Attaichnus kuenzelii revisited: A miocene record of fungus-growing ants from argentina

Abstract

Attaichnus kuenzelii is a nest of leaf-cutting ants from the Miocene of La Pampa, Argentina, described by Laza (1982) in a pioneer paper for insect palaeoichnology. The type locality was revisited to update morphological description, affinities, and palaeoenvironmental conditions. In present day outcrops, A. kuenzelii shows no mounds or high concentration of interconnected chambers as originally described, but well separated and regularly distributed chambers, which at most may compose groups of two or three. This updated morphology suggests that A. kuenzelii can be attributed to Attini other than Atta, probably species of Acromyrmex or Trachymyrmex. Associated trace fossils are large vertebrate burrows, rhizoliths, meniscate burrows, and scarce Coprinisphaera. Other specimens of dung beetle and bee trace fossils found in the same palaeosols were probably constructed by younger producers on soils developed on post-Miocene sediments, which are mostly eroded today. Evidence for this assumption is the lack of carbonate cementation and presence of organic remains in these traces. The reinterpretation of A. kuenzelii as a nest of species of Acromyrmex or Trachymyrmex modifies the original assumption of a northern subtropical palaeoenvironment during the Miocene for this area based on the distribution of Atta species. The new affinities proposed in combination with the studied palaeosols interpreted as mollisols, phytoliths, and previous palaeontological, and sedimentological evidence suggest an open palaeoenvironment under a temperate climate, dominated by grasses.