Abstract
We describe a partial skeleton of a stem group penguin from the Waipara Greensand in New Zealand, which is tentatively assigned to Muriwaimanu tuatahi. The fossil includes the first complete wing of a Paleocene penguin and informs on previously unknown features of the mandible and tibiotarsus of small-sized Sphenisciformes from the Waipara Greensand. The wing is distinguished by important features from that of all geologically younger Sphenisciformes and documents an early stage in the evolution of wing-propelled diving in penguins. In particular, the wing of the new fossil exhibits a well-developed alular phalanx and the distal phalanges are not flattened. Because the wing phalanges resemble those of volant birds, we consider it likely that the wing feathers remained differentiated into functional categories and were not short and scale-like as they are in extant penguins. Even though the flippers of geologically younger penguins may favor survival in extremely cold climates, they are likely to have been shaped by hydrodynamic demands. Possible selective drivers include a diminished importance of the hindlimbs in subaquatic propulsion, new foraging strategies (the caudal end of the mandible of the new fossil distinctly differs from that of extant penguins), or increased predation by marine mammals.
Highlights
Penguins (Sphenisciformes) are flightless, wing-propelled diving birds, in which the forelimbs attained a flipper-like shape
Even though penguins have a comparatively extensive fossil record that dates back to the earliest Cenozoic Era [3,4], little is known about the evolutionary transitions that occurred in the evolution of the penguin wing
In light of a pending revision of the fossil material, our study mainly focuses on the morphology of the wing bones, but, as detailed in the following, the new fossil elucidates other previously unknown aspects of the osteology of Paleocene stem group Sphenisciformes
Summary
Penguins (Sphenisciformes) are flightless, wing-propelled diving birds, in which the forelimbs attained a flipper-like shape. The Paleocene strata of the Waipara Greensand in New Zealand have, in particular, yielded a diverse array of stem group Sphenisciformes These fossils represent the oldest known penguins and belong to several differently-sized species. Two very large-sized penguins from the Waipara Greensand, Crossvallia waiparensis as well as an unnamed species [7,8], are only represented by leg bones These fossils yielded much new information on the skeletal morphology of the earliest stem group. The fossil is from a small-sized species and is tentatively assigned to Muriwaimanu tuatahi, which is the smallest sphenisciform species from the Waipara Greensand This assignment is based on the similar shape and concordant dimensions of the wing elements of the new fossil and those of M. tuatahi. In light of a pending revision of the fossil material, our study mainly focuses on the morphology of the wing bones, but, as detailed in the following, the new fossil elucidates other previously unknown aspects of the osteology of Paleocene stem group Sphenisciformes
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