A 165-Million-Year-Old Throat Tells a Mammalian Story

There is a fossil that carries within it not just bones but a behavior—a moment frozen in stone that reveals how an animal lived and, more profoundly, how it ate. In the Inner Mongolia region of China, paleontologists have uncovered a 165-million-year-old mammal ancestor with an exceptionally preserved throat. The creature, named Megacauda sungei, was about the size of a modern platypus, with hind limbs suited for swimming and a broad, flattened tail like a river otter. But what makes it remarkable is the anatomy of its throat, which suggests it could chew and swallow in a manner previously thought to have evolved much later .

The fossil was discovered in the Daohugou area of Inner Mongolia, a site known for its exquisitely preserved Jurassic specimens. The delicate hyoid bones and throat structures are rarely fossilized, and their survival in this specimen allowed researchers led by Zhe-Xi Luo of the University of Chicago to reconstruct, for the first time in a mammaliform, the entire throat anatomy . The hyoid bones in modern mammals form a U-shaped saddle suspended from the skull, anchoring the muscles that allow us to swallow chewed food in manageable lumps rather than gulping down entire prey like a crocodile. They also give infants the ability to suckle milk .

Megacauda has these structures. It also has a pair of small bony projections on the roof of its mouth that support the soft palate—another key adaptation for suckling and swallowing. "The mammalian ways of swallowing and suckling are unique," Luo said. "It is one of several biological adaptations of mammals that stand out from all other vertebrates" . Thomas Martin of the University of Bonn, a co-author, noted that it was this adaptation that made brain development, and eventually humans, possible .

The animal itself was a strange chimera. Its teeth were blade-like, resembling those of a seal, for cutting meat. Its pelvis was platypus-like, with long outer digits. Its tail was broad and flat, an otter's tail, for propulsion in water . The researchers believe it was a semi-aquatic predator, dipping into rivers to hunt small vertebrates and invertebrates. Tiny bone fragments found in its stomach area suggest its last meal may have been a salamander .

The discovery pushes back the origin of mammal-like swallowing and suckling to before the rise of modern mammals. "Once early mammals had this apparatus, each group could develop specialized teeth and diversify into a tremendous range of food resources," Luo explained. "They could be carnivores, insectivores, herbivores, omnivores, or anywhere in between" . The fossil also suggests that the platypus and echidna, which lack some of these features, did not lose them because they never had them—they likely had them and then lost them over evolutionary time .

For those who study the age of dinosaurs, Megacauda is a reminder that the Mesozoic was not merely a time of giant reptiles. It was also a time of extraordinary experimentation among early mammals, creatures that were small and often overlooked but were quietly evolving the biological toolkit that would one day allow their descendants to thrive in a world the dinosaurs left behind.

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Sources:

· University of Chicago News · Nature · New Scientist · BBC Wildlife Magazine