From ScienceDaily:
The fact that all living giant tortoises are insular may suggest that their evolution followed the so-called island rule: a trend toward dwarfism of large animals and gigantism of small animals on islands…
In a recent study in the journal “Cladistics,” Dr Evangelos Vlachos from the Paleontological Museum of Trelew, Argentina, and Dr Márton Rabi from the Martin Luther University Halle-Wittenberg (MLU), funded by the German VolkswagenStiftung, assembled the most comprehensive family tree of extinct and extant tortoises so far. The researchers analysed genetic data from living species together with osteological data from fossil and living tortoises.
This is the first study of such global scale to allow for investigating body size evolution in tortoises. The fossils reveal a very different picture of the past compared to the present. Giant size evolved on multiple occasions independently in mainland Asia, Africa, Europe, North and South America at different times of Earth history. However, all of these species went extinct at latest during the Pleistocene ice age.
“The fossils highlight a great number of extinct mainland giant species and suggest that the evolution of giant size was not linked to islands,” says Dr Evangelos Vlachos.
Instead, living insular giant tortoises, such as the ones from Galapagos and Seychelles, more likely represent survivors of unrelated giant species that once inhabited South America, East Africa, and/or Madagascar. Paper. (paywall) – Evangelos Vlachos, Márton Rabi. Total evidence analysis and body size evolution of extant and extinct tortoises (Testudines: Cryptodira: Pan-Testudinidae). Cladistics, 2017; DOI: 10.1111/cla.12227 More.
The researchers are unsure why giant tortoises thrived on islands but one suggestion is that they were already big and as a result, they were better able than smaller tortoises to reach remote islands.
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