The Chinese soft-shelled turtle looks like someone glued the snout of a pig onto the face of a fish, with the texture of a scrotum for good measure. But its bizarre appearance pales in comparison to an even more bizarre, and newly discovered, habit: it expels waste through its mouth.
When the turtle breaks down proteins in its liver, it ends up with an abundance of nitrogen, which it expels from its body in the form of urea. Humans are the same—we get rid of urea in the form of urine, via our kidneys. But the soft-shelled turtle has an altogether different route.
It’s well-adapted to life in the water, and lives in salty swamps and marshes. But Yuen Ip from the National University of Singapore noticed that when the turtle emerges from water, or is stranded on land during dry spells, it will plunge its head into puddles. While submerged, it rhythmically expands and contracts its mouth. Ip found that the turtle gets rid of most of its urea through its mouth rather than its kidneys, via gill-like studs in its mouth. It can breathe and get rid of waste through the same structures.
[Update: Note the comments below. Urine is more than just dissolved urea, so while the stuff the turtle spits out is similar, it's not quite the full deal.]
The turtle’s shell provides it with a formidable defence and one that is unique in the animal world. No other animal has a structure quite like it, and the bizarre nature of the turtle’s anatomy also applies to the skeleton and muscles lying inside its bony armour.
The shell itself is made from broadened and flattened ribs, fused to parts of the turtle’s backbone (so that unlike in cartoons, you couldn’t pull a turtle out of its shell). The shoulder blades sit underneath this bony case, effectively lying within the turtle’s ribcage. In all other back-boned animals, whose shoulder blades sit outside their ribs (think of your own back for a start). The turtle’s torso muscles are even more bizarrely arranged.
This body plan – and particularly the odd location of the shoulder blades – is so radically different to that of all other back-boned animals that biologists have struggled to explain how it could have arisen gradually from the standard model, or what the intermediate ancestors might have looked like. Enter Hiroshi Nagashima from the RIKEN Center; he has found some answers by studying how the embryos of the Chinese soft-shelled turtle (Pelodiscus sinensis) shift from the standard body plan of other vertebrates to the bizarre configuration of adult turtles.
By comparing the embryos to those of mice and chickens, Nagashima showed that all three species start off with a shared pattern that their last common ancestor probably shared. It is only later that the turtle does something different, starting of a sequence of muscular origami that distorts its body design into the adult version.
Clad in hard, armoured shells, turtles have a unique body plan unlike that of any other animal. Their shells have clearly served them well and the basic structure has gone largely unchanged since the time of the dinosaurs. But this unchanging nature poses a problem for anyone trying to understand how they evolved and until now, fossil turtles haven’t provided any clues. All of them, just like their living descendants, have fully formed two-part shells.
But three stunning new fossils are very different. They belong to the oldest turtle ever discovered, which lived about 220 million years ago in the area that would become China. Unlike today’s species, its mouth had a full complement of small, peg-like teeth but even more amazingly, it had a feature that distinguishes it from any other turtle, either living or extinct – it only had half a shell.
The ancient turtle was unearthed by Chun Li (no, not that one) from the Chinese Academy of Sciences, who called it Odontochelys semitestacea, a name that literally means “toothed turtle in a half-shell”. It was a small animal, just 35 cm from snout to tail, and its shell consisted of just a plastron (the bottom half) and not a carapace (the top half).
Li’s team believes that this incomplete shell represents an intermediate step along the evolutionary path to the modern version. To them, Odontochelys‘s anatomy settles debates about how the group’s distinctive shell evolved, which animals they were most closely related to and what sort of lifestyles the earliest members had. It’s a true hero in a half-shell.