Abstract
We report a small hawk-like diurnal bird from the early Oligocene (30–31 million years ago) of Poland. Aviraptor longicrus, n. gen. et sp. is of a size comparable with the smallest extant Accipitridae. The new species is characterized by very long legs, which, together with the small size, suggest an avivorous (bird-eating) feeding behavior. Overall, the new species resembles extant sparrowhawks (Accipiter spp.) in the length proportions of the major limb bones, even though some features indicate that it convergently acquired an Accipiter-like morphology. Most specialized avivores amongst extant accipitrids belong to the taxon Accipiter and predominantly predate small forest passerines; the smallest Accipiter species also hunts hummingbirds. Occurrence of a possibly avivorous raptor in the early Oligocene of Europe is particularly notable because A. longicrus coexisted with the earliest Northern Hemispheric passerines and modern-type hummingbirds. We therefore hypothesize that the diversification of these birds towards the early Oligocene may have triggered the evolution of small-sized avivorous raptors, and the new fossil may exemplify one of the earliest examples of avian predator/prey coevolution.
Highlights
Accipitrid diurnal birds of prey include 237 extant species with diverse ecological preferences and feeding habits (Thiollay 1994)
N. gen. et sp. closely resembles diurnal birds of prey in its skeletal morphology, with shared derived features including the proportions of the short beak, the presence of a pair of large fenestrae in the caudal margin of the sternum, the narrow processus lateralis and bulky omal extremity of the coracoid, the long preacetabular section of the pelvis, and the long hallux whose claw bears a large tuberculum flexorium
The new species agrees with the Accipitridae and differs from the superficially similar Falconidae—which originated in the New World and did not occur in Europe before the mid-Cenozoic (Mayr 2017)—in the proximodistally short hypotarsus of the tarsometatarsus, the large and ovate nostril, and the absence of a mandibular fenestra
Summary
Accipitrid diurnal birds of prey include 237 extant species with diverse ecological preferences and feeding habits (Thiollay 1994). Diurnal birds of prey have a scant early Cenozoic (Paleogene) fossil record and most aspects of their evolutionary history remain poorly known (Mayr 2009, 2017). The oldest fossil is from the early Eocene (50.5–52 million years ago [Ma]) of Belgium (Mayr and Smith 2019), but this specimen—a tarsometatarsus fragment— cannot be assigned to a particular accipitrid clade and does not provide insights into the ecology and feeding habits of the unnamed species it belonged to. The few species known from more diagnostic elements (mainly tarsometatarsi) represent medium-sized to larger “buzzard”- or “eagle”-type species, which presumably foraged on small mammals or other terrestrial vertebrates; most of these fossils are from the late Eocene and early Oligocene of North America (Mayr and Perner 2020). We describe a skeleton of a small raptor from the early Oligocene of Poland, which resembles sparrowhawks (Accipiter spp.) in the length proportions of the limb bones and represents a distinctive accipitrid
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