Chiroptera is one of the few modern mammal orders for which no fossil record has been associated with the Paleocene-Eocene Thermal Maximum. Despite intensive collecting efforts, the earliest remains of bats are still elusive. Archaeonycteris trigonodon from the early Middle Eocene of Messel Formation (MP11) in Germany along with Icaronycteris index and Onychonycteris finneyi, both from the late Early Eocene Green River Formation (Wa7) in Wyoming, have been recognized as representing the most primitive bats based on skeleton morphology. Very few dental features of any of these taxa have been studied in detail because upper and lower dentitions are in occlusion. Nearly one century after its initial description it has become possible to digitally reconstruct the teeth of A. trigonodon using micro-CT scanning technology. This permits characterization of the complete dentition of A. trigonodon and for the first time enables dental comparisons with A. brailloni from the middle Early Eocene of Avenay (MP8+9) in France. The early Early Eocene French locality of Meudon (?MP8+9) has also yielded a few isolated bat teeth that have never been formally described. M1 is distinctly smaller than M2, both have a deep ectoflexus but M1 is more asymmetric than M2. The centrocrista does not extend far toward the labial border and both have a complete lingual cingulum and no paraconule. Lower molars are relatively wide, especially the trigonid of m2. The entoconid of m1 is distinct and individualized whereas it is more reduced and in line with the hypoconulid on m2 and m3. The new taxon from Meudon is similar in size to Archaeonycteris? praecursor from the early Early Eocene of Silveiriha (MP7) in Portugal but differs from that taxon in having lower molars with a relatively longer trigonid and shorter postcristid. These results indicate that the diversity of archaeonycterid bats is higher than previously recognized and that diversification of this lineage began early in the Eocene.
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