From One Diver to Another: There's Something Loony about Petralca

First things first: we now have approximately half of a baby Mesozoic maniraptor preserved in amber. What a time to be alive.

Onward to the main subject of this post. The seabird Petralca from the Miocene of Austria was first described in 1987 as an auk. However, this identification was (reportedly*) not justified by any anatomical observations and other researchers have subsequently suggested that Petralca may instead belong to a different group of diving birds, the loons (or divers, if you're British). Saying anything conclusive regarding Petralca has been difficult though, given that the holotype is not particularly well preserved. Some of the bones associated with the specimen are not even preserved directly, only evident as impressions.

*I cannot confirm this for myself, as the original description is in German.

The holotype of Petralca, from Göhlich and Mayr (in press).

To uncover more information about the specimen, paleontologists Ursula B. Göhlich and Gerald Mayr initiated further preparation of the fossil as well as casting of the preserved bone impressions. Armed with the new data they collected from these ventures, they were able to better compare the skeleton of Petralca to those of definite fossil loons as well as extant loons and auks.

There's no use beating around the bush: they found that Petralca is a loon. Every available skeletal element in Petralca that could be compared to those of loons and auks was more similar to those of loons. Of particular note is that the radiale (one of the wrist bones) of Petralca has a deep and prominent notch, which is a very distinctive feature of extant loons, but is absent in auks.

Comparison between the radiale of Petralca (B), a red throated loon (Gavia stellata, C), and a razorbill (an auk, Alca torda, D), from Göhlich and Mayr (in press).

In addition to clearing up its phylogenetic affinities, this reassessment of Petralca also provides clues to how it lived. Extant loons can swim quickly underwater by propelling themselves with their feet, whereas previously known early Miocene loons, such as Colymboides minutus, don't appear to have been so specialized for diving. The humerus of Petralca, however, had very thick bone walls, which is characteristic of diving birds (including both extant loons and auks). It looks like Petralca truly lived up to its claim as a diver.

Reference: Göhlich, U.B. and G. Mayr. In press. The alleged early Miocene auk Petralca austriaca is a loon (Aves, Gaviiformes): restudy of a controversial fossil bird. Historical Biology in press. doi: 10.1080/08912963.2017.1333610