Adelomyrmex

[[Adelomyrmex genus-group]]

BIOGEOGRAPHICAL CONSIDERATIONS

The two genera of the Adelomyrmex genus-group exhibit in the Southern Hemisphere a vicariant distribution pattern with Baracidris confined to the Afrotropical part of the African continent, and with Adelomyrmex being represented in New Guinea, the Western Pacific archipelagoes of Samoa and Fiji, and in the Neotropical Region (tropical South America and Middle America). Large gaps in range are a prominent feature of the distribution pattern.

Such vicariance suggests a group that originated in the Southern Hemisphere on the Gondwanian land mass before Africa became isolated in Lower Cretaceous time (some 135 to 125 MYA) (Storey, 1995; MacLoughlin, 2001). So, the ancestral stock of the extant genera of the group could have originated no later than that time, with initial differentiation of ancestral Baracidris and ancestral Adelomyrmex coinciding with separation of the West Gondwanian African continent from its principal counterpart, South America. Such timing predates the age of the earliest known ant fossils, indicating that the Formicidae is probably older than postulated previously.

The geographical range of ancestral Adelomyrmex probably extended in later Cretaceous time from South America through East Gondwanaland (i.e., Antarctica + Australia + New Zealand). Probably events associated with climatic change (glaciation of Antarctica and drying of large parts of the Australian continent) were the driving forces in causing the present wide separation of elements of Adelomyrmex, whose extant members seem to be adapted to tropical climate, and life in mesic habitats.

If, through unfavorable climatic changes, Adelomyrmex experienced forced withdrawal from large parts of East Gondwanaland, in West Gondwanian South America, the opposite occurred, with range expansion into tropical Middle America, and extensive differentiation there (the A. tristani species complex has there many populations isolated in valleys, from Panamá to México).

In conclusion, the geographical distribution of Adelomyrmex genus-group is an interesting biogeographic puzzle, the broad outlines of which have been addressed above. This subject will be addressed in more detail, in the prospective treatment of phylogenetic and chorological relationships of the group taxa.