A goldmine of New Zealand's prehistoric natural history has just yielded two more long-lost native species – tiny flightless rails.
Scientists discovered fossil bones of what have just been described as two new species of rail near St Bathans in Central Otago, where many other ancient specimens have been unearthed previously.
The remains - found in deposits estimated to be between 16 and 19 million years old – are thought to represent the oldest flightless rails known globally.
The two new species are minuscule in relation to today's rails, which include weka, takahe and pukeko.
One was barely larger than a sparrow.
Many rail species fly and have dispersed to far-flung oceanic islands.
But rails have often evolved into flightless forms on islands, more so than any other bird group.
The world's largest flightless rails evolved in New Zealand - notably the takahe and weka.
"Flightlessness in birds is often associated with an increase in size," said Ellen Mather, a PhD student at Australia's Flinders University, who led the study describing the new discoveries.
"The weka, which is in the same family as our fossil birds and lives in New Zealand today, is about the same size as a chicken.
"The banded rail, the weka's closest flying relative, is about half that size."
The most common of the new fossil rails, which has been named Priscaweka parvales - meaning ancient weka with small wings - was a mere one twentieth of the weight of a weka and similar in size to the recently extinct Chatham rail, or Cabalus modestus.
Small flightless birds existed only in the absence of terrestrial mammal predators.
When humans discovered New Zealand, the main islands had many flightless birds including giants such as the nine species of moa, two giant geese, two adzebills, even some tiny wrens, and at least five flightless rails.
When the two rail species roamed New Zealand, the South Island was dominated by Lake Manuherikia - a "megalake" that spanned 5600sq km and was surrounded by a subtropical rainforest of plants typical of Australia and long lost from this land.
Gum trees, sheoaks, palms and cycads were common.
Today, traces of this giant lake are revealed in sediments around the town of St Bathans.
Canterbury Museum's curator of natural history and study co-author, Dr Paul Scofield, said the new St Bathans rails join a host of other fossil birds recovered from these deposits that show New Zealand has long been a land of birds.
"The discovery of these two minuscule flightless rails raises the question, 'Where did they come from?'" Schofield said.
"The new species are unlike any rail known elsewhere so their exact origin or closest relatives remain a mystery."
Another co-author, Te Papa's Alan Tennyson, added: "This fossil rail discovery reinforces New Zealand's importance in understanding the evolution of birds worldwide and we are sure that many more important fossil discoveries will be made at this site."
Other than hints of large flightless moa ancestors, these rails are the first flightless birds to be confirmed from the "St Bathans Fauna" group.
Flightless birds had been a feature of the New Zealand avifauna for millions of years longer than generally thought.
"The ongoing research into the fossil birds of New Zealand builds on that begun over 150 years ago," said Associate Professor Trevor Worthy, of Flinders University.
"It continues to throw up revelations into the timing and origins of major groups of birds that characterise modern avifaunas."
Co-author Professor Mark Archer, of the University of NSW's PANGEA Research Centre, said the discovery reinforced the fact New Zealand had long been "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 60 million years will test basic presumptions made about bird evolution in general."
The findings, published overnight in the Journal of Systematic Palaeontology, come weeks after scientists described another curious St Bathans creature: a burrowing bat that was three times the size of the average bat today.
The extinct Vulcanops jennyworthyae, which weighed about 40g, represented the largest burrowing bat known to science, and New Zealand's first new bat genus for more than 150 years.
Others in the St Bathans Fauna group have included crocodiles, terrestrial turtles, flamingo-like palaelodids, swiftlets, and several pigeon, parrot and shorebird lineages.
Most of these were probably warm-adapted species.
The group also included the enigmatic St Bathans mammal, whose existence suggested that terrestrial mammals did in fact once live in Zealandia, the recently recognised "eighth continent" that today lies mostly submerged beneath the ocean.