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The earliest ants
- The rise of the ants: A phylogenetic and ecological explanation

Ants evidently arose during the Cretaceous period at somewhat more than 100 million years ago. Their earliest known fossils fall into two groups. The first is the very primitive Mesozoic subfamily Sphecomyrminae. The second group consists of primitive members of the extant subfamily Formicinae and the poneromorph group of subfamilies, as recently divided (4), comprising the abundant and diverse Ponerinae and five other less prominent subfamilies.

The workers of Sphecomyrma, the best known sphecomyrmine genus, were a mosaic of ant and wasp traits (5, 6). Specimens have been found in deposits of Late Cretaceous amber (fossilized resin) in Asia, Siberia, and North America, hence across much of the northern supercontinent of Laurasia (7, 8).

Unlike the ants of later, Paleogene deposits, the sphecomyrmines are very rare. Among the many thousands of Cretaceous insect specimens examined from New Jersey, Canada, and Burma, only seven specimens have enough features preserved to be called definitively sphecomyrmine workers. Of the biology of these ancient ants it can be said only that they lived in mesic forests with rich floras and insect faunas. The exact timing and causes of their extinction in the final 10 or 20 million years of that era remain unknown (9, 10).

Early in the history of the sphecomyrmines, a wider radiation beyond the stem genus Sphecomyrma occurred. In addition to clearly derivative sphecomyrmine genera were a sphecomyrmine-ponerine intermediate (8), an apparent true poneromorph, a member of the advanced subfamily Formicinae (9), and a possible member of the subfamily Aneuretinae, considered to be precursors of the modern subfamily Dolichoderinae (10).

What appear to be the oldest certifiable ant fossils include Gerontoformica cretacica, from the Early Cenomanian to Uppermost Abian (Lower Cretaceous) amber of France, dated to ≈100 million years B.P. (11). Because of imperfect preservation, it cannot be placed with reference to fossil or existing subfamilies but evidently contains a mix of sphecomyrmine and more derivative traits. The Burmese amber (10), containing sphecomyrmines and a possible myrmeciine, may be of comparable or even somewhat older provenance.

In still other very early deposits have been found additional products of the initial ant radiation, which appear to be members or precursors of the bulldog ant subfamily Myrmeciinae, including the living “dawn ant” Nothomyrmecia macrops. The subfamily is today limited to Australia, with one very rare species of Myrmecia on New Caledonia. A possible but still uncertain myrmeciine or myrmeciine precursor, Cariridris bipetiolata, has been described from a single, poorly preserved specimen in the Santana Formation of Brazil, ≈110 million years old. What seem to us to be other myrmeciines are 10 rock fossils from the Upper Cretaceous Orapa deposits of Botswana (≈90 million years B.P.) (12). Several later myrmeciines of Paleogene age, Ameghinoia and Polanskiella of Argentina, Archimyrmex of the U.S. Green River Formation, and two species of Prionomyrmex recorded from the Baltic amber, bear witness to the spread of the subfamily around the world, followed by its retreat to Australia. It is reasonable to suppose that the Myrmeciinae diverged from the sphecomyrmine stem and spread extensively by Late Cretaceous or Eocene times (13, 14).

By Paleocene times, as evidenced in 10 specimens of ants from the presumed Paleocene amber of Sakhalin, dolichoderines and aneuretines (the latter with one contemporary species, Aneuretus simoni, surviving in Sri Lanka) had made their appearance alongside of poneromorphs, myrmeciines, and formicines (15).

By the Early to Middle Eocene, as revealed by the recently described Fushun amber fauna from northeastern China that is ≈50 million years or somewhat younger in age, the diversification of the major groups of ants was in full play (16). Among some 20 identifiable specimens of workers and queens is a variety of primitive ponerines and myrmicines as well as primitive members of the “formicoid” complex, comprising formicines and an assortment of what are either primitive dolichoderines, aneuretines, or both. Most also bear traits shared with their presumed sphecomyrmine or sphecomyrmine-like ancestors, including short mandibles with small numbers of teeth, circular or ovoid head shapes, and relatively unmodified middle body segments. Several sedimentary compression fossils of Early Eocene provenance recently discovered in British Columbia also represent relatively primitive poneromorphs (B. Archibald, personal communication).

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