The image above is a fictionalised version of Archelon, a prehistoric sea turtle, animated by Ray Harryhausen in the 1966 movie One Million Years BC. This World Evolution Day, let's take a look at the incredible origins of real-life sea turtles.
The story of all turtles actually begins here, in South Africa – although in a very different world. About 260 million years ago, on the shores of the ancient Karoo sea, among dunes and shrubs of a semi-arid Gondwana, lived a small reptile called Eunotosaurus africanus.
This little lizard had a unique characteristic: its ribs curved backwards and were exceptionally thick, forming an armoured dome under its skin. Eunotosaurus didn’t know this, but it was to become the ancestor of all tortoises, sea turtles and terrapins.
Descendants of Eunotosaurus are called pantestudines, of which modern turtles are a member. But we still have a long way to go before we reach today...
Why did the shell evolve?
About 40 million years later another reptile, called Odontochelys semitestacea (meaning “toothed turtle with half a shell"), had evolved along the coast of shallow oceans in what is today China.
Odontochelys was nothing like the turtles we know and love today. It had teeth, not a beak. It had claws, but not flippers. And most noticeably, it did not have an upper carapace. However, it had evolved one thing that is characteristic of turtles: a plastron – its bottom shell.
It is interesting that the lower armour of the turtle evolved before its top carapace, but it makes sense if you think about the type of predators that would have preyed on the early turtles – sharks and other ambush predators striking from below.
Enter Proganochelys. This creature, called the “beast turtle” walked the deserts of the ancient continent of Laurasia, between modern Thailand and Germany. Like its ancestor Odontochelys, it had the bottom plastron armour plates, but as a land dweller, it needed armour from above – especially as the world it roamed was now inhabited by dinosaurs.
Proganochelys evolved upper armour to protect itself from these dinosaurs. It had bony neck spikes, armoured leg plates and armoured spikes on its tail that it could swing like a club. But, it had also evolved features that still remain in most modern turtles – a solid, bony carapace and a toothless, beaked mouth.
These turtle-like ancestors are called “stem turtles”, the group that diverged into the tortoise, terrapin and sea turtle groups we recognise today. This evolution was driven by the breaking up of the ancient supercontinents and the isolation of the oceans and their currents.
Placodonts - the turtles that weren’t turtles
During the Triassic, at the same time as the early turtle ancestors were being born, a "failed experiment" of evolution occurred. These creatures were called placodonts – and although not related to turtles, they do play a role in their story.
Their evolutionary path is similar to that of turtles. Placodonts started out as marine lizards, such as Placodus, which were similar to today’s marine iguanas. Their large size was enough to prevent predation by early sharks.
As more predators emerged, placodonts began evolving armoured plates, but not made of bone as a turtle's shell is. Species of placodont, like Psephoderma, had thick armoured plates and hunted small molluscs in shallow waters. Others, like Henodus, developed teeth that allowed filter feeding as seen in today's baleen whales.
Placodonts did not survive the Triassic-Jurassic extinction event about 200 million years ago, a volcanically-induced period of climate change that resulted in the extinction of half the world’s species and allowed the dinosaurs to become the dominant land animal. The extinction of the placodonts opened their shallow-water niche, and allowed modern turtles to flourish.
The first turtles
As the ancient continent of Laurasia split up, the southern portion of what we would today recognise as the USA was low-lying, largely covered in brackish, salty swamps. About 110 million years ago, the land-dwelling stem turtles started to live semi-aquatic lives, eventually evolving into fully aquatic freshwater and saltwater turtle species.
Most of these turtles would continue to develop the armour plating of their Proganochelys ancestor, streamlining it. They lost the neck and tail armour, but their plastron armour developed to protect them from marine predators, covering their necks and preventing them from retracting their heads like a tortoise can.
Their limb armour was lost, and their claws and limbs evolved into the flippers we see today – after all, they no longer needed to walk on land. The oldest fossil of a species that palaeontologists agree is a "sea turtle" and not a "stem turtle" is Desmatochelys padillai, the oldest known sea turtle.
The ancient leatherbacks
In the shallow seas that covered much of North America 90 million years ago, a giant had evolved. This giant was called Archelon, and at 4m long with a flipper span of 4.9m it was by far the largest turtle species that has ever lived.
Archelon was a Dermocheloid or a “skin-shelled turtle”. It had a framework of bony plates, and strong neck muscles – a combination that would allow it to be fast and strong enough to hunt giant squid.
