What was the ancestor of the turtles
Where do the giant tortoises come from?
Its shell reaches a length of over one meter, in extreme cases it can weigh more than 400 kilograms: The Galapagos giant tortoise (Chelonoidis nigra) is the largest terrestrial tortoise species on earth today. Since their similarly huge relatives can all be found on islands, research has so far assumed that giant tortoises are the opposite of island dwelling in large animals. Now, however, it is becoming apparent that the evolution of these armored giants may not be as strongly tied to islands as was previously assumed.
Tortoises live in very different regions of the world - from deserts to forests. They include species such as the Greek tortoise and the Galapagos tortoise. Although naturalists have been studying giant tortoises since the time of Charles Darwin, he still puzzles them today.
Because all living giant tortoises can be found on islands, many researchers suspect that their development followed the so-called island rule: a trend towards dwarfism in large animals and giant growth in small animals on islands. However, previous work on existing giant tortoises has not provided a consistent answer to the question of their origin: some studies have suggested that the giant stature is related to the lack of predatory mammals on islands. Others concluded that the turtles must have been very large by the time they reached the remote archipelagos. Since only very few giant tortoise species still exist today, these hypotheses cannot be verified without analyzing extinct species with the help of fossils.
In their new study in the specialist magazine "Cladistics", the Argentinian Evangelos Vlachos and Márton Rabi from the Martin Luther University Halle-Wittenberg (MLU) dared to tackle precisely this desideratum: The researchers analyzed genetic data from living species, which they obtained with bone measurements combined by fossil and live turtles. Based on these investigations, they created the most extensive family tree of extinct and still living turtles to date. It is the first study on a global scale that makes it possible to trace the development of body size in turtles.
Multiple gigantic stature
From this analysis a completely different picture emerges for the past: "The fossils show a large number of extinct giant species on the mainland and indicate that the development of the giant growth was not tied to islands," says Vlachos. Instead, the giant growth developed several times independently of one another in the course of the earth's history on the mainland - in Asia, Africa, Europe, North and South America. However, by the Ice Age of the Pleistocene at the latest, all of these mainland species died out.
Another unexpected finding from the study is that the Mediterranean turtles, best known as pets, are in fact a dwarf line, as their ancestors turned out to be significantly larger.
"We are now in a position to better understand the development of this successful group. Today, however, of the approximately 43 living species, 17 are considered critically endangered and many more are endangered, mainly due to habitat loss by humans. That's a disappointing fact, "says Rabi. The species still alive today, such as those on the Galapagos Islands and the Seychelles, are likely to be survivors of unrelated giant species that were once native to South America, East Africa or Madagascar.
Many puzzles remain unsolved
"Giant tortoises are perhaps better adapted to island life because they endure the water and food shortages better than smaller species during their journeys across the ocean for longer periods of time. There are reports of giant tortoises that have been sighted drifting 740 kilometers from the coast." , continues Rabi.
"We suspect that warmer climates and evolutionary pressures from predators play a role in the development of the giant, but the picture is complex and our samples of fossil evidence are still limited," says Vlachos. What led to the extinction of these mainland giants remains unclear. For Ice Age species, it may have been a combination of the threat from predators, including humans, and climate change. It is also unclear what drives the turtles to develop into giant forms again and again if the island rule does not apply. (red, November 5, 2018)
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