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100 Ma sweat bee nests reported herein are the oldest evidence of crown bees. A new phylogeny for short-tongued bees, calibrated with these nests dated with 40Ar/39Ar, attests for the first time for a late Albian rapid diversification of bees along with angiosperms. Such hypothesis lacked paleontological support until this study. The new ichnospecies Cellicalichnus krausei, which was found along with wasp trace fossils and new beetle trace fossils in the Castillo Formation of Patagonia, represents typical Halictini nests composed of sessile cells that are attached to main tunnels. According to geological, paleosol, paleobotanical, and ichnological data, bees, and angiosperms cohabited in an inland and dry environment comparable to an open dry woodland or savanna, under warm-temperate and semiarid-subhumid climate, in the Southern Hemisphere by the Albian., Competing Interests: The authors have declared that no competing interests exist.

The new ichnospecies Celliforma curvata is described to include curved fossil bee cells from Argentina, Uruguay and the USA. The upper part of the cell (neck) of the new ichnospecies is curved, and accordingly, it can be attributed to bees of the subfamily Diphaglossinae ( Colletidae). The oldest record of C. curvata, from the early Eocene of North America (52-49 Ma), provides a minimum age for the appearance of this subfamily, in accordance with an already proposed calibrated phylogeny. It is also proposed that these fossil cells could be used for future calibrations of molecular clocks. C. curvata indicates that Diphaglossinae had a widespread distribution, from southern Utah to extra- Andean Patagonia at 42° S. In contrast, extant representatives reach only 38° S in this region. Diphaglossinae were more extended southwards in the past thanks to better environmental conditions in extra- Andean Patagonia. [ABSTRACT FROM AUTHOR]