Elephant birds: First DNA study of fossil eggshells hints new lineage

The birds that laid the eggs weighed up to 1 ton.
Sade Agard
Artist’s impression of elephant birds and reconstructed eggshell using fragments
Artist’s impression of elephant birds and reconstructed eggshell using fragments

Alicia Grealy/Gifford Miller 

A DNA examination of fossil eggshells more than 1,000 years old provided new insights about extinct elephant birds from Madagascar, according to a Nature Communications paper published February 28.

The research advances our understanding of the evolution and distribution of elephant birds on the island. It also raises the possibility of a previously undiscovered lineage of the bird. 

Elephant birds: Were they the largest birds to have ever lived?

Elephant birds: First DNA study of fossil eggshells hints new lineage
A fragment of elephant bird eggshell alongside that of an emu's (black)

The large elephant birds, some of the heaviest known flightless animals to have ever roamed this planet, once called Madagascar home. Several theories exist about why and when they went extinct 1,000 years ago and how much humans were involved. 

There may have been four species of elephant birds based on the few bones unearthed. Still, since this group was initially described 150 years ago, its evolutionary history has remained a mystery.

When skeleton fossils are scarce, examination of ancient DNA retained in eggshells is a promising source of knowledge. For instance, earlier studies have proven that the kiwi and elephant birds are related.

Now, Alicia Grealy and colleagues report the first phylogeographic analysis of elephant birds using eggshell mitochondrial DNA (aDNA) to revisit elephant bird taxonomy and evolutionary history.

They examined 960 pieces of elephant bird eggshell discovered at 291 different sites in Madagascar. In line with the period in which these birds lived, the eggshells ranged in age from 6,190 to 1,290 years.

The eggs were estimated to weigh between 0.86 and 10.47 kilograms (bigger than some dinosaur eggs!) by measuring the thickness of the eggshells. Additionally, the size of the birds who laid them was between 41 and 1,000 kilograms.

According to the DNA study, elephant birds in southern Madagascar had lower genetic diversity and fewer species than believed from skeletal fossils. The authors speculate that northern Madagascar may be home to an undiscovered lineage of elephant birds, where skeleton remains have yet to be discovered.

The findings highlight the complexity of the elephant bird's evolutionary history and show how vital information can be preserved in eggshells.

Why did elephant birds go extinct?

Elephant birds and other well-known megafauna species are thought to have gone extinct shortly after making contact with early modern humans due to being over-harvested as a food supply. Evidence for this includes the discovery of fossilized remains with cut marks. 

That said, there is also proof that people coexisted with birds for thousands of years before they went extinct around a thousand years ago. This has prompted further questions about how Madagascar truly lost its unique fauna.

Study abstract:

The systematics of Madagascar’s extinct elephant birds remains controversial due to large gaps in the fossil record and poor biomolecular preservation of skeletal specimens. Here, a molecular analysis of 1000-year-old fossil eggshells provides the first description of elephant bird phylogeography and offers insight into the ecology and evolution of these flightless giants. Mitochondrial genomes from across Madagascar reveal genetic variation that is correlated with eggshell morphology, stable isotope composition, and geographic distribution. The elephant bird crown is dated to ca. 30 Mya, when Madagascar is estimated to have become less arid as it moved northward. High levels of between-clade genetic variation support reclassifying Mullerornis into a separate family. Low levels of within-clade genetic variation suggest there were only two elephant bird genera existing in southern Madagascar during the Holocene. However, we find an eggshell collection from Madagascar’s far north that represents a unique lineage of Aepyornis. Furthermore, divergence within Aepyornis coincides with the aridification of Madagascar during the early Pleistocene ca. 1.5 Ma, and is consistent with the fragmentation of populations in the highlands driving diversification and the evolution of extreme gigantism over shorts timescales. We advocate for a revision of their taxonomy that integrates palaeogenomic and palaeoecological perspectives.

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