Madagascar’s elephant birds have been extinct since around the year 1000 CE. At up to three metres tall and some weighing over 500 kilograms, they were the largest birds to ever live.
Elephant birds are an important link in understanding the evolution and diversification of birds. As members of the ratite group, they are related to other large, flightless birds such as Australia’s emus and cassowaries. But their closest living relatives are the tiny kiwis of New Zealand.
Reminders of giants
CSIRO researcher Alicia Grealy now works with National Research Collections Australia at CSIRO. She studied the eggshells of elephant birds during her PhD at Curtin University and said skeletal remains are relatively rare compared with eggshells.
"Elephant bird eggshells are in abundance on the island of Madagascar, with some beaches seeming paved with broken eggshells,” she said.
"Eggshells offer a unique opportunity to study these birds, to learn more about the origins of biodiversity and about extinction. The island of Madagascar itself is also important for understanding evolutionary processes.”
“The first question we wanted to answer is how many species of these giant birds existed? This has been very difficult for scientists to answer using skeletal morphology. There are gaps in the fossil record and it has been difficult to extract DNA from skeletal remains. As a result, some scientists argued for eight species fitting into two genera, which is the level up from species. Other researchers suggested there were four species fitting into three genera.”
Ancient DNA from skeletal remains and eggshell had already revealed elephant birds’ kinship with kiwis. Could it also help delineate species relationships within elephant birds?
The paleogenomics of elephant birds
DNA doesn’t survive well in hot, humid climates like Madagascar. The skeletal remains of elephant birds have been a poor source of the material. But eggshells are another story. Their thick bioceramic helps protect the DNA and proteins trapped inside from degradation.
Alicia and her team sourced 960 elephant bird eggshell fragments from Madagascar. They ranged in age from around 1300 to 6200 years.
“Using ancient DNA techniques, we were able to assemble 17 complete and four partial mitochondrial genomes of elephant birds. We also managed to sequence ancient proteins extracted from the eggshells. This took two years of painstaking laboratory work,” she said.
The team put the eggshell DNA results together with DNA results from skeletal remains to construct a family tree. Genetic differences were found to be associated with eggshell characteristics, like thickness and pore density.
“At the time of their extinction there were likely three species belonging to two different families, and this was supported by both the genetic and protein results. Although skeletal morphology had suggested there were more species, we think this was due to extreme differences between males and females of the same species.
“Based on analysis of some newly discovered eggshell fragments, we think there was also a separate sub-species living in the north of Madagascar.
“Our hope is that the research community will formally revise the taxonomy of elephant birds to integrate the insights from eggshells.”
The DNA also provided clues about how the past environmental conditions in Madagascar shaped their evolution.
“We used the amount of genetic difference between species to estimate when they split from a common ancestor; for the two families, this happened around 30 million years ago. At this time, populations of small elephant birds may have adapted to a changing climate, eventually becoming different species—some remaining small and others becoming quite large.”
“But more recently, a second split happened among the larger of the elephant birds, which coincided with another period of climate change around 1.5 million years ago, leading to the evolution of an even larger species. Between this time and their extinction, one species, Aepyornis maximus, doubled in size. This shows extreme gigantism can evolve over very short timescales, a surprising result,” Alicia said.
As well as looking at ancient DNA, ancient proteins and eggshell morphology, the team also measured stable isotopes from the eggshell, which revealed elephant bird diet. It was a mixture of shrubs, succulents and grasses.
“We found that elephant bird species had different diets, suggesting they adapted to unique ecological niches. This is consistent with our interpretation that changes in the environment, including vegetation, may have driven speciation.”
The paper ‘Molecular exploration of fossil eggshell uncovers hidden lineage of giant extinct bird’ was published today in Nature Communications.