Stilwell’s penguin

Stilwell’s penguin is one of the earliest penguins known. Fossils were recovered from 62.5-60-million-year-old (late early to middle Paleocene) marine deposits of the Takatika Grit, at Maunganui Beach, Chatham Island (Wharekauri / Rēkohu).

The holotype, NMNZ S.47312, consists of associated fossils of a single individual, including a left tarsometatarsus, a left radius, and a caudal vertebra, which are all held at Museum of New Zealand Te Papa Tongarewa. Another four specimens have been referred to this species, and are also held there. Only one of these specimens consists of a single bone (left coracoid), whereas the others comprise of several skeletal elements and similarly represent associated skeletons of individuals. Together, these fossils have allowed the description of a large portion of the skeleton of this species.

Compared to all living and extinct penguins, Stilwell’s penguin was medium-sized, and likely did not stand taller than a modern king penguin, at less than 1 metre in height. Although it is among the most ancient of penguins known, Stilwell’s penguin was completely aquatic, as all modern penguins are today. Its robust and flattened fore-limb bones indicate that this species was well-adapted for underwater wing-propulsion and was aerially flightless. However, its wings were not as stiff and flipper-like as many geologically younger penguins. The tarsometatarsus bone of the lower leg was short and wide, which better resembles the bones of modern penguins compared to those of aerially flighted close relatives (such as diving petrels), in terms of proportions. Today, penguins use their highly specialised wings to efficiently dive underwater, to great depths at considerable speeds, in a medium much denser than air. Their short tarsometatarsi contribute to the structure of their feet, which are used as rudders for steering while swimming, as well as to rest upon, walk on, and propel themselves from prone positions. Despite the great age of Stilwell’s penguin, the osteological adaptations apparent in its skeleton imply a degree of underwater diving proficiency, and display many characteristics fundamental to the basic penguin body plan.

Stilwell’s penguin lived only a few million years after the Cretaceous-Paleogene mass extinction event, in a region that now corresponds to the Chatham archipelago (Wharekauri / Rēkohu). Around 60 million years ago, this archaic penguin would have inhabited the shoreline of ephemeral islands and adjacent nearshore or shallow-marine environments that existed in this area, bounded by deep oceans north and south of the Chatham Rise. Surface water temperatures at the time are likely to have been sub-tropical to tropical.

Fossils of several other species of ancient penguin are known from the eastern coast of New Zealand’s South Island, during a similar time interval to Stilwell’s penguin. Many of these were much larger, affording them better thermal insulation, as well as a greater capacity for deep diving. Some contemporary giant forms had stout tarsometatarsi similar to Stilwell’s penguin, which would have additionally contributed to a hydrodynamic body-shape, aided in heat-loss prevention, and possibly also augmented their underwater ‘flying’ ability through steering. While these may have allowed the largest penguins to attain a competitive edge, the short tarsometatarsus in Stilwell’s penguin would have conferred different diving capabilities compared to similarly-sized ancient penguins with more elongate tarsometatarsi such as the Waipara penguin (Muriwaimanu tuatahi), and facilitated ecological niche separation. Stilwell’s penguin is unique in that it possessed a shortened tarsometatarsus, within an overall body size range comparable to that of modern penguins.

Stilwell’s penguin contributes to the large diversity of the world’s earliest penguins, which are all solely aquatic and relatively specialised for underwater locomotion. The highly derived form of this species, well-adapted for life in the aquatic environment, provides further evidence that the penguin lineage diverged from that of their closest relatives, the Procellariiformes, in the Upper Cretaceous, rather than the Paleocene.

The genus name, Kupoupou, means “diving bird” in Te Re Moriori, in recognition of the Rēkohu provenance of the fossils. The specific epithet stilwelli is after palaeontologist Jeffrey D. Stilwell, who organised and led field work parties in recovering the fossil material attributed to this taxon.