Podiceps sociatus (Navás, 1922) sensu Olson, 1995 Thiornis is a fossil genus of Middle Miocene grebe known from a nearly complete specimen from Libros, Spain.

[1][2] For a while the whereabouts since its description was unknown, but unbeknownst to the scientific community, the slab was on display in the Paris Museum at least since 1930 where it was finally reexamine for a 1995 redescription by American biologist and ornithologist Storrs L.

[2] The holotype of Thiornis is known from a virtually completed specimen that majority of it consists of postcranial material and even feather impressions (MNHN 1930-1).

[2] Navás (1922) had initially classified Thiornis as a rallid in the Fulicini tribe with the moorhens, a classification that was supported by subsequent authors until Olson (1995) found the bird to be a grebe instead.

Storer (2000) argued that the size and dimensions of the bones are not good at determining phylogenetic relationships among grebes alone, thus Thiornis should either be considered a valid genus or at least a species of Tachybaptus.

[1] Despite the age of the fossil, the anatomy of Thiornis suggests an ecology that is not unlike that of those of modern grebes as the group as a whole are homogeneous at the osteological level.