CONSIDER the floating fortress that is a sea turtle. It seems to fly gracefully through the water even though it is encased in heavy armour. This combination of poise and protection evolved like any wonder of the animal kingdom, yet until recently the fossils we had found made it seem as if they emerged fully formed hundreds of millions of years ago. In fact, scientists who study the evolution of these animals have a running joke: turtles might as well have come from space.
In the past few years, however, truly ancient fossils have been discovered and they are helping to unravel this real-life just-so story. As we uncover fresh clues, we are learning that the tale of how the turtle got its shell reads nothing like we had previously guessed.
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In the UK, the word “turtle” usually refers to the reptiles that swim in the sea, but these are just one branch of a larger group that includes species that burrow in desert sand and slog through swamps. Whether we are talking about a Galapagos tortoise, green sea turtle or red-eared terrapin at the pet shop, they all belong to an order called Testudines – which zoologists tend to call the turtles or chelonians.
All living animals in this order are united by their shell and the modifications required to live inside this box of bone. Their upper ribs are fused to the inside of their shell and their shoulder joints are set inside their ribs. This anatomical form is unprecedented among vertebrates. Imagine how your arms would move – or rather wouldn’t – if your shoulders were inside your ribcage. “It is the shell and associated features, such as the position of the limb girdles inside it, that makes turtles so unusual,” says palaeontologist at the Smithsonian National Museum of Natural History in Washington DC.
Less obvious but equally confusing are the holes in turtle skulls. Or rather the lack thereof. Look at the back of almost any other reptile skull – whether it is a gecko or a Tyrannosaurus rex – and you will see a pair of openings for jaw muscle attachments. Turtle skulls, with their toothless beaks, don’t have these holes, making them an oddity that lacks a clear connection to any other group of reptiles.
These features stretch back into deep time. Around 13 million years ago, a turtle aptly known as Stupendemys grew as large as a small car. About 75 million years ago, lived Archelon, which at 4.6 metres from head to tail is the largest turtle ever documented. Yet these are youngsters compared with Proganochelys, the first specimen of which was found in roughly 220-million-year-old rock in Germany and named in 1887.
Spiky on top
For decades, Proganochelys stood as the most primitive turtle known. Yet it was unquestionably a turtle, from its spiky shell to the arrangement of its shoulders and lack of holes in the rear of its skull. This wasn’t much help to those trying to figure out how turtles evolved. Researchers wanted to find the remains of a creature that was on its way to becoming a turtle, but the earliest examples looked fully formed and offered no clue to the appearance of a turtle ancestor.
“For well over a century, most zoologists could not even imagine what a proto-turtle would have looked like,” says Sues. Eventually, some palaeontologists began to guess that they should look out for a fossil with blobs of bony armour on its back and abdomen, the first hint of a shell. Some called it the polka dot ancestor.
Things changed abruptly in 2008, when Chun Li at the Chinese Academy of Sciences in Beijing and his colleagues that didn’t look quite like any other. Named Odontochelys, the turtle was found in ancient ocean sediments in China that were about the same age as Proganochelys. Yet this fossil was a turtle in a half shell, a primitive animal protected on the abdomen but with no carapace on top. This hinted that turtles evolved their shells in piecemeal style, the bottom half first. Odontochelys had teeth, too – it was clearly a stage between turtles as we know them and their more ancient ancestors.
“This discovery released palaeontologists from their self-imposed search image for a proto-turtle with shells and instead focused the search to animals with broadened ribs,” says Tyler Lyson at the Denver Museum of Nature and Science. The finds have been rapidly piling up since then. In 2015, Sues and his colleague Rainer Schoch named Pappochelys, a more ancient turtle with a lizard-like appearance. Three years later, Li body but a toothless, beaked skull.
Forgotten fossil
Palaeontologists also realised that they had known about one of the most ancient turtles all along – but miscategorised it. Named in 1892, Eunotosaurus was thought to be a lizard from 260 million years ago that was unremarkable except for its broad ribs. But when it was compared with the newly discovered early turtles, it . All these fossils add up to a reasonably clear picture of how the turtle got its shell. “The origin and early evolution of the turtle body plan has become one of the best-documented examples of an evolutionary transition in the fossil record,” says Sues.
