Detailed comparisons with extant and fossil species of Sphe niscus suggest that the available postcranial bones of S. mui zoni sp. nov. morphologically correspond best with those of S. urbinai, aside from that the latter is distinctly larger. Some shared features are found to be exclusive for these two species within Spheniscus, such as the lack of a ventrocaudally pro jecting lip proximally bordering the pneumotricipital fossa on the humerus, or a tarsometatarsuswhich trochlea III is dorsally at about the same level as the lateral condyle of trochlea VI or slightly surpasses it. Although of almost identical size, S. muizoni sp. nov. can be distinguished by several osteological features from S. chilensis, such as the limb bone proportions of the carpometacarpus (Fig. 6) and morphological differences at the humerus, carpometacarpus, femur, tibiotarsus, and tarso− metatarsus. Based on the morphological similarity of S. mui zoni sp. nov. and S. urbinai and their stratigraphical succes sion within the Pisco Formation, it can be supposed that the first gave directly rise at least to the latter.
S. muizoni sp. nov. is the only known penguin species from the Cerro la Bruja locality and is unknown from any older or younger deposit in or outside the Pisco Formation. As pen guins are shorebirds—breeding on land (or ice) and feeding in fairly adjacent marine waters (del Hoyo et al. 1992), the pen guin fossils complement the exclusively marine vertebrate fauna represented at Cerro la Bruja, comprising fossil taxa of marine fishes (Teleostei) and sharks (Selachii), a turtle and a crocodile, different marine mammals such as fossil odonto cetes (Kentriodontidae, Pontoporiidae),mysticetes (Cetotheri idae, Balaenopteridae), pinnipeds (Phocidae), and a boobie (Aves, Sulidae; Muizon 1988: table 1).
To date, all fossil penguins reported from the different levels of the Pisco Formation belong to the extant genus Spheniscus. The today living species of Spheniscus include: S. mendiculus (Galapagos), S. humboldti (coast of Peru and Chile), S. magellanicus (coast of Chile and Argentina, Islas Malvinas or Falkland Islands), and S. demersus (coast of South Africa and Namibia; del Hoyo et al. 1992).
However, S. muizoni sp. nov. from the latest middle/earli est late Miocene of Cerro la Bruja is not only the most ancient penguin species in the Pisco Formation, but also the strati graphically oldest record for the extant genus Spheniscus in general.
The fossil record indicates a faunal change of Spheniscus specieswithin the stratigraphically older sequence of the Pisco Formation. S. muizoni sp. nov., exclusive in the oldest deposits (latest middle/earliest late Miocene, 13–11 Ma) is replaced by the persistant S. urbinai, existing from the late Miocene to the early Pliocene (9 to 3.5Ma) deposits (S. megaramphus is only known from a 6 Ma old locality). This replacement might be linked to changes of the climatic conditions at the Pacific coast of South America during the latest middle Miocene. Tsuchi (2002: 269) dated a short warm episode in between cool epi sodes of the surface marine climate of the latest middle Mio cene at ca. 11.5 Ma, which is supported by deposits rich in warm water planktonic foraminifera in northern Chile. Extant penguins (on a species level) require a relatively stable water temperature (del Hoyo et al. 1992: 144) and change of water temperature (e.g., as in the phenomenon known as El Niño) is known to have far reaching effects on the Humboldt and Galapagos Penguin. It is not known yet, if there is a faunal consequence also within some other fossil vertebrate groups represented in Cerro la Bruja and younger localities in the Pisco Formation because the vertebrate fauna of Cerro la Bruja has not been studied in detail as yet.
Apart from Spheniscus, there is only very scanty penguin material known from other Peruvian localities, which is de scribed as Palaeospheniscus sp. and Spheniscidae indet. (Acosta Hospitaleche and Stucchi 2005). However, the latter comes from deposits older than the Pisco Formation, the lower middle Miocene Chilcatay Formation and the upper Eocene–lower Oligocene Otuma Formation, respectively. Outside the Pisco Formation, fossil representatives of Sphe niscus have been reported only from Chile: S. chilensis from the late Pliocene and Spheniscus sp. from the late Miocene early Pliocene. There was only one report on a fossil Sphe niscus outside of South America, S. predemersus Simpson, 1971 described from South Africa. This species was, however, later referred by Simpson (1975b) to the new genus Inguza. Thus, the fossil record of Spheniscus is restricted to the lat est middle/earliest late Miocene to the late Pliocene of South America, more precise to the Pacific coast of South America (Peru and Chile). The modern distribution of Spheniscus cor relates with cold marine currents—the west−east directed Cir cumpolar Antarctic Current and their northern upturns along the west−coasts of South America (Humboldt Current) and southern Africa (Benguela Current). However, the restriction of the Mio−Pliocene record of Spheniscus to the Pacific coast of South America was not due to a geographic barrier, as the Drake passage was already open by the late middle Eocene around 40 Ma ago (Scher and Martin 2006). However, no re cord of Spheniscus is known along theAtlantic coasts of South America or southern Africa during the Neogene. However, the presence of this genus cannot be ruled out if their restriction to the Pacific coast during the Neogene is linked to other ecologi cal or alimentary factors—perhaps interrelated to the still open Panama Isthmus (closure starts during the early Pliocene, about 4–3 Ma, Kameo and Sato 2000) and thus corresponding marine currents, which might had an effect on the distribution of potential prey. In all extant Spheniscus species pelagic school fish (mostly anchovies) is the dominant prey; cephalo pods and crustaceans are subordinate (del Hoyo et al. 1992). Because the skull of S. muizoni sp. nov. is lacking, no informa tion is available on the shape of the bill, which varies at least in some extant penguin genera and their preferred diet. Neither fossil nor extant Spheniscus species show any obvious interre lation of body size and latitude of their habitat.