Colossal tortoises: Climate change and the evolution of Europe’s largest ‘modern’ reptiles
Benjamin Kear (Australia) and Georgios Georgalis (Greece)
Most people are familiar with the famous giant tortoises of the Galapagos Islands – isolated oddities evolving in the absence of predators on a remote tropical paradise. However, as little as 5mya, continental landmasses (including Europe, Africa and India) also had their own species of giants. However, these were nearly three times the size of their modern cousins, probably close to the mass of a small car, and would have rivalled some dinosaurs for being among the most colossal reptiles of all time.
The best preserved gigantic tortoise fossils (as opposed to the merely ‘giant’ ones) have been found in Mediterranean Europe, particularly France, Greece and Spain, and were described in the scientific literature as early as 1877. Yet, despite an impressive chronicle of discoveries, the inter-relationships between these different kinds of gigantic tortoises are far from adequately understood. The present, albeit tentative, consensus is that there are at least three separate lineages, all of which achieved maximal body size at about the same point in geological time.
Cheirogaster, the genus found in Europe, has a long fossil history stretching back some 50mys to the Eocene and includes up to 11 species. It is represented by a virtually complete skeleton (with a shell well over a metre long) unearthed from Pliocene deposits (about 5mys old) near Perpignan, in southern France. Also, several impressively big skulls, limb bones and other elements (suggesting animals of about 2m carapace length) from late Miocene and Pliocene sediments (around 5 to 8mys old) have been found on the eastern Aegean islands of Samos, Lesvos, and localities around Thessaloniki and Athens, in Greece.
Together, these specimens provide a picture of what the extinct gigantic tortoises would have looked like – imagine a scaled up version of a Galapagos tortoise, but with a flatter shell, bony, armour-plated skin (presumably for protection against predators such as sabre-toothed cats) and a blunt-snouted head, with prominent forward facing eyes.
The gigantic tortoises of Africa and India would have looked very similar, but have been placed in separate genera – Centrochelys and Megalochelys respectively. Unlike Cheirogaster, these forms are known mainly from fragmentary remains, although, they too indicate gargantuan proportions – there are anecdotal accounts of carapaces the size of Volkswagens from the remote deserts of North Africa! Both Centrochelys and Megalochelys survived until the Pliocene-Pleistocene boundary (3Ma), and Centrochelys, at least, seems to have been fairly widespread with remains recovered from Morocco, Tunisia, East Africa and the Middle East (Palestine and Iran). Indeed, a comparatively tiny (only 60cm long), possibly related species, Geochelone (Centrochelys) sulcata (also known as the African desert or spurred tortoise), still lives on in central Africa today.
As mentioned previously, the suggested inter-relationships between these chelonian behemoths are a little sketchy. Traditionally, Cheirogaster was thought to have shared an African origin with Centrochelys and subsequently dispersed into Europe, sometime before the late Eocene (about 50mya – the oldest occurrence of Cheirogaster). However, this evolutionary scenario contrasts with recent palaeogeographical reconstructions and DNA sequence analyses of modern European tortoises (including the football-sized Testudo and Eurotestudo, commonly found in pet shops).
These indicate the absence of an effective Arabian-Anatolian land bridge until the early Miocene (about 15Ma) and, therefore, a probable Asian origin for all European fossil tortoise taxa before the influx of the diminutive Testudo and Eurotestudo from Africa less than 10Ma (during the late Miocene). Therefore, the development of gigantism seems to be an independent feature that evolved multiple times in several quite distinct tortoise lineages.
Because the record of European gigantic tortoises spans a broad stratigraphical interval (about 35 million years – late Eocene to Pleistocene), it is possible to map the evolution of large body size over time. The most massive Cheirogaster specimens (carapace length of around 2m) are known from the Upper Miocene to Upper Pliocene (3 to 8Ma) and coincide with the appearance of equally gigantic tortoises in Africa and the Near East.
Interestingly, the Miocene-Pliocene transition is marked by the onset of cooler, dryer climates in the Afro-Eurasian region and the spread of open woodlands and grasslands throughout Mediterranean Europe. Therefore, selection for gigantism might represent an adaptation for maintaining higher metabolic activity (through ‘gigantothermy’) in the face of unfavourable environmental conditions. That and/or a shift towards consumption of cellulose-rich plant matter (for example, grasses and woody plants), which requires a large fermentative gut to break it down.
Whatever the driver, it is clear that by about 2 to 3Ma (during the Pleistocene) the European gigantic tortoises had all but disappeared. Again, climate change may have played a role because, from the beginning of the Pleistocene, rapid temperature fluctuations, popularly known as the ‘Ice Ages’, brought glaciations to northern Europe. These would have destroyed habitats and perhaps prompted the extinction of the gigantic tortoises, together with many of their more ‘tropically-adapted’ mammalian contemporaries.
About the author
Dr Benjamin Kear is a vertebrate palaeontologist with La Trobe University in Melbourne, Australia and Research Associate of both Monash University in Melbourne and the South Australian Museum in Adelaide. Georgios Georgalis is a palaeontology researcher with the Aristotle University of Thessaloniki, Greece and research affiliate of the University of Pilsen in the Czech Republic.