Sphecomyrma
Sphecomyrma is an extinct genus of ants which existed in the Cretaceous approximately 79 to 92 million years ago. The first specimens were collected in 1966, found embedded in amber which had been exposed in the cliffs of Cliffwood, New Jersey, by Edmund Frey and his wife. In 1967, zoologists E. O. Wilson, Frank Carpenter and William L. Brown, Jr. published a paper describing and naming Sphecomyrma freyi. They described an ant with a mosaic of features—a mix of characteristics from modern ants and aculeate wasps. It possessed a metapleural gland, a feature unique to ants. Furthermore, it was wingless and had a petiole which was ant-like in form. The mandibles were short and wasp-like with only two teeth, the gaster was constricted, and the middle and hind legs had double tibial spurs. The antennae were, in form, midway between the wasps and ants, having a short first segment but a long flexible funiculus. Three additional species, S. canadensis, S. mesaki and S. nexa, were described in 1985, 2005, and 2024, respectively.
The genus is among the earliest known ants to roam the earth and at the time was considered to be the evolutionary link between ants and wasps. It was suggested that ants diverged from tiphiid wasp ancestors, but later studies show that they originate from a different clade. It shows resemblance to extant primitive ants such as Nothomyrmecia and members of the tribe Aneuretini. Some scientists, however, doubted the nature of these ants and believed they were wasps due to the possible absence of the metapleural gland in Sphecomyrma and short scapes which are key diagnostic traits for ants. Additional specimens collected proved Sphecomyrma was an ant as the metapleural gland was identified. Further fossil evidence, along with its slender body and large compound eyes, suggest that they were epigaeic, foraging socially above ground and out in open areas. The sphecomyrmines, including these ants, most likely vanished by the end of the Mesozoic.
Taxonomy
Before the discovery of the first Sphecomyrma fossils, there were no fossil records of any ants from Cretaceous amber and the oldest social insects at the time extended back to the Eocene epoch; the earliest known ant at the time was described from a forewing found in the Claiborne Formation in Tennessee. The great diversity of ant fossils found in the Baltic amber and Florissant shales of the Oligocene and in the Sicilian amber of the Miocene have prompted entomologists to search for ants of Cretaceous age that may link ants and non-social wasps together. Such a link may shed light on the early origins of ants, but no fossils of any social insect existed before S. freyi was discovered, thus the early evolution of ants remained a mystery. Only a single hymenopterous Upper Cretaceous fossil has been the subject of possible significance to the evolution of aculeate wasps and ants. A single forewing was discovered in Siberia and described in 1957 as Cretavus sibiricus, with the author noting that the wing venation closely resembles those of bethylid or scoliid wasps, of which these families of wasps have close connections with the origin of ants. However, there were several problems: as the fossil was only a single wing, scientists could not explain or answer whether or not the insect had key diagnostic body traits that would even place it within the subclade Aculeata.In 1966, the first fossils of S. freyi were collected by Edmund Frey and his wife during a trip collecting mineralogical specimens in the Magothy exposure near Cliffwood Beach in Raritan Bay, New Jersey. They found a large deep red piece of amber embedded in clay containing a number of insects, including some Diptera flies. The approximate age of the fossils dates back to the Cretaceous, 92 million years ago. Donald Baird of Princeton University first notified Carpenter about the recent discovery, and David Stager of the Newark Museum arranged to transfer the specimens to be studied and examined. At the time, the existence of ants of Cretaceous age was significant, but the ants were most likely not detected as much of the amber around the site was previously collected. The discovery of the ants confirmed the existence of Cretaceous ants, and the appearance of the workers closely matched the speculations of what Mesozoic ants looked like. As a result of these finds, E. O. Wilson and colleagues erected the new subfamily Sphecomyrminae and designated the genus Sphecomyrma as the type genus in an article they published in 1967 in the journal Science. The published paper included the first description of S. freyi, which was named after Edmund Frey and his wife. A holotype and a paratype of the species were collected and moved to the Museum of Comparative Zoology, but the holotype was accidentally destroyed. However, a neotype specimen numbered AMNH-NJ-112 was collected from the Sunrise Landing site near Brunswick in 1994, and was subsequently donated to the museum. The holotype was destroyed because the amber was accidentally cracked in half, separating the two workers from each other and later stored in a wooden cabinet in an uncovered drawer with other fossil insects for 30 years; the piece eventually deteriorated, appearing more dark and fractured.
Although the hypothesised description of what Mesozoic ants looked like was somewhat accurate when compared to actual specimens, some characteristics were inaccurate. They believed that ant-like mandibles first appeared before the development of an ant-like petiole, but such case was proven false when it was opposite on S. freyi specimens. Based on drawings, it was suggested that Mesozoic ants had long mandibles with multiple teeth, toothed tarsal claws and a broadly jointed petiole. Examination of collected specimens, however, shows that these ants had very short mandibles, toothless tarsal claws and a separated petiole.
