Scientific name: Meiolania sp.
When did it become extinct? The last of these turtles is thought to have become extinct about 2,000 years ago.
Where did it live? The bones of these extinct turtles have been found on Lord Howe Island, 600 km from mainland Australia and the islands of New Caledonia.
There would be very few people who would fail to recognize a turtle, such is the familiarity of these unusual reptiles. Although the fossil record is full of peculiar beasts, it has been said that the turtles are among the oddest vertebrates to have ever lived. Al though their skeleton has the same bones as any other vertebrate, they are put together in a very different way. Their body is protected by a bony shell, which is, essentially, a hugely modified rib cage. The strength of this external carapace depends on the species, but it ranges from the leathery dome of the soft-shelled turtles to the almost impregnable shell of the giant tortoises. Also unique is the position of the hip and shoulder girdles, as they are found inside the rib cage. These animals are most familiar for being able to withdraw their heads and legs into the safe confines of their shells. The way they withdraw their head allows scientists to identify two groups of turtle: the cryptodires and the pleurodires. The latter are often called side-necked turtles because they bend their long necks into an S shape to keep their heads out of harm’s way. The turtles that people often keep as pets fall in the first group, the cryptodires, and these can pull their heads right into their shells by bending their necks below the spine.
There’s no doubt that some of the turtles, especially the land-dwelling species, are very slow, lumbering creatures, characteristics that are often linked to evolutionary failure and poor adaptability. However, nothing could be father from the truth for the turtles. These shelled reptiles are a successful group of animals that have been around since the Triassic—at least 215 million years (and probably considerably longer)—which makes them much older than the lizards and snakes. Not only are they ancient, but they are among the very few living reptiles that have become almost completely amphibious, only leaving the water to lay eggs (some species of snake also only leave the water to lay eggs). Today, there are around 300 turtle species, ranging from tiny, 8-cm tortoises all the way up to the oceangoing giant, the leatherback turtle (Dermochelys coriacea), which can be 3 m long and weigh 900 kg.
Even though some truly bizarre turtles are still with us today, they pale in insignificance compared to an immense, land-living turtle that only became extinct in the last couple thousand years. This was the horned turtle, and in life it must have been an astonishing animal. The horned turtle was around 2.5 m long, and it must have weighed in the region of 500 to 700 kg. By comparison, the largest living land-dwelling turtle is the Galápagos tortoise (Geochelone nigra) at about 300 kg and 1.2 m long. Imagine a horned turtle alongside a Galápagos tortoise and you get an idea of the size of this extinct beast. Not only was the horned turtle big, but it also had a very bizarre appearance. Sprouting from its skull were large horns and spikes, the longest of which grew from toward the back of the head and could reach a span of 60 cm. This formidable forward armory was combined with the typical tortoise carapace and a heavily protected tail that also sported spines. The horns of this extinct turtle made it impossible for the head to be pulled into the shell during times of danger. It is possible that these horns were used by the turtle to defend itself, but we don’t know what predators lurked on the islands where these extinct reptiles lived. Male giant turtles can be quite aggressive to one another during the breeding season, and maybe the extinct giant used its horns and tail spikes to fight other males for the right to mate. As with other island animals, the horned turtles may have grown to great size because there was very little in the way of threats in their isolated home terrain. Alternatively, great size is a simple yet effective defense against many predators. The truth is that we’ll never know the evolutionary force behind the incredible size and appearance of these turtles.
What we can be more sure of is their diet. Large land-dwelling turtles are slow, heavy animals, so fast-moving animal prey is out of the question. We know that the Galápagos tortoise and other terrestrial giant turtles are herbivores that eat a wide range of plant matter. The horned turtle was obviously unsuited to climbing trees or rearing up on its back legs to reach lofty vegetation, so it must have been dependant on the unique, low-growing plants that grow on New Caledonia and the surrounding islands. All living turtles lay eggs, and we can assume that the horned turtle was no different, but how it laid them and where will never be known for certain. Perhaps it excavated a pit before laying its eggs and forgetting about them.
It is amazing to think that these giant, bizarre turtles roamed some of the isolated islands of the western Pacific into very recent geological times, but exactly why they died out is another mystery. We do know that island animals have suffered badly at the hands of humans, and we can be almost certain that the first thing to spring to the mind of the first human who saw these shelled giants was, “Can I eat it?” A slow moving turtle, regardless of its size, is no match for humans and their various weapons. Lord Howe Island and New Caledonia are small areas of land, and they could never have supported large populations of such big animals; therefore it is very likely that when humans did discover the horned turtle, they wiped them out in a matter of centuries, or possibly even decades.
• Apart from the way that living turtles bend their necks to hide their heads, we can divide them another way into three groups: there are marine forms, with legs modified into flippers, for example, the leatherback turtle; terrestrial forms, with thick, pillar – like legs, for example, the Galápagos tortoises; and semiaquatic forms, for example, terrapins and snapping turtles.
• Many of the living species of turtle may soon follow the horned giant to extinction as they are incredibly endangered. Some of the very rare species only survive in small populations on isolated islands, while the oceangoing species are at risk from fishing hooks, drift nets, and direct hunting. Without complete and active protection, it is very likely that some of the most amazing turtles could be extinct within 30 years.
• As turtles led such slow lives, they are among the most long-lived of the all the vertebrates. The Galápagos tortoise can live to be at least 150 years old. One famous, long lived radiated tortoise (Geochelone radiate) was presented to the Tongan royal family in 1777 by none other than Captain Cook. Known as Tu’i Malila, this tortoise died in 1965, at age 188. The longevity of an immense turtle like the horned giant can only be guessed.
• Further back in the fossil record, in the age of the dinosaurs, there were other extinct turtles that were truly enormous. One of these, Archelon, is only known from 70-million years old fossils. It was about 4 m long, and the span of its flippers was around 4.5 m. Fully grown, Archelon probably weighed in the region of 2 to 3 tonnes. Its large head and powerful bite appear to be suited to eating shelled mollusks such as the extinct ammonites.
Further Reading: Gaffney, E. S. “The Postcranial Morphology of Meiolania platyceps and a Review of the Meiolaniidae.” Bulletin of the American Museum of Natural History 229 (1996): 1–166; Gaffney, E., S. Hutchison, J. Howard, F. A. Jenkins, and L. J. Meeker. “Modern Turtle Origins: The Oldest Known Cryptodire.” Science 237 (1987): 289–91.