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| Aenigmatorhynchus rarus Mayr & Smith, 2026 |
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.
Keywords: Aenigmatorhynchus rarus, gen. et sp. nov., Aves, beak morphology, fossil birds, evolution
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| Skulls of Aenigmatorhynchus rarus from the latest early or earliest middle Eocene of Messel in Germany. (a) The holotype specimen (SMF-ME 11857A) coated with ammonium chloride. (b), (c) µCT scans of the holotype. (d) The holotype before it was transferred to artificial resin (photograph by Bruno Behr). (e), (f) a referred skull of Ae. rarus in the Pohl collection (PBP-MES−590A+B); coated with ammonium chloride. Abbreviations: car, cartilago arytenoidea; cdo, condylus occipitalis; hyo, hyoid apparatus; ios, ventral margin of interorbital septum; jug, jugal bar; lmd, articular (caudal) end of left mandibular ramus; lqd, left quadrate; pal, palatine (os palatinum); ppo, processus postorbitalis; rmd, articular (caudal) end of right mandibular ramus; rps, rostrum parasphenoidale; rpt, right pterygoid; rqd, right quadrate; sym, symphysis mandibulae; tra, tracheal rings. The scale bars equal 10 mm. |
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| Skull and cervical vertebrae of the new avian species Aenigmatorhynchus rarus from the Messel fossil site in Germany. This fossil consists of slab and counter slab and is a referred specimen in a private collection. It was coated with ammonium chloride to enhance the contrast; the scale bar equals 10 mm. |
Aves Linnaeus, 1758
Order and family incertae sedis
Aenigmatorhynchus, gen. nov.
Diagnosis: The new taxon is characterized by a long, straight, mediolaterally narrow and pointed beak, which reaches about 75% of the entire skull length; the mandibular symphysis is very long and measures about half the length of the mandible; the rami mandibularum run in parallel in the rostral half of the mandible; the mandible exhibits prominent processus coronoidei and its tip forms a trough-like dorsal sulcus. The latter two features probably represent autapomorphies of the new taxon.
Etymology: The genus name is derived from aenigma (Lat.), riddle—which in turn is derived from αἴνιγμα (Gr.), speaking in riddles—and ῥύγχος (Gr.), beak; the taxon name refers to the unusual character distribution shown by the mandible of the new species.
Aenigmatorhynchus rarus, sp. nov.
Etymology: The species epithet is derived from rarus (Lat.), rare, in reference to the fact that the holotype skull currently is the only record of the species in a public collection.
Gerald Mayr and Krister Smith . 2026. A remarkable Beak Morphology in a Bird Skull from the Eocene of Messel (Germany) signifies unusual feeding specializations. R Soc Open Sci. 12(6): 250620. DOI: doi.org/10.1098/rsos.250620 [25 Jun 2025]
