Abstract
AbstractOne of the most remarkable differences between Paleogene penguins and their living relatives is the shape and length of their beaks. Many of the Eocene and Oligocene penguins have a thin and elongated spear-like bill, which contrasts with the proportionally shorter and more robust bill of most living species. These differences suggest an important shift in their feeding strategies. This study explores the morphological disparity on the skull of penguins, emphasizing bill morphology and it relationship with feeding habits. For this, the skulls of 118 species of aquatic birds, including 21 fossil and living penguins, were analyzed using two-dimensional geometric morphometric. The results show that, unlike what has been reported for modern birds overall, in penguins and Aequornithes, bill elongation is related to a reduction of the braincase. The discriminant analysis shows that there are significant differences between penguins that feed near or far from the coast and between those that consume nectonic and planktonic prey, identifyingMadrynornisas the only extinct form with a possibly planktonic diet. Additionally, it is clear that Paleogene penguins occupy a region of morphospace unexplored by most diving birds, with the western grebe being their closest modern analogue. This is consistent with the hypothesis that giant penguins hunted by harpooning and not by biting as living forms do, signaling a significant change in the habits of those birds leading to the emergence of their crown group.
Highlights
Though very little was known about the cranial morphology of stem penguins until the twenty-first century, we know that one of the most striking differences between extant penguins and their Paleogene relatives can be seen in their bills (Fig. 1)
The first PC mainly describes the proportion between braincase size and bill length; whereas the second PC describes the position of the braincase with respect to the tomial edge of the bill, as well as the general bill sinuosity (Supplementary Fig. 2A)
Whereas basal-most penguins like Waimanu and Perudyptes seem to have had comparatively shorter dagger-like bills, resembling the condition observed in loons and large grebes, the acquisition of a hyperelongated bill in penguins seems to be related to the appearance of giant taxa
Summary
Though very little was known about the cranial morphology of stem penguins until the twenty-first century, we know that one of the most striking differences between extant penguins and their Paleogene relatives can be seen in their bills (Fig. 1). Many stem penguins shared a distinctive and extremely elongated spear-like bill (Ksepka and Ando 2011), representing more than two-thirds of the skull length This contrast with the shorter and more robust beak present on most modern penguins suggests an important shift in their feeding strategies. The first included a partial beak described as straight and overall similar to the long beak of the modern king penguin, whereas the second is a straight and pointed fragment of jaw with an unusually long symphysis. The latter specimen was so different from any modern taxa that Marples was not even sure whether it belonged to a penguin. The subsequent discovery of Waimanu (Slack et al 2006), Icadyptes (Clarke et al 2007), and Inkayacu (Clarke et al 2010) showed that a narrow and slender pointed bill was common among Paleogene taxa, and this is considered as a character shared by most of the stem penguins (Ksepka and Ando 2011)
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