Introduction
Penguins (Aves: Sphenisciformes) are icons of Antarctic wildlife. Nevertheless, they are not restricted to this part of the Southern Hemisphere, and most species occur in more northerly areas (Davis & Renner Reference Davis and Renner2003). The longest latitudinal range (across nearly 75° latitude) of penguin rookeries comprises coastal West Antarctica, the Scotia Arc and the Pacific coast of South America (Jadwiszczak Reference Jadwiszczak2009, fig. 3). Taking into account plate tectonics, a distributional pattern similar to that present in extant Sphenisciformes can be observed in the record of Eocene (55.8–33.9 Ma) penguins. Their remains are known from as far south as the Antarctic Peninsula (mainly isolated bones) and as far north as Peru (Jadwiszczak Reference Jadwiszczak2009, Clarke et al. Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010).
The most diverse assemblage of these seabirds comes from the Eocene La Meseta Formation of Isla Marambio (Seymour Island; northern Antarctic Peninsula), particularly from its upper part corresponding to the late middle and late Eocene (Jadwiszczak Reference Jadwiszczak2010, fig. 2). Individuals from four species (two genera differing in morphology and frequency), Anthropornis nordenskjoeldi Wiman, 1905, A. grandis (Wiman, 1905), Palaeeudyptes gunnari (Wiman, 1905), P. klekowskii Myrcha, Tatur & del Valle, 1990, were larger than the largest modern penguins (Aptenodytes forsteri Gray; Jadwiszczak Reference Jadwiszczak2001). The record of Eocene penguins from South America comprises a partial skeleton (slightly smaller than that of A. forsteri) from the late middle Eocene of Argentine Tierra del Fuego (Clarke et al. Reference Clarke, Olivero and Puerta2003), ten bones of both small-bodied and ‘giant’ birds (undetermined sphenisciforms and Palaeeudyptes sp. respectively) from the middle to late Eocene of the Magallanes Region, Chile (Sallaberry et al. Reference Sallaberry, Yury-Yáñez, Otero, Soto-Acuña and Torres2010), and partial skeletons of three species from the middle and late Eocene of Peru: a relatively large-bodied Perudyptes devriesi Clarke et al., Reference Clarke, Ksepka, Stucchi, Urbina, Giannini, Bertelli, Narváez and Boyd2007 as well as two ‘giant’ penguins, Icadyptes salasi Clarke et al., Reference Clarke, Ksepka, Stucchi, Urbina, Giannini, Bertelli, Narváez and Boyd2007 and Inkayacu paracasensis Clarke et al., Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010 (Clarke et al. Reference Clarke, Ksepka, Stucchi, Urbina, Giannini, Bertelli, Narváez and Boyd2007, Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010). Inkayacu paracasensis, diagnosed mainly on the basis of five “autapomorphies within Sphenisciformes” (i.e. unique features), is of particular interest because its remains include partial feathering (Clarke et al. Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010). Presented here are bones from the late Eocene of the Antarctic Peninsula demonstrating that some of postulated (Clarke et al. Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010) autapomorphies of Inkayacu are actually synapomorphies shared with taxa represented by these Antarctic specimens. This finding substantially expands knowledge of the source region for the second oldest equatorial ingression by Palaeogene penguins.
Two Antarctic penguin bones discussed here come from the upper La Meseta Formation (north-north-west side of the unit Telm7; Jadwiszczak Reference Jadwiszczak2010, fig. 1), Seymour Island (64°17′S, 56°45′W). They are permanently deposited at the Institute of Biology, University of Białystok, Poland; abbreviated IB/P/B (Jadwiszczak Reference Jadwiszczak2006).
Results
Three of the five autapomorphies found by Clarke et al. (Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010) in I. paracasensis have their counterparts in some Antarctic specimens assigned by me to so-called giant sphenisciforms (Jadwiszczak Reference Jadwiszczak2006). The dorsal surface of the Peruvian premaxilla has paired grooves meeting at the midline, and the same structure is conspicuous in an Antarctic specimen (IB/P/B-0167) attributed to an individual from the genus Anthropornis or (more probably) Palaeeudyptes (Fig. 1; since this bone is long though not massive it probably belonged to P. klekowskii). Moreover, a distal femur of Palaeeudyptes gunnari (IB/P/B-0103, a relatively well-preserved fossil; see Jadwiszczak Reference Jadwiszczak2006) quite closely resembles that of I. paracasensis: both possess a distally tapering medial condyle and a tab-like process on the ventral surface (Fig. 2). The above-mentioned features are slightly more pronounced in Inkayacu, but taking into account variability observable in femora assigned to Palaeeudyptes, I would not regard them as autapomorphies. Another aspect of the medial condyle morphology, its considerable width in the Peruvian species, is less obvious. However, judging from the preserved parts of the articular surface (Fig. 2b), P. gunnari probably did not differ substantially in respect to this character from Inkayacu.
Distal upper jaw, IB/P/B-0167, of the Eocene penguin (?Palaeeudyptes klekowskii) from Isla Marambio. a. Dorsal view. b. Close-up of its most intriguing part in dorsal view.
Distal femur, IB/P/B-0103, of the Eocene penguin (Palaeeudyptes gunnari) from Isla Marambio. a. Medial view. b. Ventral view. c. distal view. The fossil was coated with ammonium chloride sublimate prior to photography.
Discussion
Clarke et al. (Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010) placed I. paracasensis in a clade of Eocene giant penguins together with two Antarctic species of Palaeeudyptes (the larger P. klekowskii and smaller P. gunnari) and a problematic Burnside “Palaeeudyptes” from New Zealand. Considering the overall interspecific similarity confirmed by a phylogenetic analysis (Clarke et al. Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010) and remarks above, it seems obvious that I. paracasensis and two La Meseta “palaeeudyptids” were very closely related.
Such a conclusion also implies that a second early-stem penguin dispersal to low latitudes (Clarke et al. Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010) originated in the Antarctic Peninsula region. It followed a slightly earlier dispersal from the Antarctic (completed by the middle Eocene) and preceded an equatorial ingression from New Zealand (by the late Eocene; Clarke et al. Reference Clarke, Ksepka, Stucchi, Urbina, Giannini, Bertelli, Narváez and Boyd2007). Interestingly, the supposed close relationship of the discussed taxa enables considerations on Palaeeudyptes plumage colours (the feathering may have been predominantly gray and reddish-brown; Clarke et al. Reference Clarke, Ksepka, Salas-Gismondi, Altamirano, Shawkey, D'Alba, Vinther, DeVries and Baby2010). However, to verify that, more quality fossils are needed, and a recent discovery of a semi complete skeleton of P. gunnari (Hospitaleche & Reguero Reference Hospitaleche and Reguero2010) testifies that not only isolated bones can be collected in Antarctica.
Acknowledgements
I am indebted to Stig Walsh (National Museums Scotland) who commented on an early version of this manuscript, and Magdalena Siemieniuk (University of Białystok, Poland) who helped produce the figures. The manuscript benefited from constructive reviews by Carolina Acosta Hospitaleche (Museo de La Plata, Argentina) and Tatsuro Ando (Ashoro Museum of Paleontology, Japan).
