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. Two additional species, S. canadensis and S. mesaki, were described in 1985 and 2005, 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.