In ancient Flores, an island in eastern Indonesia, “hobbit”-size humans shared the landscape with an immense bird. At more than five feet tall, the Ice Age stork Leptoptilos robustus would have towered over the three-foot-tall Homo floresiensis, who lived more than 60,000 years ago.
Paleontologists previously thought the big bird was a flightless species that had adapted to live in an isolated island ecosystem. But newly analyzed fossils including wing bones, presented today in the journal Royal Society Open Science, have changed the story. Despite the stork’s size, its 12-foot wingspan likely would have allowed it to soar overhead.
This new realization prompted paleontologists to revise what they previously thought about the anatomy and behavior of L. robustus. Rather than a hunter of small prey, the new study suggests the bird was probably a scavenger like other prehistoric, flying storks that are known to have relied on herbivore carcasses for their meals, just like the marabou stork of sub-Saharan Africa does today. The Flores stork’s preference for carcasses may even explain why the animal ultimately went extinct.
In addition to huge birds, the island was home to a species of Stegodon, an extinct close relative to elephants, which only grew to four feet tall at the shoulder. “The giant storks were reliant upon them for a large part of their diet,” says University of Bergen paleontologist Hanneke Meijer, lead author of the new study. She points out that bones of Stegodon were found alongside the bird bones in a cave, where the birds would have been unlikely to venture without enticement.
When the Stegodon disappeared, Meijer and colleagues propose, so did L. robustus. Other animals on the island that relied on the mammals as a food source, such as Komodo dragons, managed to survive elsewhere. But the extinction of L. robustus coincided with major changes on Flores, triggered by a period of warming near the end of the Ice Age. “Our hypothesis is that when Stegodon became extinct, the whole ecosystem collapsed,” Meijer says.
Paleontologists were able to create this new view of the giant Flores stork thanks to 21 bones, including parts of the wing, found in Liang Bua cave. This rocky shelter might have been a way for animals, such as Stegodon, to escape the heat and get a drink—but carnivores could have taken advantage of the situation to snag an easy meal. The remains of prey killed by a Komodo dragon or Homo floresiensis would have been a tempting snack for the scavenging storks, which then might have perished inside the cave and been buried there, preserved in the fossil record until scientists dug up the bones tens of thousands of years later.
Islands can be intense natural laboratories for evolution. The relative isolation can lead organisms to adapt in very different ways than those in the greater expanses of Earth’s continents. According to a phenomenon called the island rule, for example, large species often become smaller to subsist on more limited resources, whereas animals that are often small—such as rodents and lizards—grow to unprecedented sizes.
When it was first described in 2010, the Flores stork was thought to be part of this pattern. The bird was originally envisioned as a unique, flightless giant that had adapted to stalking smaller prey in the island’s forests. By revealing that the Flores stork could fly, however, the new study suggests the animal was probably not a case of unusual island evolution, but part of a family of giant storks that once soared over much of the world.
“I think my perception of L. robustus has changed very much in line with my career,” says Meijer, who studied some of the first specimens of the giant bird. The original set of bones, she says, were big and strange, seemingly fitting with the idea that island life alters creatures in unexpected ways.
But the discovery of the animal’s wing bones presented a different picture.
A giant in the sky
Liang Bua cave preserves a treasure trove of paleontological and archaeological specimens, including the remains of Homo floresiensis and Homo sapiens, stone tools used by both species, and a menagerie of animal bones.
The first bones of L. robustus were discovered in 2004, but it took many years more for experts to collect and catalog more remains from the animal. It wasn’t until the new study that Meijer and colleagues put all the pieces together to assemble a more complete picture of the animal.
If the Flores stork had been flightless, the bird’s wing bones would have been smaller and shown anatomical signs that they were no longer used for flight. Paleontologists have seen this time and again among the extinct, carnivorous “terror birds,” emus and their relatives, and various other land birds that evolved after the extinction of the dinosaurs 66 million years.
When wing bones of the Flores stork were identified in the collection from Liang Bua cave, Meijer says, “they looked like functional wing bones and nothing like the bones of flightless species.” Those finds inspired Meijer and colleagues to rethink the life of the giant bird.
“You are thinking about how they would have behaved and interacted with the other species at Liang Bua,” she says, “almost like you get to know an animal on a personal level.” Each skeletal piece recovered from the cave is another part of the puzzle.
The new analysis “shows that our understanding of the fossil record is constantly improving, and that our initial interpretations about a fossil animal’s anatomy and behavior are preliminary hypotheses subject to revaluation,” says University of Cambridge paleontologist Daniel Field, who was not involved in the study. Such revisions not only help paleontologists better understand how and why species evolved, they also provide new insights into an organism’s extinction.
By surveying the distribution of giant storks across prehistoric Africa and Eurasia, for example, the new study also finds that L. robustus was likely one of the last surviving species of these once-plentiful birds. Clinging to an island refuge between the Indian and Pacific Oceans, the giant birds ultimately died out, but left clues to their story within the cave floor of Liang Bua.