Abstract
We report the skull of a new avian species from the latest early or earliest middle Eocene fossil site Messel in Germany. Aenigmatorhynchus rarus, gen. et sp. nov. is characterized by a long, straight, and pointed beak, as well as a mandible with prominent processus coronoidei, a very long symphysis, closely adjacent cristae tomiales, and a narrow dorsal sulcus along the tip. This unusual character mosaic impedes a straightforward phylogenetic assignment. In its proportions, the mandible is superficially similar to that of extant stilts (Himantopus, Recurvirostridae) and oystercatchers (Haematopus, Haematopodidae), but some features preclude an assignment of Ae. rarus to these and other charadriiform taxa. The ventral ossification of the rostrum suggests comparisons with long-beaked taxa of the Aequornithes and Telluraves, but again several features conflict with a position of Ae. rarus within either of these clades. Even though an unambiguous phylogenetic placement is not possible, the new fossil expands the avifauna of the Messel site and exhibits a distinctive beak morphology, which is not found in extant birds and indicates a specialized foraging behaviour as yet unknown in birds.