Ancestors of Archelon radiated across the globe, and for a time they were the dominant large animal of the ocean. These Dermocheloids diversified, taking advantage of the gaps left in ecosystems by the end-Cretaceous extinction (yup, the one that killed the dinosaurs) and began to occupy niches in the environment that we don't typically associate with sea turtles today - Alienochelys had broad, crushing plates that it used to grind up shellfish and squid (the same feeding technique used by many rays today), and Ocepechelon, which had a long tube-shaped jaw that it used for suction hunting (just like a Knysna seahorse).
As marine mammals, like dolphins and whales, evolved about 20 million years ago, the soft-shelled Dermocheloid turtles were all hunted or outcompeted to extinction. All except one – the leatherback turtle (Dermochelys coriacea). The leatherback's unique lifestyle of feeding on jellyfish, a very nutrient-poor food source, left them out of the evolutionary competition and helped them survive - the fast metabolisms of large mammals could not thrive on jellyfish alone.
While the ancient ancestors of the leatherback were diversifying - other groups of stem turtles continued to perfect an armoured shell, eventually adapting to a streamlined body plan where the plastron and carapace were both strong and light.
Ctenochelys acris, an 80 million-year-old species from the southern USA, is a likely ancestor to all modern-day sea turtles, apart from the leatherback. It possesses characteristics of modern sea turtles and freshwater snapping turtles - for example it had large back flippers which would have been used for propulsion, something modern sea turtles don't have.
The first "true" shelled sea turtles emerged about 40 million years ago. These turtles are called Chelonian turtles – simply meaning “shelled turtles”. This armour allowed them to survive fierce marine predators, including sharks, lepidosaurs and eventually mammals. This knack for survival is why we still see six Chelonian species alive today, whereas almost every other type of sea turtle has gone extinct. The turtles that emerged in this time are very similar to the species alive today.
About 34 million years ago, the earth underwent a climate change event called the Eocene-Oligocene Transition where the world went from very warm to quite cool, with the ice caps reforming. The ancestor of modern green sea turtles (Chelonia mydas) at this time was likely a generalist omnivore, but localised changes in climate affected its food source, so we see specialised feeding techniques evolving: algae-grazing green turtles and carnivorous, sea cucumber-hunting flatback turtles (Natator depressus). Both these turtles are still functionally omnivores, but their behaviour dictates their diet.
The other four modern sea turtles - loggerheads (Caretta caretta), hawksbills (Eretmochelys imbricata), olive ridleys (Lepidochelys olivacea) and Kemp's ridleys (Lepidochelys kempii) - are all closely related, likely stemming from a carnivorous common ancestor that closely resembled the loggerhead turtle. Like the green and flatback turtles, the differences in diets and feeding methods used by these four species suggests that speciation was due to environmental changes that affected their food sources.
All four of these sea turtles are "technically" omnivores, although in reality they stick to rather rigid diets. Loggerheads have powerful jaws and beaks for crushing molluscs and shellfish, hawksbills are able to live almost entirely off of sea sponges and olive ridleys feed on a combination of jellyfish, fish eggs, echinoderms and shrimp. Kemp's ridley turtles may be the only truly omnivorous sea turtle, feeding on molluscs, jellyfish, fish and seaweed, but juveniles have special adaptations for hunting crabs.
Modern sea turtles
In the past 3 million years, the face of the earth has changed. Uplifting of Central America - previously entirely submerged under the sea, cutting off populations of turtles in the Pacific and Atlantic oceans - and upwellings of cold currents at the Cape of Good Hope and Tierre Del Fuego (southernmost tip of South America) prevent many of these warmth-dependant animals from rounding the tips of the continents. This isolation of the oceans, while not far enough in the past to cause speciation, has affected the global sea turtle gene pool.
Loggerhead turtles have been most affected - today there are several genetically distinct populations of loggerheads, most notable when comparing Pacific and Atlantic loggerheads - we are watching the early stages of speciation. Green turtles are also following a similar trend, one that is more visible due to their comparatively more diverse shell markings. Although green turtles are recognised as one species, a quick search reveals the debate surrounding them and the names "Agassizi turtle", "black sea turtle" and "Galápagos green turtle" all appear as proposed subspecies.
In more recent times, human beings have been the significant driving force in the genetics of sea turtles, due to large-scale hunting and collection of turtle eggs for food.
For almost a hundred million years sea turtles and their ancestors have roamed the oceans. They survived the asteroid that wiped out the dinosaurs. They survived the evolution of mammals. They even survived the ice ages. But now, mankind is quickly driving them towards extinction. Let's do our part to ensure the protection of the seven remaining endangered species of sea turtle.