Fossil of 'platypus-like' animal from dinosaur era unearthed in China

With a body like a platypus, tail of an otter and a canine head, a fossil found in China shows the earliest signs of a mammal-like animal with a semiaquatic lifestyle.

Megacauda sungei lived about 168 to 163 million years ago in the Jurassic period when massive dinosaurs dominated the planet.

A new study published in Nature places Megacauda on the mammaliaforms family tree under the extinct Docodonta branch, early cousins of true mammals.

Weighing in at about 1 to 2 kilograms, Megacauda is the largest of the Jurassic mammaliaforms.

It was about the same size as a modern platypus (Ornithorhynchus anatinus).

Senior study author and palaeontologist Zhe-Xi Luo from the University of Chicago, said the discovery of Megacauda suggested the capability to swim originated early in mammal evolution.

"Semiaquatic mammals are not uncommon, in today’s world; there are 140 living mammal species that are riparian-dependent," he said.

How similar to a platypus was Megacauda?

Palaeontologist Suzanne Hand, an emeritus professor at University of New South Wales who was not involved in the study, said there were a lot of morphological parallels between Megacauda and platypus.

"Such as foot, ankle and pelvis morphology, expanded ribs and rib cage, webbed feet, flattened tail and exceptionally thick fur, to name just a few features," she said.

Professor Luo said there were also a lot of differences to the platypus.

"Unlike the platypus … Megacauda had strong canine and sectorial teeth for carnivorous feeding in water.

"We were excited that the fossil was stunningly complete — a vast majority of bones appear to be preserved."

The fossil was found in 2015, in the south-east of Inner Mongolia, from a fossil-bearing layer of rock called the Haifanggou Formation.

Professor Luo said this deposit was "teeming with salamanders".

"Because Megacauda has carnivorous teeth, they can feed on small fishes and salamanders."

Further study of the internal microstructure of Megacauda's limb bones may provide further information about how it lived, according to Professor Hand.

"Including how much time it might have spent foraging in water versus on land," she said.

But the fossil already provides some other details on mammalian evolution beyond furry forays into the water.

The preservation of soft tissues in the throat of the fossil shows it was developed for milk feeding as an infant and swallowing food as an adult, features also seen in modern mammals.

What's so good about being semiaquatic?

Professor Hand said there were several potential reasons for a semiaquatic lifestyle to have evolved independently many times among mammals and pre-mammals.

"Evolutionary incentives for this include the exploitation of a previously untapped, abundant and rich food resource, reduced resource competition from land-living relatives, and new opportunities for dispersal, resulting in significant habitat and range extensions.

"Semiaquatic animals have also been shown to survive catastrophic events significantly better than fully terrestrial animals."

Megacauda takes the earliest-known semiaquatic mammaliaforms crown from Castorocauda, a prehistoric beaver-like creature, which lived in the same region between 164 and 159 million years ago. 

The earliest-known semiaquatic true mammal, Dongoconodon platycauda, was found in a 120 million-year-old Chinese fossil deposit.

The animal, which may have looked a bit like a rakali, lived in the early Cretaceous and was estimated to be 40 centimetres long and weigh 190 grams, scientists reported last week in Nature Communications.

In Australia, the earliest semiaquatic monotreme, Kryoryctes cadburyi, lived about 100 million years ago.

Modern platypuses diverged from echidnas and other mammals about 55 million years ago, but the oldest platypus fossil found is only about 3.8 million years old.