A newly described bird from prehistoric Japan was an odd duck, so to speak. The enormous swan lived in the sea, sporting stubby wings that it possibly used to create a cradle on its back for offspring.
at Kyoto University and at the Gunma Museum of Natural History in Japan analysed a fossilised skeleton that was excavated in 2000 in the Usui river in central Japan. The riverbed鈥檚 marine deposits date back to the Miocene epoch more than 11 million years ago.
Comparing the skeleton to modern swans, the team identified it as a new genus and species, dubbed Annakacygna hajimei. A second, larger new species from the same genus was identified from a fossilised leg bone found along the nearby Kabura river in 1995 and named Annakacygna yoshiiensis.
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The Annakacygna genus differs considerably from modern swans, with the two extinct species being flightless sea-dwellers with a suite of bizarre adaptations seemingly cobbled together from other water birds.
A. hajimei was the size of a modern black swan (Cygnus atratus), says Matsuoka. A. yoshiiensis, however, was about 30 per cent larger, on par with Trumpeter swans (Cygnus buccinator), the largest of North America鈥檚 swan species. But Annakacygna had a substantially heavier build than modern swans, with dense bones. The researchers think this more massive frame probably provided stability as the birds navigated choppy seas.
Annakacygna were also 鈥渉ead heavy鈥�, with big, broad bills for filter-feeding surface plankton, says Matsuoka.
The swan鈥檚 short wings were unusual. The forearm bones were just over half the length of the upper arm bones. In modern swans, these bones are about the same length. The flightless bird鈥檚 fossilised bones possessed physical traces of highly developed muscles that, in today鈥檚 swans, are only engaged when folding the wings.
Flexible shoulder joints allowed the wings to stretch upwards along the back, but the wrist wasn鈥檛 able to flex much beyond a set angle. The muscles and joints of the wing may have helped produce flapping courtship displays, and when combined with the flexible, muscular tail arching upwards, the wings may also have made a cradle to transport hatchlings. Modern swans sometimes give cygnets 鈥減iggybacks鈥� with similar positioning of the wings, but Annakacygna may have taken the behaviour to an extreme.
Such great specialisation in feeding and reproduction, processes critical for an organism鈥檚 survival and legacy, make Annakacygna the 鈥渦ltimate bird鈥�, says Matsuoka.聽Future research may uncover why the genus went extinct, he says.
鈥淭he fact that such an animal existed is the biggest surprise,鈥� says Matsuoka.
Albert Chen at the University of Cambridge says 础苍苍补办补肠测驳苍补鈥檚 combination of anatomical features 鈥� such as its loon-like feet and duck-like bill 鈥� is unique among birds.
鈥淭hese are some of the most remarkable fossil birds 鈥� indeed, among the most remarkable dinosaurs 鈥� to be named in recent times,鈥� says Chen.
搁别蹿别谤别苍肠别:听Bulletin of the Gunma Museum of Natural History
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