Madagascar’s extinct elephant birds – the largest birds to have ever lived – have attracted public interest for hundreds of years. Little is known about them because of large gaps in the skeletal fossil record.
A new study published today in Nature communications used ancient molecules extracted from fossilized eggshells to reveal surprising new insights into the biology of these flightless giants. How many species were there? Where did they live? What did they eat?
Answers to these questions contribute to understanding the origin and loss of species, which is especially important today as global biodiversity continues to decline rapidly.
Incredibly large birds
As a biodiversity hotspot, the island nation of Madagascar is a natural test tube for studying evolution and extinction. The numerous species of now-extinct megafauna that once roamed there play a key role in further understanding these processes.
One such group was the elephant birds, about which precious little has been gathered since they were first described 150 years ago.
Along with the ostrich of Africa, the emu and cassowary of Australia, the rhea of South America, and the moa and kiwi of New Zealand, Madagascar’s elephant birds (or vorobatra in the Malagasy language) were large, unrelenting ratites. They disappeared about 1,000 years ago, soon after humans first settled in Madagascar.
These were truly huge birds, with some weighing over 700 kg and standing up to 3 meters tall. Their eggs, which weighed 10 kilograms, were 150 times the size of a hen’s egg.
Elephant birds (Aepyornithiformes) have been the stuff of legend for hundreds of years, with early sightings possibly the origin of the mythical creature, the roc (or rukh), and inspiring writers such as HG Wells. British naturalist David Attenborough also took a special interest in elephant birds, documenting his journey for answers about his own elephant bird egg in Attenborough and the Giant Egg.
In recent years, elephant birds have been found to be more closely related to the chicken-sized kiwi bird—a result that has changed our view of bird evolution.
A patchy record
However, there is still debate about how many species of elephant birds actually existed. At one time, 16 species were named based on differences found between skeletal fossils. In the 1960s, this was reduced to seven species, and the most recent revision classified elephant birds into four species. But why the controversy?
Although these birds became extinct relatively recently, the skeletal fossil record through time and space is patchy. Madagascar’s climate can be very hot and humid, which is not conducive to the preservation of biological material.
When the bones are incomplete or fragmented, it can be difficult to distinguish between different species – and sometimes, the bones are not preserved at all, as in northern Madagascar where there were reports of eggshells but no bones.
Modern DNA technology can help overcome this obstacle. In the same way that we can identify people or tell how they are related to each other by comparing their DNA, ancient DNA from fossils can help identify unknown specimens or reveal relationships within and between species.
The more differences there are between the DNA of two organisms, the more distantly related they are. These differences can then be used to estimate when species evolved, which provides clues as to how and why. But, like the elephant bones themselves, the DNA within them is not well preserved.
Thick and abundant egg shell
This is where the eggshell comes in. Compared to other birds, the elephant egg shell is very thick, so the DNA trapped inside is better protected. Eggshells are also far more abundant than bones, with fragments thickly scattered on beaches along the Madagascar coastline where these birds probably once nested.
In addition to preserving DNA and proteins, the eggshell retains “stable isotope” signatures that can be matched to those of plants and animals from the environment to get an idea of what the birds ate and drank.
Physical characteristics of the eggshell (such as thickness and pore density) can also reveal information about egg size, bird size, nesting environment, nesting behavior and can sometimes be used to distinguish between groups of birds.
With the help and support of local Malagasy people, our team collected hundreds of 1,000-year-old fossilized eggshells from all over Madagascar. We examined their thickness, microstructure (via micro-CT scanning), DNA and proteins, and their stable isotopes. We screened hundreds of eggshell fragments to find 21 with sufficient DNA to reconstruct a family tree using eggshell and bone samples of known identity.
A mysterious origin
We found that there weren’t as many elephant bird species as previously thought – there was very little genetic difference between the samples. We believe that some of the size and shape differences observed in skeletal fossils are simply differences between males and females and not differences between species. It is common among voles for females to be much larger than males – and to be confused with different species.
But the most surprising thing is that we identified a mysterious eggshell from the northern part of Madagascar that belonged to a new lineage of large elephant birds that weighed 230 kg and laid 3 kg of eggs. Although these birds were closely related to the elephantidae of central Madagascar, they were genetically distinct and had a different diet. They were also separated by about 1,000 km in distance and 1.5 km in altitude.
This proves that elephant birds lived in northern Madagascar, where no skeletons have ever been found, a result reminiscent of finding a new hominid from DNA analysis of an unknown fossil.
These were not the only unexpected findings: we also identified potential drivers of speciation (formation of new species) and extreme gigantism in elephant birds. As Madagascar became drier and cooler during the last ice age, the vegetation changed and elephant birds may have adapted to new niches. This led to the evolution of the largest species in a rapid and recent time frame – within the last 1.4 million years, a fraction of their evolutionary history.
These findings show how ancient eggshell DNA is a promising avenue for studying the evolution of extinct birds. It contributes to understanding Madagascar’s past biodiversity—an important step toward understanding how to conserve its unique species in the future.
Alicia Grealy et al, Molecular exploration of fossil eggshell reveals hidden lineage of giant extinct birds, Nature communications (2023). DOI: 10.1038/s41467-023-36405-3
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Reference: DNA from fossil eggshells reveals how extinct elephant birds lived (2023, March 4) retrieved March 4, 2023 from https://phys.org/news/2023-03-dna-fossil-eggshells-reveals-extinct.html
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