S. freyi remained as the sole member of Sphecomyrma until a fossil closely resembling the species was collected in Canadian amber deposits in 1985. Wilson provided the first description of the fossil, naming it S. canadensis. He notes that the close resemblance of the fossils to S. freyi in key characteristics strongly support its inclusion in Sphecomyrminae. He further notes that the specimens are the first ants recorded in Canadian amber, but a vast diversity of insects have been found in the amber prior to the discovery, stretching as far back as the 19th century. The discovery of the specimens from Canada indicates that the subfamily was widespread through much of the northern hemisphere during the late Cretaceous. In 2005, new fossils of an undescribed Sphecomyrma ant and S. freyi were collected in the White Oaks outcrop in Sayreville, New Jersey. The specimens were later donated to the American Museum of Natural History and studied by palaeoentomologists Michael Engel and David Grimaldi, who both provided the first description of the undescribed ant in an American Museum Novitates journal article, naming it Sphecomyrma mesaki. The age of these fossils are estimated at 79 to 92 million years.
In 1987, Russian palaeoentomologist Gennady M. Dlussky elevated the subfamily at family level, renaming it as Sphecomyrmidae to accommodate Sphecomyrma and other fossil insects he studied throughout the Soviet Union. This placement was only short-lived as Wilson, with new morphological evidence, reduced the family back to subfamily level and all studied Cretaceous ants were put into Sphecomyrma or Cretomyrma. However, Dlussky and Russian palaeoentomologist Elena B. Fedoseeva retained their classification and Sphecomyrma remained under Sphecomyrmidae. Their reasons were that the first segments of the antenna were too short to be geniculate, and the structure of the mandibles. As geniculate antennae allow brood and food manipulation, or even sociality, it is impossible to classify Sphecomyrma and relatives as ants. Despite the authors' notes, they did not cite any study as to how ants manipulate items and such with their antennae, but he may have implied that the apices, a feature found on the antenna, may have been too far away from the mandibles. The close distance of the apices from the mandibles would allow manipulation of food or antennation with other nestmates. Another issue was whether or not a metapleural gland was present, a unique feature only found in ants. Despite this and previous claims made by Dlussky saying that sphecomyrmines including Sphecomyrma were most likely solitary or semi-communal, the confirmed existence of the metapleural gland through newly collected fossils confirms that Sphecomyrma and relatives were definitely social. The evidence against Dlussky eventually reinstated Sphecomyrma and the subfamily as members of Formicidae in 1997, although some sources published before 1997 did not formally recognise Sphecomyrminae at family level. Additional doubts surrounding the nature of Sphecomyrma and relatives emerged when a 1999 paper concluded that the fossil ants found in Cretaceous amber in New Jersey were closer to wasps than they were to ants. This was quickly dismissed due to overwhelming evidence supporting their placement within the Formicidae, and the fact the authors cited unpublished cladograms and disregarded the key diagnostic traits found in the ants.
Evolution
Ants of this genus are considered to be the most primitive within the family Formicidae. The body presents a wasp-like structure but with several ant-like characteristics. These ant-like characteristics, however, are primitive compared to more modern ants, and thus it is intermediate with other primitive ants and aculeate wasps. The presence of the metapleural gland, the nodiform, the structure of the petiole and its general physical appearance of an ant concludes Sphecomyrma species are ants rather than wasps; the absence of the metapleural gland would mean that it is most likely a wasp instead of an ant. It is not exactly known which group of wasps are the ancestors of Sphecomyrma, but members of the family Thynnidae, particularly those in the genus Methocha are strikingly similar to Sphecomyrma. Wilson placed the genus closest to the Tiphiidae among extant wasps, but a later study published in 1975 derived the ants from a later clade and not to the Tiphiidae. More recently, ants are considered to have evolved from a lineage within the aculeate wasps, and a 2013 study suggests that they are a sister group of the Apoidea, and the sister group to this lineage is likely the Scoliidae.Sphecomyrma may hold a close relationship with extant primitive ants. The Australian dinosaur ant is recognised as the most primitive living ant today, and both ants closely resemble each other. The Eocene genera of Aneuretini also resemble Sphecomyrma ants, which are believed to be the ancestors of Dolichoderinae. The subfamily Myrmeciinae was thought to be the ancestor of Aneuretini because the elongated mandibles, were considered primitive and short mandibles were derived. This theory was proven false after Sphecomyrma specimens had small mandibles. Currently, phylogenetic analyses recognise Sphecomyrma as a sister group to modern living ants, meaning that is a stem-group formicid. This means that it is more closely related to the ants in contrast to any other organism, but the crown-group formicids are more closely related to each other and distantly related to Sphecomyrma. By the end of the Mesozoic, the sphecomyrmines including these ants vanished.