Osteology, variation, and adaptations of the oldest Eurasian little owl Athene vallgornerensis, with implications for the evolutionary history of the genus Athene (Aves: Strigidae).

Abstract

Extant little owl (Athene noctua) and its relatives are iconic strigiform birds (Aves: Strigiformes) with a very poorly known evolutionary history. The anatomy and adaptation of early members of this group are poorly understood, as the fossil record of the genus Athene is very fragmentary prior to the Middle Pleistocene. The fossil taxon Athene vallgornerensis represents the earliest known species of Athene owls in Eurasia. Previously documented exclusively from the Lower Pleistocene of Mallorca and the Iberian Peninsula, this species was until now known from only a few postcranial elements, leaving its osteological characterization, adaptations and phylogenetic affinities highly uncertain. Here, I report a very rich collection of more than 350 fossil bones, including several partial skeletons, belonging to A. vallgornerensis from the Early Pleistocene (Calabrian, ~1.8-1.6 Ma) of Crimea (Eastern Europe). This material makes A. vallgornerensis the best-known pre-Late Pleistocene fossil owl species, permits the first comprehensive osteological description and reveals a mosaic morphology of A. vallgornerensis, partly similar to that of modern A. noctua, Athene brama and Athene cunicularia. Most importantly, the remarkably shortened hind limb (and tarsometatarsus in particular) aligns A. vallgornerensis with the phylogenetically basal and ecologically distinctive Athene blewitti, suggesting this fossil species could occupy a more arboreal or forest-edge niche than the open-country A. noctua and A. brama. The large sample size allows for the first quantitative assessment of individual variation in an Early Pleistocene European Athene population. The tarsometatarsus, in particular, exhibits remarkable intraspecific variability hinting at a high degree of phenotypic plasticity that may have served as raw material for adaptive evolution in the genus. These findings not only confirm the taxonomic validity of A. vallgornerensis but further provide critical insights into the early evolutionary history of Athene owls in Eurasia.