Fossils of surprisingly tiny flightless Central Otago birds present a mystery

Wednesday, 28 February 2018

The tiny extinct rail - weighing 30-40g - overshadowed by a regular duck. Artist's impression courtesy Gavin Mouldey

Fossils of some tiny flightless prehistoric birds found in Central Otago are raising intriguing questions for scientists.

The fossils come from two species that lived 16-19 million years ago, were members of the rail (Rallidae) family, and one is barely larger than a sparrow.

Rails include swamphens, moorhens, coots and crakes, and are common around wetlands. The world's largest rails evolved in New Zealand, notably the flightless takahe and weka.

The more common of the new fossil rails is named Priscaweka parvales, meaning ancient weka with small wings, and was just 1/20th of the weight of a weka.

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When the two newly-identified species were alive, the climate and landscape was very different from that of today.

'At that time, a 5600-square kilometre mega-lake dominated the landscape of New Zealand's South Island. It was surrounded by a subtropical rainforest,' a statement from Flinders University said. 'Plants typical of Australia and long lost from New Zealand, such as eucalypts, casuarinas, palms and cycads, were common.'

Along with the Flinders' researchers, others involved in the project were from the University of NSW, Canterbury Museum and the Museum of New Zealand. Their findings were published in the Journal of Systematic Palaeontology.

The fossils were found along the banks of Mata Creek and the Manuherikia River, near St Bathans in Central Otago. They are among the St Bathans Fauna found in sediments near the base of the early Miocene (19 to16-million-years-old) Bannockburn Formation. Other St Bathans Fauna include crocodiles, turtles and a mammal that lived on land.

Professor Mike Archer, from the UNSW Pangea research centre, said the latest discoveries emphasised New Zealand's natural history as one of the world's 'most extraordinary engines driving bird evolution'.

'Charting how lineages like these rails have changed through time on an island that has been geographically isolated for over 80 million years will test basic presumptions made about bird evolution in general,' Archer said.

It's thought the two newly-identified species came from Australia, having flown the 1500km across the Tasman Sea. 'However, the new species are unlike any rail known elsewhere so their exact origin or closest relatives remain a mystery,' the statement said.

The published study added that although New Zealand was at a similar distance from Australia in the early Miocene as now, dispersal was perhaps less difficult then. Geological evidence suggested that in the early Miocene, there were other islands and seamounts in the Tasman Sea that could have acted as stepping stones.

The discovery of the fossils of the two species showed flightlessness in New Zealand birds was not a recent phenomenon, although relatively large flightless members of their evolutionary descendents were, the study said.

The abundance of flightless birds in New Zealand before human colonisation had been attributed to a lack of mammalian predators that lived on land.

So far, no mammalian terrestrial predators were known from any New Zealand fossil site, and the St Bathans mammal was an enigma. It had caused a reconsideration of the idea bird-dominated ecosystems only arose in the absence of non-chiptoran (non-bats) mammals, the paper said.

'Its presence apparently did not preclude the development of flightlessness among small birds, suggesting that this mammal, or these mammals, were unlikely to have strongly predated upon them,' the paper said.

While the St Bathans Fauna did include predatory birds, as well as the crocodiles, their presence also did not seem to have precluded the evolution of flightlessness in rails during the early Miocene.

There was substantial climatic cooling in New Zealand not long after the St Bathans deposits were formed. That was most likely responsible for the extinction of the tropical fauna and flora, and could have caused the extinction of the St Bathans rails.