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Bats are among the most recognizable, numerous, and widespread of all mammals. But much of their fossil record is missing, and bat origins remain poorly understood, as do the relationships of early to modern bats. Here, we describe a new early Eocene bat that helps bridge the gap between archaic stem bats and the hyperdiverse modern bat radiation of more than 1,460 living species. Recovered from ∼50 million-year-old cave sediments in the Quercy Phosphorites of southwestern France, Vielasia sigei 's remains include a near-complete, three-dimensionally preserved skull—the oldest uncrushed bat cranium yet found. Phylogenetic analyses of a 2,665 craniodental character matrix, with and without 36.8 kb of DNA sequence data, place Vielasia outside modern bats, with total evidence tip-dating placing it sister to the crown clade. Vielasia retains the archaic dentition and skeletal features typical of early Eocene bats, but its inner ear shows specializations found in modern echolocating bats. These features, which include a petrosal only loosely attached to the basicranium, an expanded cochlea representing ∼25% basicranial width, and a long basilar membrane, collectively suggest that the kind of laryngeal echolocation used by most modern bats predates the crown radiation. At least 23 individuals of V. sigei are preserved together in a limestone cave deposit, indicating that cave roosting behavior had evolved in bats by the end of the early Eocene; this period saw the beginning of significant global climate cooling that may have been an evolutionary driver for bats to first congregate in caves. [Display omitted] • A new bat species is described from a 50-million-year-old cave deposit in France • Its well-preserved fossils include the oldest uncrushed skull of a bat yet known • This stem bat appears to have been capable of advanced (laryngeal) echolocation • The fossils suggest that advanced echolocation predates the crown bat radiation When and how echolocation first evolved in bats is contentious. Hand et al. report on an uncrushed, 50-million-year-old bat skull that provides significant new data to inform the debate. Measurements of the inner ear of the French stem bat Vielasia suggest that advanced (laryngeal) echolocation may have evolved before the crown bat radiation. [ABSTRACT FROM AUTHOR]