The alleged early Miocene Auk Petralca austriaca is a Loon (Aves,Gaviiformes): restudy of a controversial fossil bird

We restudy the holotype specimen of the alleged fossil auk Petralca austriaca from early Miocene marine deposits of the Austrian locality Pucking, which was considered the earliest European representative of Alcidae (auks). The specimen is a partial skeleton consisting mainly of wing bones on two slabs. A recent re-preparation yielded new data on the skeletal morphology of Petralca, which allow more detailed comparisons with auks and loons. Our study shows that the taxon is clearly distinguished from auks in various skeletal features and can be confidently identified as a loon (Gaviiformes). Petralca resembles Colymboides in overall morphology of the wing bones, and clearly is a stem group representative of Gaviiformes. However, some features indicate that it is more closely related to the crown group taxon Gavia than to the stem group taxa Colymboides and Colymbiculus. Unusually thick bone walls of the limb bones indicate well-developed diving capabilities for Petralca austriaca.