Thursday 9 January 2014

Fossil Kiwi from the Miocene of South Island, New Zealand.

Kiwi (the term is Māori and can be plural or singular without modification) are small Ratite Birds from New Zealand, considered to be evolutionarily convergent with small Mammals due to their fur-like feathers, burrowing, nocturnal lifestyle and scent-led foraging strategy. Like other Ratites (Ostriches, Rheas, Cassowaries and Emus, plus the extinct Elephant Birds of Madagascar and Moa of New Zealand) Kiwi are flightless, with the females laying large eggs which are then raised exclusively by the males. This breeding strategy is particularly notable in the Kiwi, as the single large egg laid is approximately 25% of the mother's mass. This has led to speculation that Kiwis have evolved from much larger Birds (other than the Chicken-sized Kiwi all other Ratites are large animals), and that the large egg size is an evolutionary quirk caused by the shrinking of adult Bird size in an environment free of egg predators. Despite being endemic to New Zealand, which has had no land connection to any other land mass for around 55-82 million years, Kiwis are thought to be more closely related to the Cassowaries and Emus of Australia than to the extinct Moa (another Māori word that can be plural or singular without modification) of New Zealand, and to have shared a common ancestor with these Australian Birds around 53.5 million years ago, making their distribution somewhat of a mystery.

In a paper published in the Proceedings 8th International Meeting of the Society of Avian Paleontology and Evolution on 10 December 2013, a team of scientists led by Trevor Worthy of the School of Earth and Environmental Sciences at the University of Adelaide, describe a new species of fossil Kiwi, from the Early Miocene (16-19 million years ago) of St Bathans in Central Otago, towards the southern end of New Zealand's South Island.

The new Kiwi species is named Proapteryx micromeros, where 'Proapteryx' means 'before Kiwi' and 'micromeros' means 'little thigh'. The species is described from a right femur missing the distal condyles (bottom end) and a separate left quadrate (part of the jaw joint). These bones appear to be from a Bird somewhat smaller than the modern Kiwi, probably less than 500 g, which Worthy et al. suggest may have been able to fly (this is not totally surprising as modern Ratites are thought to be descended from flying ancestors), which could help to explain how the Kiwi came to colonize New Zealand (whichwas closer to Australia in the Miocene)

The right femur of Proapteryx micromeros in (A) medial, (B) cranial and (C) caudal views. Scale bars are 10 mm. Worthy et al. (2013).

Left quadrate of Proapteryx micromeros in (A) medial, (B) lateral and (C) anterolateral views. Abbreviations: cml, cond. mandibularis lateralis; cmm, cond. mandibularis medialis; cp, cond. pterygoideus; cr, crista; po, proc. orbitalis; ts, tuber. subcapitulare. Worthy et al. (2013).

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