr C, editor. Encyclopedia of social insects. Cham: Springer; 2020.

    Google Scholar 

  61. Paul J, Roces F, Hölldobler B. How do ants stick out their tongues? J Morphol. 2002;254(1):39–52.

    Article  Google Scholar 

  62. Eisner T, Happ G. The infrabuccal pocket of a formicine ant: a social filtration device. Psyche J Entomol. 1962;69:107–16.

    Article  Google Scholar 

  63. Barden P, Perrichot V, Wang B. Specialized predation drives aberrant morphological integration and diversity in the earliest ants. Curr Biol. 2020;30:3818–24.

    Article  CAS  Google Scholar 

  64. Petersen-Braun M. Buschinger, Entstehung und Funktion eines thorakalen Kropfes bei Formiciden-Kniginnen. Insect Soc. 1975;22:51–66.

    Article  Google Scholar 

  65. Glancey BM, Glover A, Lofgren CS. Thoracic crop formation following dealation by virgin females of two species of Solenopsis. Fla Entomol. 1981;64:454.

    Article  Google Scholar 

  66. Meer R, Glance BM, Lofgren CS. Biochemical changes in the crop, oesophagus and postpharyngeal gland of colony-founding red imported fire ant queens (Solenopsis invicta). Insect Biochem. 1982;12:123–7.

    Article  Google Scholar 

  67. Casadei-Ferreira A, Fischer G, Economo E. Evidence for a thoracic crop in the workers of some Neotropical Pheidole species (Formicidae: Myrmicinae). Arthropod Struct Dev. 2020;59: 100977.

    Article  CAS  Google Scholar 

  68. Hölldobler B, Maschwitz U. Der Hochzeitsschwarm der Rossameise Camponotus herculeanus L. (Hymenoptera, Formicidae). J Comp Physiol. 1964;50:551–68.

    Google Scholar 

  69. Tulloch G, Shapiro J, Hershenov B. The ultrastructure of the metasternal glands of ants. Bull Brooklyn Entomol Soc. 1962;57:91–101.

    Google Scholar 

  70. Maschwitz U, Koob K, Schildknecht H. Ein Beitrag zur Funktion der Metathoracaldrüse der Ameisen. J Insect Physiol. 1970;16:387–404.

    Article  CAS  Google Scholar 

  71. Yek S, Mueller U. The metapleural gland of ants. Biol Rev. 2011;86:774–91.

    Article  Google Scholar 

  72. Hölldobler B, Engel-Siegel H. On the metapleural gland of ants. Psyche. 1985;91:201–24.

    Article  Google Scholar 

  73. Bagnères A, Morgan E. The postpharyngeal glands and the cuticle of Formicidae contain the same characteristic hydrocarbons. Experientia. 1991;47:106–11.

    Article  Google Scholar 

  74. Soroker V, Vienne C, Hefetz A, Nowbahari E. The postpharyngeal gland as a gestalt organ for nestmate recognition in the ant Cataglyphis niger. Sci Nat. 1994;81:510–3.

    CAS  Google Scholar 

  75. Gama V, Cruz Landim C. Estudo comparative das glândulas do sistema salivar de formigas (Hymenoptera, Formicidae). Naturalia. 1982;7:145–65.

    Google Scholar 

Download references

Acknowledgements

We thank Mr. Robert J. O’Flynn of the YKLP for proofreading and Mr. Ye Liu for the artistic reconstruction.

Funding

This study was supported by the following grants from the Natural Science Foundation of Yunnan Province: 2015HA021 and 2019DG050.

Author information

Authors and Affiliations

Authors

Contributions

YZ: conceptualization, methodology, software, data curation, writing—original draft, formal analysis, visualization. XJ: conceptualization, writing—review and editing, investigation. GZ: conceptualization, writing—review and editing, investigation. HM: methodology, data curation, formal analysis. XL: software, writing—review and editing, investigation. HH: investigation. RH: conceptualization, methodology, writing—review and editing, investigation. YL: conceptualization, methodology, writing—review and editing, supervision, funding acquisition. All authors read and approved the final manuscript.

Corresponding authors

Correspondence to Hao Ran or Yu Liu.

Ethics declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Additional information

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

Additional file 1: Figure S1.

Artistic reconstruction of two alate females of Zigrasimecia sp. Table S1. Summary of studies related to the preservation of fossilized internal organs (present study marked in yellow).

Additional file 2: Video S1. 3D model of the thorax of Zigrasimecia sp. in lateral view.

Additional file 3: Video S2. 3D model of the thorax of Zigrasimecia sp. in dorsal view.

Additional file 4: Video S3. 3D model of the thorax of Zigrasimecia sp. in posterior view.

Additional file 5: Video S4. 3D model of the gaster of Zigrasimecia sp. in lateral view.

Additional file 6: Video S5. 3D model of the gaster of Zigrasimecia sp. in dorsal view.

Additional file 7: Video S6. 3D model of the gaster of Zigrasimecia sp. in posterior view.

Additional file 8: Video S7. 3D model of the head of Zigrasimecia sp. in lateral view.

Additional file 9: Video S8. 3D model of the head of Zigrasimecia sp. in frontal view.

Additional file 10: Video S9. 3D model of the head of Zigrasimecia sp. in top view.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Zhuang, Y., Xu, W., Zhang, G. et al. Unparalleled details of soft tissues in a Cretaceous ant. BMC Ecol Evo 22, 146 (2022). https://doi.org/10.1186/s12862-022-02099-2

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://doi.org/10.1186/s12862-022-02099-2

Keywords

  • Amber
  • Cretaceous
  • Fossil
  • Micro-CT
  • Soft-tissue preservation
  • Zigrasimecia