165-Million-Year-Old Swimming Mammal Relative Had Three-Rooted Fangs

A Jurassic hunter that paddled after its food tipped the scales at 2.2 to 4.4 pounds (1 to 2 kilograms), heavier than any other mammal relative known from that period, researchers report in the journal Nature. Every upper fang sat on three roots, something not seen in any mammal on record.

Researchers named it Megacauda sungei. Its throat bones show where muscles attached that mammals today rely on for swallowing, drinking and suckling. In other words, pieces of our throat design were working well before true mammals arrived, according to Earth.com.

One skeleton, one museum

Everything known about Megacauda comes from a lone skeleton recovered in Inner Mongolia, China. The rock around it is 163 to 168 million years old, placing it in the middle Jurassic. The Paleontological Museum of Liaoning keeps the specimen.

Scientists from Shenyang Normal University, Shandong University of Science and Technology, the University of Chicago and the University of Bonn worked on the description, with Thomas Martin (Bonn), Zhe-Xi Luo (Chicago) and Chang-Fu Zhou (Shandong) in the lead.

The animal was a docodont, part of a vanished branch sitting right next to true mammals on the family tree. “Big tail” is what the name means, a nod to its wide, flattened tail. It had roughly double the mass of its nearest known cousin and was 150 to 200 times bulkier than a miniature docodont dug up nearby. Plenty of early mammal relatives were only a few grams.

Teeth for grabbing slippery prey

Its top canines were about 60 percent longer than the bottom pair. Ridges and furrows running along them likely helped them survive stabbing into prey. Toward the back of the jaw, pointed cheek teeth worked like shears, a bit like those of a seal. The team sees it as a top hunter in its waters, feeding on salamanders, fish and small invertebrates.

A body for water and burrows

Its build suggests an aquatic life. Seen from above, the flat tail resembled a giant otter’s more than the rounded paddle of a beaver or platypus. Long back feet ended in long outer toes, as in a platypus or sea otter, and the hip shows muscle scars that a platypus uses to pull its legs in tight while swimming. The forelimbs, meanwhile, were suited to digging.

Put together, the researchers say it lived a lot like a platypus: catching food in the water and resting in dens on land. It likely swam less powerfully than an otter or beaver, since its tail muscles stopped short of where they extend in those animals.

Reading the throat from bone

The prize of the fossil is the roof of the throat, which CT scans captured in 3D. At the back of the skull they found a tiny bony hook. In marsupials and placental mammals, a muscle loops around that hook to stiffen the soft palate, the bendy rear part of the mouth’s roof. With the palate held firm, the tongue can push against it when a newborn feeds or an adult takes a drink.

Next to the hook, a groove probably held a second muscle. Mammals now use that one to raise the throat against the soft palate as they swallow, so food does not end up in the nose. Saddle-shaped throat bones like those of many modern mammals round out the picture, though the skull also held on to older ridges from earlier relatives.

Because muscle tissue does not turn to stone, the team worked out the missing muscles by comparing attachment sites with living species, an opossum among them. Their reading is that swallowing, drinking and nursing gear showed up as a package.

Why it matters for the story of mammals

No earlier mammal relative is known to have spent part of its life in water. Other docodonts climbed trees or tunneled like moles, and across roughly 100 million years the group reached every northern continent. Martin said the group took up “very different lifestyles at an incredibly early stage.”

The combination of swimming and digging evolved again about 120 million years later in pantolestids, an extinct mammal group, and once more about 30 million years after that in the family of otters and weasels.

What is still unclear

It is a single specimen, so every finding hangs on it. The body weight comes from the skull and from comparisons with platypuses of similar size, and the palate and throat muscles are reconstructions from bone. Bones alone cannot show whether Megacauda nursed its young or how often it drank. Settling those questions would take more docodont fossils with complete throats, and the feeding anatomy of the first true mammals to compare against.

Source: Earth.com