A remarkable beak morphology in a bird skull from the Eocene of Messel (Germany) signifies unusual feeding specializations.

Abstract

We report the skull of a new avian species from the latest early or earliest middle Eocene fossil site Messel in Germany. <i>Aenigmatorhynchus rarus,</i> 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 (<i>Himantopus</i>, Recurvirostridae) and oystercatchers (<i>Haematopus</i>, Haematopodidae), but some features preclude an assignment of <i>Ae. rarus</i> 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 <i>Ae. rarus</i> 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.