DNA from fossil egg shells reveals how extinct elephant birds lived

Photo credit: Gifford Miller, author provided

Madagascar’s extinct elephant birds – the largest birds that have ever lived – have attracted public interest for hundreds of years. Little is known about them due to large gaps in the skeletal fossil record.

A new study published today in nature communication used ancient molecules extracted from fossil eggshells to gain 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 our understanding of species emergence and loss, which is particularly important today as global biodiversity continues to decline rapidly.

Amazing big 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 understanding these processes.

One such group were the elephant birds, about which very little is known since they were first described over 150 years ago.

Along with Africa’s ostrich, Australia’s emu and cassowary, South America’s nandu, and New Zealand’s moa and kiwi, the elephant birds of Madagascar (or Vorompatra in the Malagasy language) were large, flightless ratites. They became extinct about 1,000 years ago, shortly after humans first settled Madagascar.

These were truly huge birds, some weighing over 700 kg and reaching 3 m in height. Their eggs, weighing 10 kg, were 150 times the size of a hen’s egg.

Elephant birds (Aepyornithiformes) have been the stuff of legends for hundreds of years, with early sightings possibly giving rise to the mythical creature Roc (or Rukh) and inspiring writers such as HG Wells. British naturalist David Attenborough was also particularly interested in elephant birds and documented his search for answers about his own elephant bird egg at Attenborough and the giant egg.

In recent years, elephant birds have been found to be most closely related to the chicken-sized kiwi bird — a finding that has changed our view of bird evolution.

DNA from fossil egg shells reveals how extinct elephant birds lived

An elephant bird egg (Aepyornis maximus) reconstructed from fragments at a market in Toliara (southwest Madagascar). In life, some species of elephant birds laid eggs weighing up to 10 kg – the volume of about 150 chicken eggs and larger than dinosaur eggs. Photo credit: Gifford Miller

A patchy record

However, there is still debate as to how many species of elephant bird actually existed. At one time, 16 species were named based on differences found between skeletal fossils. In the 1960s, that number dropped to seven species, and the most recent revision classified elephantbirds into four species. But why the controversy?

Although these birds became extinct relatively recently, the skeletal fossil record is patchy through time and space. The climate of Madagascar can be very hot and humid, which is not conducive to the preservation of biological material.

When bones are incomplete or fragmented, it can be difficult to tell different species apart—and sometimes bones are not preserved at all, as in far north Madagascar, where there are reports of eggshells but no bones.

Modern DNA technology can help overcome this barrier. Much like how we can identify humans or determine how related they are 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 they are related. These differences can then be used to estimate when species evolved, giving clues as to how and why. But just like the elephant bird bones themselves, the DNA within them is not well preserved.

Thick and ample eggshell

This is where the eggshell comes into play. Compared to other birds, the eggshell of elephant birds is very thick, so the DNA enclosed in it is better protected. Egg shells are also much more common than bones, with fragments densely strewn across the beaches along the coast of Madagascar where these birds are thought to have once nested.

DNA from fossil egg shells reveals how extinct elephant birds lived

Artist’s rendering of elephant birds in their natural habitat. Photo credit: Bonnie Koopmans, author provided

In addition to preserving DNA and proteins, the eggshell preserves “stable isotope” signatures that can be matched to those of surrounding plants and animals to get an idea of ​​what the birds ate and drank.

Physical characteristics of the eggshell (such as thickness and pore density) can also provide information about egg size, bird size, nesting environment and nesting behavior, and can sometimes be used to distinguish between groups of birds.

With the help and support of Malagasy locals, our team collected hundreds of 1,000-year-old fossil eggshells from across Madagascar. We have examined their thickness, microstructure (by 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 pedigree using eggshell and bone samples of known identities.

A mysterious parentage

We found that there were not as many species of elephant birds as previously thought – there were very little genetic differences between specimens. We believe that some of the size and shape differences seen in the skeletal fossils are only differences between males and females and not differences between species. It is common among ratites for females to be much larger than males – and to be confused with other species.

Even more surprising, however, is that we identified a mysterious eggshell from far north Madagascar that belonged to a novel lineage of large elephant birds that weighed 230 kg and laid 3 kg eggs. Although these birds were closely related to the elephant birds of central Madagascar, they were genetically different and had different diets. They were also separated by a distance of about 1,000 km and an altitude of 1.5 km.

This proves that elephant birds lived in the far north of Madagascar, where skeletons have never been found, a finding reminiscent of the discovery of a new hominid from DNA analysis of an unknown fossil.

These weren’t the only unexpected findings: we also identified potential causes of speciation (formation of new species) and extreme gigantism in elephant birds. As Madagascar became drier and cooler during the last ice age, vegetation changed and elephant birds may have adapted to new niches. This led to the evolution of the largest species in a rapid and short time frame – within the last 1.4 million years, a fraction of their evolutionary history.

These results show that ancient DNA from eggshells is a promising way to study the evolution of extinct birds. It contributes to our understanding of Madagascar’s past biodiversity – an important step in understanding how its unique species can be conserved in the future.

More information:
Alicia Grealy et al, Molecular Exploration of Fossil Eggshell Reveals Hidden Lineage of Extinct Giant Bird, nature communication (2023). DOI: 10.1038/s41467-023-36405-3

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Citation: DNA from fossil egg shells 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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