Several species of the gigantic reptiles originally roamed alongside their much smaller counterparts in the southwestern region of Madagascar, which was a hotspot for tortoises.
Now, researchers may complete that picture with a new species. Astrochelys rogerbouri, which the scientists estimated to have had a carapace of around 50 centimeters long, would have been one of the island’s giant tortoises.
Despite the fact that the fossilized leg bone on which its description was based was found more than a century ago, it was recognized as the juvenile of the gigantic tortoise Aldabrachelys abrupta due to its intermediate size.
The extinct reptile was only recently discovered to be a distinct species thanks to DNA testing on the fossil, and scientists named the reptile after a former colleague.
Co-author of the study and former curator of fossil reptiles and birds at the Museum, Dr. Sandra Chapman said that she is proud to pay respect to the late scientist Roger Bour as part of this effort, which named the new species Astrochelys rogerbouri after him.
In the early 2000s, Roger frequently visited the turtle exhibits at the museum, where Dr. Chapman had multiple encounters with him.
Astrochelys rogerbouri, like many other Indian Ocean tortoises, was probably eliminated as soon as people arrived on the islands, either with the early residents of Madagascar from Southeast Asia or more recently with European colonists.
The new species’ description is a part of a larger investigation, which was released in the journal Science Advances. It shows that the tortoises living throughout the Indian Ocean evolved during two distinct dispersals, with turtles found in the Seychelles being only distantly related to those that were once found on Mauritius.
It is thought that the earliest tortoises to inhabit the islands of the Indian Ocean originated on the mainland of Africa some 40 million years ago. Living giant tortoises are occasionally seen washed up on beaches today, supporting the theory that the creatures spread by floating on the water.
Although Madagascar is closer to the African continent than the Mascarene Islands, genetic study of tortoise bones by the researchers indicates that the Mascarene Islands were the first to be colonized by reptiles. The Cylindraspis tortoises first settled on now-submerged islands like Saya de Malha before moving on to Mauritius, Rodrigues, and Réunion in the south.
The first tortoises didn’t reach Madagascar until more than 10 million years later. Eventually, a particular group of Aldabrachelys tortoises would travel 2,000 kilometers to the Seychelles, then across to the island of Aldabra.
For millions of years, these tortoises would rule the Indian Ocean islands. Giant tortoises with shells over a meter long would have inhabited the islands together with other animals that were around a tenth of their size if there were no humans or huge carnivores to compete with.
These species coexisted despite the fact that they were all herbivores and numerous. According to the researchers, the “extraordinary” variety that would have been present in southwest Madagascar, where up to five species were coexisting, was caused by the fact that they each fed a different kind of plant and so filled a separate niche.
By trampling seedlings, these tortoises are thought to have maintained the ecosystem’s balance of forest and savannah, decreased the risk of fire through grazing, and helped disperse the huge seeds of local baobab trees.
All of this would change once humans arrived. Although estimates for the time of the island’s earliest settlement range from 10,000 to 1,500 years ago, there is a consensus that the early settlers came from what is now Malaysia and Indonesia.
Most of Madagascar’s distinctive megafauna, which included gorilla-sized lemurs and pygmy hippos, had already been pushed to extinction by around 1,000 years ago. Human hunting, the development of farms, and drought have all been put out as potential explanations.
Astrochelys rogerbouri, as well as other huge species like Aldabrachelys grandidieri and Aldabrachelys abrupta, are likely to have gone extinct during this time period. Giant tortoises were also in danger.
The Mascarene Islands’ Cylindraspis tortoises survived until the presence of Europeans in the 1600s, and by the 1840s, it is thought that all species were gone due to harvesting for food by passing sailors.
According to co-author and head of the study team Professor Uwe Fritz, they typically imagine that humans just started eradicating species in recent times. In reality, people altered their environment and exploited local food supplies early on.
The ecological equilibrium of these islands was significantly disturbed as a result of the disappearance of the majority of the giant tortoise species in the western Indian Ocean.
While Mascarenes now has no native tortoises, Madagascar nevertheless has five species that reside in various parts of the island. Four species are listed as Critically Endangered; however, Bell’s hinged tortoise, which is widespread in central Africa and is thought to be at low risk of extinction.
These species are on the verge of extinction as a result of habitat loss for agriculture; for example, there are only thought to be 400 ploughshare tortoises left in the wild. Due to the demand from collectors and the scarcity of these tortoises, wild tortoises are being illegally seized for the exotic pet trade.
Tortoises might be a key component in restoring Madagascar’s biodiversity if these concerns are successfully managed. For instance, while fire poses a significant threat to island conservation, the tortoises might aid in reducing this danger by selectively consuming and removing potentially combustible plants.
Given that the Aldabra gigantic tortoise and Madagascar’s extinct gigantic turtles have common origins, several scientists have also proposed bringing the Aldabra giant tortoise to the island to help with this.
This initiative is modeled after one that was carried out on the island of Rodrigues and is credited for reestablishing seed distribution routes and combating invasive species without endangering local plant life.
Knowing where extinct tortoises formerly roamed might help with any future reintroduction initiatives by pointing out potential relocation places.
Senior Reptile Curator Patrick Campbell at the Museum said that one of the key reasons researchers conduct this sort of study is to preserve the species that are existing today. Giant tortoises play a crucial role in the environment, supporting other species like certain trees by partially digesting their seed coats.
This helps with plant germination and dissemination, and without these reptiles, there probably wouldn’t be as many trees on these islands.
In order to create a more complete picture of the former habitat of these gentle giants, the researchers have urged for more genetic examination of tortoise bones.
Sources:
Kehlmaier, C., Graciá, E., Ali, J. R., Campbell, P. D., Chapman, S. D., Deepak, V., Ihlow, F., Jalil, N. E., Pierre-Huyet, L., Samonds, K. E., Vences, M., & Fritz, U. (2023). Ancient DNA elucidates the lost world of western Indian Ocean giant tortoises and reveals a new extinct species from Madagascar. Science advances, 9 (2), eabq2574. https://doi.org/10.1126/sciadv.abq2574
https://phys.org/news/2023-01-species-large-tortoise-century-mistaken.html