This means that we can now retell how the turtle got its shell with more confidence, just as we are also learning the truth about other famous evolutionary tricks (see “Real just-so stories”). During the Permian period, from about 298 to 252 million years ago, lizard-like reptiles such as Eunotosaurus were digging into the ground for food or shelter. Those with broader ribs would have been more efficient diggers, and this conferred an advantage that was passed down.
After that, turtle ancestors evolved an abdomen shell, perhaps to protect and stabilise their internal organs from the stresses of digging. In later species, bits of bone began to form along the skin of their backs, creating a more enclosed carapace. The motions required for digging are similar to those that turtles use to swim, and so burrowing into the ground
pre-adapted turtles to be competent swimmers when that evolutionary niche became available.
Yet the case of turtle shell evolution isn’t completely closed. In particular, there is still debate about whether the top half of the shell evolved before or after turtles began swimming. Increasingly shelled-in turtles would have had protection from attack while suspended in the water, but it is unclear whether that was a reason they took to a life aquatic or rather a downstream effect of that move.
“Around 13 million years ago, a turtle called Stupendemys grew as large as a small car”
Part of the reason we don’t have firm answers here is that there are tens of millions of years between Eunotosaurus, Odontochelys, and Proganochelys, and we don’t know what came in between. We also know little about the role of extinction events. Eunotosaurus and its early turtle relatives lived about 10 million years before the worst mass extinction of all time. Yet we have only the wispiest grasp of how and why turtles survived such cataclysmic times.
Researchers will undoubtedly dig out more undiscovered turtle ancestors in the years ahead. And now these discoveries won’t enter an evolutionary information vacuum. Each newly discovered shell fossil is a time capsule that builds on a story we have already begun to read.
“When black cows were painted with stripes, they avoided attack from biting flies”
Real just-so stories
Fresh evidence from fossils and genomic analysis is teaching us how several strange wonders of the animal kingdom truly came about
How the whale lost its legs
It is thought that whales trace their lineage back to deer-like creatures that ventured into the water to feed many millions of years ago. At some point between then and now, whales lost their legs. As evolutionary transitions go, it is fairly extreme. Fossil discoveries over the past decades have begun to paint a picture of how it happened.
About 50 million years ago, early whales began to evolve from animals that swam by kicking both sets of legs, doggy-paddle style. Later forms had shorter legs with webbed hands and feet, a powerful tail and a long snout.
At first, keeping their legs gave these early whales the option of returning to land, perhaps for resting or breeding. But then whale ancestors began to swim by undulating their bodies, using their front proto-flippers to steer. At this point it seems their hind limbs got in the way of their tails. Those animals with the smallest hind limbs became the best swimmers, and the rest is history.
How the panda got its thumb
Humans are part of a select group of animals with an opposable thumb. We tend to forget that pandas are also members of the club – well, sort of. Next to the five toes on their front paws, they have a stubby modified wrist bone that they use like a thumb. Giant and red pandas both have it. But how come?
Genomic analysis recently identified two genes involved in limb development . Mutations in the equivalent genes in humans and mice can disrupt skeleton formation. It looks like this is what happened to both species of panda independently, but the bone mutation turned out to be useful.
Both the red and giant panda feed on bamboo in a way that means dexterity is highly beneficial to them. Being able to select young shoots and strip out the tender core of stems means panda diets are about 50 per cent protein, despite them being vegetarian.
How the zebra got its stripes
Scientists have come up with many explanations for why zebras evolved stripes, ranging from camouflage to display signals to heat regulation. But over the past few years evidence has been quietly mounting for a different and unexpected possibility.
A 2019 study of captive zebras and horses in the UK revealed that . Horses that were dressed like zebras – wearing black and white striped cloth coats – also had fewer horseflies land on them, compared with horses wearing either black or white coats.
Why the stripes make it less likely that the flies will land isn’t yet clear. Still, fake zebra stripes seem to protect other animals too. When Japanese black cows were painted with zebra-like stripes they were also . The authors of this study suggest that painting black-and-white stripes on livestock could be an alternative method of pest control.
This raises the question of why other animals didn’t evolve stripes to ward off flies, too. It may be that selection pressures for avoiding biting flies were higher for zebras because in the region of Africa where they live, biting flies can transmit deadly disease-causing parasites.
Layal Liverpool
