Two of the strangest animals on Earth – the platypus and the echidna – have just become even stranger, thanks to a new study on a single fossil that provides important clues about their ancient ancestors. According to scientists, the finding may change what is currently known about the evolution of the most primitive mammals still living on Earth.
The platypus and the echidna, found in Australia and New Guinea, belong to the group of monotremes – the only group of mammals that lays eggs. They are called "monotremes" because they have a single opening in their bodies used for multiple functions: Laying eggs, urinating, and defecating. This opening is called the "cloaca."
The platypus – a mix of beaver and duck with a venomous spur
The semi-aquatic platypus resembles a cross between a duck and a beaver, with a broad bill, webbed feet, and a flat tail. It hunts underwater, is toothless, and secretes milk through the skin instead of nursing through nipples. The male platypus has a venomous spur used in mating battles rather than for defense or capturing prey, and thus platypus venom is intended to cause pain, not death. It gathers food underwater with eyes, ears, and nostrils closed and sealed from water entry. The bill functions as a sensory detector based on electric signal detection, allowing the platypus to capture its food, which consists of worms, tadpoles, small crustaceans, and a variety of aquatic insects and larvae. The food is stored in the platypus’s cheeks and eaten on land.
The echidna – a body that secretes milk and cools itself with mucus
The echidna lives only on land, is covered in spines, has backward-facing hind legs that help it dig, and like the platypus, has no teeth or nipples. Instead, milk is secreted through the skin and the young lick it. It has a narrow, long snout that serves as both nose and mouth. Its tongue is long and sticky, and it has no teeth at all. In danger, the echidna lies on the ground with its exposed belly pressed to the earth and all its spines pointing outward toward the predator. The echidna cannot sweat like humans to cool its body, nor pant like dogs. Instead, it uses mucus bubbles to cool itself.
So what does this have to do with Cadbury chocolate?
A new study published in the prestigious journal PNAS presents new findings on the subject. A team led by paleontologist Susan Hand from the University of New South Wales revealed the internal structure of a rare and unique fossil of Kryoryctes cadburyi – an ancient mammal species that lived over 100 million years ago and is considered an ancestor of the platypus and echidna.
The name, composed of Greek words, means "the digger from a cold region" – kryo ("cold") and oryktes ("digger"). The name "cadburyi" was given as a tribute to the Cadbury chocolate company, which supported the excavations at the site, and also because the fossil’s dark color resembled chocolate.
The fossil – an upper arm bone (the humerus) – was discovered back in 1993 at Dinosaur Cove in southeastern Australia. Externally, it resembles the bone of a terrestrial echidna more than that of an aquatic platypus, but advanced 3D scans revealed a surprise.
The internal structure of the bone actually resembled that of the platypus: Thick walls and a small central cavity, a combination that leads to a heavier bone and reduces the animal’s buoyancy in water. This kind of structure is typical of animals that use bone weight to dive and hunt. In contrast, the terrestrial echidna has much thinner and lighter bones.
The finding supports the hypothesis that Kryoryctes cadburyi was a common ancestor of the two species, and that it may have lived at least part of the time in water. “Our research suggests that the semi-aquatic lifestyle of the platypus began more than 100 million years ago,” said Hand, “and that the echidna returned to a terrestrial lifestyle at a much later stage.”
There are quite a few examples of mammals transitioning from land to water – whales and dolphins, for instance, evolved from land mammals that shared a common ancestor with hippos. In contrast, cases of reverse transition – from water to land – are much rarer and require significant changes in the skeletal system, limbs, and body weight. According to the researchers, the echidna’s strange backward-facing hind legs may be a remnant of a navigation mechanism used in water.