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A giant stork would have soared through the skies of prehistoric Indonesia.
Measuring almost two metres tall, Leptoptilos robustus would have lived in a unique ecosystem which included the mysterious miniature human relative Homo floresiensis.
Over 50,000 years ago, giant carnivorous storks would have competed with ancient hominins for food on a tropical island.
Described in 2010, Leptoptilos robustus is an extinct bird species which stood almost two metres tall. Until recently, it was thought to have been confined to the ground, but the discovery of more bones now shows it was able to fly.
It forms a part of an unusual group of Pleistocene animals found on the island of Flores, Indonesia, which is renowned for unique species including elephant-like mammals known as Stegodon and the human relatives Homo floresiensis, otherwise known as 'hobbits', who stood just over a metre tall.
Dr Hanneke Meijer, who led a study describing the new fossils, says that the storks' ability to fly would have made them a formidable competitor for these hominins.
'Although we know little about the diet of H. floresiensis, they might have been scavengers,' Hanneke explains. 'To gain access to carcasses, they would have had to compete with these large birds, vultures, and Komodo dragons.'
'This would have been tough competition for a one-metre-high hominin, and it is not unlikely that they were often left picking up the leftovers.'
The researchers found evidence that L. robustus may have been the last in a lineage of giant marabou stork which were gradually wiped out as our Homo sapiens ancestors spread across the world.
The island of Flores is located in southeast Indonesia, near to the island of Komodo. It is the home of the Komodo dragon as well as a number of unique species such as the Flores giant rat.
The biodiversity of the island has its roots in a process known as insular miniaturisation, where a lack of predators and resources can cause normally large species to shrink. This process has repeated itself across evolutionary history, from tiny Transylvanian dinosaurs to small Sicilian elephants and compact Cozumel raccoons.
On Flores, this process led to the evolution of shorter species and subspecies of the megaherbivore Stegodon. Elsewhere in the world these animals grew to almost four metres high, but on Flores the smallest, Stegodon sondaari, was only around 1.2 metres tall.
Collectively, the Stegodon species provided the basis of Flores' ecosystem during the Pleistocene as the herbivores were preyed upon by other animals on the island. Among them was H. floresiensis, discovered in Liang Bua cave in 2003, which was one of the latest living species of non-human hominin.
Though its ancestry is uncertain, H. floresiensis evolved to become smaller on Flores, where it is estimated to have lived between 100,000 and 50,000 years ago. Based on its skull and evidence of cut marks on Stegodon bones, it is believed to have eaten meat and other soft foods.
The hominin would have competed for meat with other species, something L. robustus's relatives still do today.
'We think that L. robustus's lifestyle was a carrion eater quite similar to that of the extant marabou stork L. crumenifer,' Hanneke says. 'This African species is known to be opportunistic and often gathers with vultures at large herbivore carcasses. It can be seen squabbling with vultures over pieces of meat, and I imagine that similar scenes played out at Liang Bua as well.'
When it was first described, only four bones of L. robustus were discovered. It was assumed, based on its size, that it would not have been able to fly like other giant, island-specific birds such as the moa and elephant bird.
The newly excavated bones, including parts of the skull and wing, reveal that the bird would likely have flown after all.
'We assumed initially that it was a bird with limited flight abilities that spend most of its life on the ground,' Hanneke explains. 'Our new research shows that this was not the case, and it has changed the way we look at this animal.'
'It likely soared to great heights using warm air currents in search of food, mostly Stegodon carcasses, and nested in colonies in the tops of large trees.'
The researchers suggest that these colonies may have been found around the Liang Bua site, where the bones of adults and juveniles were found. Floodwater from the nearby river is believed to have left standing water in the cave and provided drinking water for the area's fauna.
This would have provided a useful ambush site for Komodo dragon to attack prey, which in turn would have drawn scavengers such as L. robustus to the site.
Using the new fossils, the researchers hope to work out the evolutionary history of the giant stork. The leg bones of L. robustus are very similar to L. falconeri, an extinct species known from Africa, Europe and Asia, which the scientists suggest could be the ancestor of the region's storks.
Finding out how the giant storks evolved may also provide a deeper insight into the island's ecosystem during the Pleistocene.
'One of the main questions that remain is when the giant storks arrived on Flores, as we have so far only found bones from between 50-70,000 years ago,' Hanneke says. 'The similarities with the older L. falconeri, however, suggest that they might have arrived in the region earlier.'
'Excavations in the So'a Basin in central Flores have yielded older bones of Stegodon and hominins between 700,000 and 800,000 years old, so L. robustus may have been part of this older ecosystem as well. We haven't found any bones yet, but we are looking for them.'
Whenever the species evolved, the arrival of Homo sapiens coincides with the extinction of the giant stork around 50,000 years ago.