Fossil egg shells reveal evolution of elephant birds

elephant birds

The elephant birds, which were once native to Madagascar, could grow up to three meters in length. © Bonnie Koopmans/ Nature

Until humans came to Madagascar around 1000 years ago, huge flightless birds, the elephant birds, lived on the island. Their evolutionary history is only incompletely known from bone finds. A new analysis using DNA analyzes from fossil egg shells now shows that genetic diversity in southern Madagascar was probably lower than previously thought. Instead, the results point to a previously unknown lineage of elephantbirds in the north - a region not previously known for elephantbird fossils.

According to skeletal finds, the giant elephant birds of Madagascar could reach a size of around three meters and weigh several hundred kilos. Although they were first described as early as 1851, their biodiversity and the evolutionary relationships between different elephant bird species remain controversial to this day. The basis for the description of most species are single, incomplete skeletal remains from southern and central Madagascar. Based on these finds, science initially assumed that there were eight species in two genera. Recent studies, however, suggested a subdivision into four species in three genera.

eggshells
Fossil egg shells of elephant birds on a beach in Madagascar. © Gifford Miller

Information from fossil eggshells

But even this classification may now have to be revised. "Using a molecular analysis of fossil eggshells more than 1000 years old, we have for the first time described the phylogeography of elephant birds and gained new insights into the ecology and evolution of these flightless giants," writes a team led by Alicia Grealy of Curtin University in Bentley, Australia.

The new analysis is based on more than 960 fragments of fossil elephant bird egg shells. “These were collected from 291 locations in southern, central and, for the first time, northern Madagascar,” the team said. Many of the eggshell fragments were found near sites where skeletal fragments had previously been discovered. "This suggests that the eggshells are likely associated with the same taxa that have been described from skeletal material from the same geographic areas."

Diversity in the south lower than assumed

On the one hand, Grealy and her team analyzed the structure and shape of the eggshells. "Measurements of the thickness of the eggshell reveal three morphotypes," the researchers report. In southern Madagascar, they found two different types of eggshells - one that is less than 1.1 millimeters thick on average, and one that is more than three times as thick, with an average thickness of 3.32 millimeters. "We estimate that the eggs with the thinnest shell weighed about 860 grams and were laid by a roughly emu-sized bird weighing about 41 kilograms," the team writes. "The eggs with the thickest shell are estimated to have weighed over ten kilograms and were laid by a bird weighing around 1,000 kilograms." The team found a third shell type in northern Madagascar, with an average thickness of 1.95 millimeters between the two guys from the south and probably came from a bird weighing about 230 kilograms.

In addition to the external studies, Grealy and her team analyzed the ancient mitochondrial DNA preserved in the eggshells. "Our results indicate that the genetic diversity of elephant birds in southern Madagascar was lower than previously thought," the authors said. Thus, in southern and central Madagascar there were probably only two distinct genera of elephant birds, each comprising only one species in that region: a small one known as Mullerornis and a large one called Aepyornis. However, the third genus, Vorombe, proposed only in 2018, is actually large members of Aepyornis, according to genetic analysis. The researchers assume that Aepyornis exhibited pronounced sexual dimorphism, with the females sometimes being almost twice as large as the males and therefore incorrectly classified as a separate genus based on the skeletal remains.

New Lineage in the North?

In contrast, the eggshells found in the north show a genetic profile that indicates new diversity within the genus Aepyornis. Although Grealy and her team cannot say with certainty whether this is a new species based on the data available so far, there are clear differences to the species found in the south of the island. The northern specimens could possibly be representatives of the species Aepyornis hildebrandti known from central Madagascar, which was smaller than the southern species Aepyornis maximus and, according to previous assumptions, only occurred in a very limited area at altitudes above 1500 meters. If the northern eggs are actually from A. hildebrandti, the putative distribution area would be extended by almost 1000 kilometers to the north.

According to the genetic analyses, the divergence of A. maximus and A. hildebrandti coincides approximately with the beginning of the Quaternary, when the climate on Madagascar became cooler. This may have encouraged A. maximus gigantism in the cooler south, while smaller specimens spread to the warmer north. The observed low genetic diversity could later have been a reason why the elephant birds could not adapt to the environmental changes caused by humans and died out.

"Our results contribute to the understanding of how elephant birds lived and functioned in the unique ecosystems of Madagascar and confirm that ancient DNA from eggshells is a promising avenue to study the evolution and extinction of terrestrial megafauna," the research team said.

Source: Alicia Grealy (Curtin University, Bentley, Australia) et al., Nature Communications, doi: 10.1038/s41467-023-36405-3

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