100 Ma sweat bee nests: Early and rapid co-diversification of crown bees and flowering plants

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.

本文报道的100处百万年前（Ma）的隧蜂（sweat bee）巢穴，是迄今发现的冠群蜜蜂（crown bees）最古老的化石实证。基于经氩-氩（40Ar/39Ar）定年的这批巢穴校准构建的短舌蜂（short-tongued bees）全新系统发育树，首次证实了阿尔布阶（Albian）晚期蜜蜂与被子植物（angiosperms）协同快速辐射演化的事件。在此项研究开展之前，该假说始终缺乏古生物学实证支撑。在巴塔哥尼亚卡斯蒂略组（Castillo Formation）中发现的全新遗迹种（ichnospecies）*Cellicalichnus krausei*，与黄蜂遗迹化石及新型甲虫遗迹化石伴生产出，其结构为典型的隧蜂族（Halictini）巢穴：由依附于主隧道的固着式细胞巢组成。综合地质、古土壤（paleosol）、古植物学及遗迹学多维度数据可知，阿尔布阶时期的南半球，蜜蜂与被子植物曾共存于类似开阔干旱林地或稀树草原的内陆干旱生境中，彼时气候为暖温带半干旱-半湿润型。