Sphenisciformes (Ksepka & Clarke, 2010)

Park, Travis & Fitzgerald, Erich M. G., 2012, A review of Australian fossil penguins (Aves: Sphenisciformes), Memoirs of Museum Victoria 69, pp. 309-325 : 310

Origin of Sphenisciformes

The fossil record of penguins is one of the longest and relatively complete of any of the neornithine groups, potentially allowing scientists to test hypotheses regarding the physical drivers of vertebrate evolution e.g. climate change, palaeoceanography ( Baker et al., 2006; Ksepka & Thomas, 2012), biogeography ( Clarke et al., 2007), secondary adaptation to water ( Thomas & Fordyce, 2007), and stratigraphic calibration of molecular divergence estimates ( Slack et al., 2006). Thus the penguin fossil record informs broader issues in macroevolution.

The oldest known penguin, Waimanu manneringi Jones, Ando and Fordyce, 2006 , is from the early Paleocene (60.5 – 61.6 Ma) of New Zealand ( Slack et al., 2006). Although archaic, it is clearly a penguin and already flightless. Simpson (1946) summarised previous theories of penguin evolution (e.g. Lowe, 1933) and concluded that penguins evolved directly from a volant ancestor, with no intermediate terrestrial stage. Molecular data estimate the divergence of Sphenisciformes from their sister taxon, Procellariiformes, about 71 Ma during the Cretaceous ( Baker et al., 2006; Brown et al., 2008). Slack et al., (2006) wrote that the origins of Sphenisciformes took place 90–100 Ma as part of the Late Cretaceous neornithine radiation. It has been proposed that once the loss of aerial flight had occurred the adaptation of penguins to an aquatic lifestyle occurred rapidly due to the opening of ecological niches left by the extinction of most marine reptiles at the end of the Cretaceous and the intensive selection pressures of entering a new “adaptive zone” ( Fordyce & Jones, 1990).

Some fossil species reached giant sizes (e.g. Anthropornis nordenskjoeldi Wiman, 1905 , Pachydyptes ponderosus Oliver, 1930 ) of 1.5 – 1.6 m in standing height ( Jadwiszczak, 2001), far exceeding that of today’s largest species Aptenodytes forsteri Gray, 1844 (emperor penguin), which rarely exceeds 1.0 m ( Friedmann, 1945; Stonehouse, 1975; Ksepka et al., 2012). Nonetheless, estimated heights of giant taxa may be overestimates following the first discovery of body proportions in a nearly complete stem penguin (Ksepka et al., 2012). These giant species (and potentially all stem species) are thought to have fed on fish, using their slender bills to spear large prey ( Olson, 1985; Myrcha et al., 1990, 2002; Ksepka et al., 2008), This contrasts with extant species, which tend to have shorter beaks ( Aptenodytes is an exception) and feed on smaller fish ( Zusi, 1975). This trophic specialisation is thought to have occurred relatively late in penguin evolution (Ksepka & Bertelli, 2006), with elongate, narrow beaks representing the ancestral condition ( Clarke et al., 2007). Fossil feathers are known from Inkayacu paracasensis Clarke et al., 2010 , a species from the Eocene of Peru. These show not only that the key features of penguin wing feathers had evolved early in penguin evolution, but that this particular species was reddish-brown and grey, considerably different from the iconic black and white colouration of extant penguins ( Clarke et al., 2010).

Considerable effort over the last two decades has been aimed at resolving penguin phylogeny including extinct taxa ( Sibley & Ahlquist, 1990; Baker et al., 2006; Gianni & Bertelli, 2004; Bertelli & Gianni, 2005; Bertelli et al., 2006; Ksepka et al., 2006; Slack et al., 2006; Walsh & Suarez, 2006; Clarke et al., 2007; Acosta Hospitaleche et al., 2007, 2008). There is a general consensus between morphological and molecular data ( Fig. 2 ), apart from the issues of where the phylogenies are rooted ( Livezey, 1989), and the timing of the divergence of the crown Spheniscidae ( Clarke et al., 2007) . Basal penguins form a paraphyletic group, with higher morphological disparity compared to crown Spheniscidae ( Davis and Renner, 2003) . This is most likely due to the relatively recent common ancestry and broadly similar feeding ecology of the crown Spheniscidae ( Zusi, 1975) . The timing of the crown clade’s divergence from stem Sphenisciformes is still unresolved, as molecular and morphological data give different estimates of ca. 41 Ma (Middle Eocene) and 11–13 Ma (Middle-Late Miocene), respectively ( Baker et al., 2006; Göhlich, 2007). All pre-Miocene taxa are stem Sphenisciformes (Ksepka & Clarke, 2010) , rendering the fossil record incongruent with the ancient divergence estimated from molecular data.

Figures

Figure 2. Temporally-calibrated phylogeny of Sphenisciformes modified from Ksepka and Ando (2011) and Ksepka et al. (2012), showing major trends in Sphenisciform evolution and the first occurrence of Sphenisciformes in each region with extant species. Stem sphenisciform branches are shown in black and crown clade Spheniscidae branches are shown in blue. Thick horizontal bars on branches indicate stratigraphic range maxima and minima. Abbreviations: AF, Africa; AN, Antarctica; AU, Australia; Ma, million years ago; SA, South America.