Introduction

Two dominant structural types in the lower molars of insectivorous bats have been described and their evolutionary implications interpreted: these are known as the nyctalodont and myotodont conditions (Menu and Sigé, 1971). Although previously noted as differential characters by some authors (e.g., Lavocat, 1961), these structures had not been the subject of extensive study among bats. Since then, intermediate conditions, interpreted as transitional evolutionary steps between the two patterns, have been reported in natural populations of both living and fossil bats.

Among the oldest known bats, a different but characteristic pattern in the posterior structure of the lower molar is exhibited by various species, and is interpreted here to represent the primitive condition of chiropteran lower molars. It is the pattern displayed by the most archaic bats, notably within, but not restricted to, archaeonycterids, although not all of them. These archaic bats, known only as fossils, are reported from the Early Eocene of various and presently disjunct regions of the world. The condition is less commonly displayed by younger, more derived and taxonomically diverse fossil bats. The classic fossil bat genus Necromantis Weithofer, 1887, now more accurately dated as Middle to Late Eocene in age (Maitre et al., 2007; Maitre, 2008; Hand et al., Chapter 6, this volume), well exemplifies this archaic lower molar structure, and the name necromantodonty is used here to typify it.