Penguins (Aves: Sphenisciformes) hold much interest for many people, including (but not limited to) scientists. According to results of molecular studies, penguin history began in the Cretaceous, but the oldest bones assigned to these birds are Paleocene in age. The first fossil representative of Sphenisciformes formally described was Palaeeudyptes antarcticus, and this event took place 150 years ago. Since that time, several dozens of species have been erected, though not all of them have stood a test of time. The 21st century entered new dynamics into the paleontology of penguins, and (importantly) it concerned both the new material, and new theories. This paper summarizes what we currently know about extinct penguins and indirectly suggests the most promising areas for further research.

The tarsometatarsus, a compound bone from the lower leg in birds, is the most important skeletal element in fossil penguin taxonomy, especially in the case of early members of this group. However, any attempt to go beyond the problem of mere classification obviously requires the better understanding of osteological traits under consideration. This in turn touches on the issue of interplay between bone and concomitant soft−tissue structures, such as muscles, tendons and vessels. This paper focuses on the more holistic comprehension of the tarsometatarsal section of the Eocene penguin foot, based on the analysis of the myology and the vascular system of its modern counterparts. A number of graphical reconstructions are provided with a discussion of the role of the hypotarsus and inter− metatarsal foramina.

Defining species boundaries, due to morphological variation, often represents a significant challenge in paleozoology. In this paper we report results from multi− and univariate data analyses, such as enhanced clustering techniques, principal coordinates ordination method, kernel density estimations and finite mixture model analyses, revealing some morphometric patterns within the Eocene Antarctic representatives of Palaeeudyptes penguins. These large−sized birds were represented by two species, P. gunnari and P. klekowskii , known mainly from numerous isolated bones. Investigations focused on tarsometatarsi, crucial bones in paleontology of early penguins, resulted in a probability−based framework allowing for the “fuzzy” partitioning the studied specimens into two taxa with partly overlapping size distributions. Such a number of species was supported by outcomes from both multi− and univariate studies. In our opinion, more reliance should be placed on the quantitative analysis of form when distinguishing between species within the Antarctic Palaeeudyptes .

The present review aims to improve the scope and coverage of the phylogenetic matrices currently in use, as well as explore some aspects of the relationships among Paleogene penguins, using two key skeletal elements, the humerus and tarsometatarsus. These bones are extremely important for phylogenetic analyses based on fossils because they are commonly found solid specimens, often selected as holo− and paratypes of fossil taxa. The resulting dataset includes 25 new characters, making a total of 75 characters, along with eight previously uncoded taxa for a total of 48. The incorporation and analysis of this corrected subset of morphological characters raise some interesting questions considering the relationships among Paleogene penguins, particularly regarding the possible existence of two separate clades including Palaeeudyptes and Paraptenodytes , the monophyly of Platydyptes and Paraptenodytes , and the position of Anthropornis . Additionally, Notodyptes wimani is here recovered in the same collapsed node as Archaeospheniscus and not within Delphinornis, as in former analyses.

The synsacrum is an important element of the axial skeleton in birds, both volant and flightless. Little is known about the maturation of this complex bone in penguins. In this work, the supposedly ontogenetically youngest known synsacrum of early penguins was described. The analysis of this specimen, collected within the Eocene La Meseta Formation of Seymour (Marambio) Island, Antarctic Peninsula, revealed that this bird had attained at least the fledging stage of growth. Studies of three mature synsacra recovered from the same formation focused on the synsacral canals and, using indirect reasoning, their contents. These analyses revealed that the lumbosacral intumescence of the spinal cord and its extensions, the transverse canals, had been developed roughly like those in extant penguins (and also swifts and cormorants). The neural spine extensions (a non−nervous tissue) tracing the transverse grooves of the dorsal wall of the synsacral canal are currently considered as involved in the control of walking. The presented data suggest that such a sense organ gained its current penguin configuration by the late Eocene.

The fossil record of the Antarctic penguins is dated to the late Paleocene of Seymour (Marambio) Island, but the largest sphenisciforms, genera Anthropornis and Palaeeudyptes , originate from the Eocene La Meseta Formation. Here, the most complete large−scale reconstruction of a limb skeleton (a whole wing and a partial hind leg) of a Paleogene Antarctic penguin is reported. All bones are attributable to a single individual identified as Anthropornis sp. The comparative and functional analyses of the material indicate that this bird was most probably well−adapted to land and sea while having a number of intriguing features. The modern−grade carpometacarpal morphology is unique among known Eocene Antarctic species and all but one more northerly taxa.