Gerontoformica
Gerontoformica is an extinct genus of stem-group ants. The genus contains thirteen described species known from Late Cretaceous fossils found in Asia and Europe. The species were described between 2004 and 2016, with a number of the species formerly being placed into the junior synonym genus Sphecomyrmodes.
Gerontoformica is known from over thirty adult fossil specimens which are composed of complete adult female workers and queens.[1] The first fossil was discovered preserved as an inclusion in a transparent chunk of Charentese amber.[2] The amber is thought to have been formed from resins of the extinct Pinales tree family Cheirolepidiaceae and possibly from the living family Araucariaceae. Paleoecology of the ambers indicates the shore to mangrove type forests were of a subtropical to warm temperate climate, with occasional dry periods[3] The ambers are recovered from deposits exposed in quarries, road constructions, and beach exposures in the Charente-Maritime region of coastal France, notably at Archingeay.[2] Dating of the amber has been done through pollen analysis and it is generally accepted to be approximately 100 million years old.[3][4]
The majority of described fossils have been found and described from Burmese amber. The Asian specimens were recovered from unspecified deposits in the Hukawng Valley of Kachin State, Myanmar.[5] Burmese amber has been radiometrically dated using U-Pb isotopes, yielding an age of approximately 99 million years old, close to the boundary between the Aptian and Cenomanian.[5] The amber is suggested to have formed in a tropical environment around 5° north latitude and the resin to have been produced by either an Araucariaceae or Cupressaceae species tree.[6]
A team of French researchers headed by André Nel and G. Perrault published the 2004 type description of the new genus and species in the journal Geologica Acta. The genus name Gerontoformica was coined as a combination of the Greek word Geronto from the old age of the ant and Formica, a genus of ants. The genus had a single species the Charentese amber G. cretacica.[2]
In 2005 American paleoentomologists David Grimaldi and Michael Engel described a new Sphecomyrminae genus, Sphecomyrmodes based on a fossil found in Burmese amber. The name formed with the suffix -oides, meaning "with the form of" and the name Sphecomyrma, type genus of Sphecomyrminae. The new genus and single species S. orientalis were separated from Gerontoformica based on the 2004 type description which interpreted the G. cretacica antennae as having an elongated scape.[7] A second species, S. occidentalis, was added to Sphecomyrmodes in 2008 based on two specimens from Charentese amber.[8]
In a 2014 PLOS ONE paper Phillip Barden and David Grimaldi, a series of nine new Burmese amber species were described and placed into Sphecomyrmodes: S. contegus, S. gracilis, S. magnus, S. pilosus, S. rubustus, S. rugosus, S. spiralis, S. subcuspis, and S. tendir. All the species were based on fossils in the private collection of James Zigras and loaned to the American Museum of Natural History for paleoentomologists to study.[5]
