Fossil bee nests, coleopteran pupal chambers and tuffaceous paleosols from the Late Cretaceous Laguna Palacios Formation, Central Patagonia (Argentina)

The Late Cretaceous Laguna Palacios Formation in Central Patagonia (San Jorge Basin), southern South America, is composed of tuffaceous deposits supplied by periodical volcanic ash falls partly reworked by rivers, on broad plains. Variations in ash-fall rates allowed the formation of stacked, mature paleosols, which are one of the most characteristic features of this formation. The mature paleosols show well-developed horizons, ped structure and bear an intricate network of trace fossils mostly produced by small roots and invertebrates. Two different insect trace fossils could be recognized in this formation: sweat bee nests and coleopteran pupal chambers. Fossil bee nests are composed of inclined tunnels with cells attached to them by means of short necks, a typical construction of bees of the subfamily Halictinae. Similar halictine constructions were reported from the Cretaceous of the USA. Coleopteran pupal chambers are discrete, ovoid structures, having an internal cavity with a smooth surface, and an outer wall of lumpy appearance composed of different layers of soil material. They are commonly constructed by the larvae of different families of Coleoptera. Similar trace fossils were previously reported from the Asencio Formation (Late Cretaceous–Early Tertiary) of Uruguay and from the Djadokhta Formation of Mongolia (Late Cretaceous). These trace fossils constitute some of the only paleontological data from the Laguna Palacios Formation, allowing inferences about its paleoecology, paleoclimatology and paleogeography. Ecological preferences of Halictinae, as well as some features of the nests, suggest a temperate, seasonal climate and an environment dominated by low vegetation for the Laguna Palacios Formation, which is also compatible with sedimentologic and pedogenic evidence. The morphology of the nests, typical of North American halictinae, adds more evidence to the hypothesis of the existence of faunal interchange between North and South America by the Late Cretaceous. The fossil nests constitute some of the oldest evidence of bees in the fossil record, the third known record of bees of Cretaceous age and the first for the Southern Hemisphere. The two traces described are, together with those of Dakota and the Gobi Desert, the only trace fossils from paleosols of Cretaceous age that can be certainly attributed to insects.