Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation

Flight first The Green River formation in Wyoming has produced many important fossils, including Icaronycteris index , which for over 40 years has been regarded as the oldest known bat. Its cranial features suggest that it could locate its insect prey by echolocation. This fuelled a spirited debate between proponents of the 'flight-first', 'echolocation-first' and 'tandem-development' hypotheses of bat evolution. New Green River bat fossils — including two near-complete skeletons, a cast of one of which is shown on the cover — looks to have settled the matter in favour of flight first. The new species is the most primitive bat known. It had fully developed wings and was clearly capable of powered flight, but the morphology of the ear region suggests that it could not echolocate, making it a possible intermediate link between bats and their non-flying, non-echolocating mammalian ancestors. Limb characteristics, including robust hind legs and retention of tiny claws on all of its elongate fingers, indicate that the new bat may have been an agile climber. Bats (Chiroptera) represent one of the largest and most diverse radiations of mammals, accounting for one-fifth of extant species 1 . Although recent studies unambiguously support bat monophyly 2 , 3 , 4 and consensus is rapidly emerging about evolutionary relationships among extant lineages 5 , 6 , 7 , 8 , the fossil record of bats extends over 50 million years, and early evolution of the group remains poorly understood 5 , 7 , 8 , 9 . Here we describe a new bat from the Early Eocene Green River Formation of Wyoming, USA, with features that are more primitive than seen in any previously known bat. The evolutionary pathways that led to flapping flight and echolocation in bats have been in dispute 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , and until now fossils have been of limited use in documenting transitions involved in this marked change in lifestyle. Phylogenetically informed comparisons of the new taxon with other bats and non-flying mammals reveal that critical morphological and functional changes evolved incrementally. Forelimb anatomy indicates that the new bat was capable of powered flight like other Eocene bats, but ear morphology suggests that it lacked their echolocation abilities, supporting a ‘flight first’ hypothesis for chiropteran evolution. The shape of the wings suggests that an undulating gliding–fluttering flight style may be primitive for bats, and the presence of a long