Take a good look at the figure above, which comes from Mayr (2009). It shows the fossilised tarsometatarsus (the fused long bone of the foot) of a bird from the late Oligocene of Patagonia. This may be one of the single most mysterious specimens in the fossil record. It represents all we know to date of Cladornis pachypus, described by Argentinean palaeontologist Florentino Ameghino in 1895. The appearance of the bone, being very broad and flat relative to its length, is quite bizarre and does not much resemble the tarsometatarsus of any other known bird.
The first thing that should be pointed out is that, whatever it was, Cladornis was a large bird. The specimen is not completely preserved (part of the proximal end of the bone has been lost) but its overall shape suggests that its original length was probably not too much longer than what we have. As such, the tarsometatarsus was probably comparable in length to that of a large pelican. However, it was much wider relative to length than that of a pelican, suggesting the possibility of a more robust bird. The shape of the bone's end indicates that the toes would have been widely spaced, and it may have even approached a zygodactyl arrangement (with two toes pointed rearwards and two forwards, like a modern parrot*) (Mayr 2009).
*When explaining this to my partner, I suggested that he imagine a parrot the size of a pelican. He shuddered and declared that he would rather not.
When Ameghino (1895) first described Cladornis, he interpreted it as an aquatic bird and suggested a relationship to the penguins, albeit in an extinct family Cladornidae (later authors would correct this to Cladornithidae). Later, noticing that it was preserved in association with terrestrial mammals, he declared that it was not marine and was possibly even terrestrial (he also included another species from the same formation, Cruschedula revola, in the Cladornithidae; this species is based on part of a scapula and there is no telling if it was related to Cladornis or not). He still maintained its relationship to the penguins (Ameghino 1906). Ameghino had a bit of a thing for trying to find the origins of all major modern vertebrate groups in his native South America (one of his other works was a book arguing for an Argentinean origin of humans) and it is possible that this was in play here. Nevertheless, the idea of a 'Patagonian land penguin' held sway until Simpson's (1946) review of the fossil penguins, in which he declared that Cladornis was "so very unlike any other penguin, recent or fossil, that I can only consider its reference to that group as erroneous".
This left Cladornis' taxonomic position completely up in the air (the question of whether Cladornis itself could get up in the air is, of course, currently completely unswerable). Wetmore (1951) included Cladornis in the Pelecaniformes, because...reasons. The closest he gave to an explanation was, "The only suggestion that has come to me is that possibly they may belong in the order Pelecaniformes, in which I have placed the family tentatively in the suborder Odontopteryges, where it is located with two others of almost equally uncertain status. This allocation is wholly
tentative and is no indication of belief in close relationship in the three diverse groups there assembled". He would later move Cladornis into its own suborder, Cladornithes, and no close relationship to the 'Odontopteryges' (now the Pelagornithidae) has been suggested since. Our current understanding of bird phylogeny finds Wetmore's remaining 'Pelecaniformes' to correspond to three or four independent clades (the Pelecanidae, Suliformes, Phaethontidae and probably Pelagornithidae) so his assignment of Cladornis to this group becomes almost completely uninformative.
Which is pretty much where we're forced to leave things. Mayr (2009) included Cladornis in his chapter on 'land birds', with other taxa discussed in this chapter belonging to the clade Telluraves. However, this was motivated more by a lack of any idea what to do with it otherwise than anything else (it is possible that Cladornis' sub-zygodactyly played a role, but not all zygodactylous birds belong to the Telluraves). I did notice a similarity in proportions between the Cladornis tarsometatarsus and the corresponding bone in the large phorusrhacid Brontornis, making me wonder if anyone had ever compared the two, but this may well be only superficial. Most recent authors have assumed that the Cladornis tarsometatarsus is simply too weird, too unique, for any resolution of its affinities to be reached without first finding more complete remains of the animal.
Ameghino, F. 1895. Sur les oiseaux fossiles de Patagonie et la aune mammalogique des couches a Pyrotherium. Boletín del Instituto Geográfico Argentino 15 (11–12): 501–602.
Ameghino, F. 1906. Enumeración de los Impennes fósiles de Patagonia y de la Isla Seymour. Anales del Museo Nacional de Buenos Aires, serie 3, 6: 97–167.
Mayr, G. 2009. Paleogene Fossil Birds. Springer.
Simpson, G. G. 1946. Fossil penguins. Bulletin of the American Museum of Natural History 87 (1): 1–99.
Wetmore, A. 1951. A revised classification for the birds of the world. Smithsonian Miscellaneous Collections 117 (4): 1–22.