The "top predator" from the Jurassic is relatively small in size: Megacauda sungei measures just half a meter from its nose to the tip of its tail. 165 million years ago, the furry ancestor to today's mammals hunted its prey in water. The cusps of its teeth were also curved backwards to enable it to catch its slippery prey. However, the true evolutionary revolution is its soft palate, which separates the oral cavity from the pharynx while also enabling swallowing and breathing. This is an important requirement for suckling as a survival advantage. The findings by researchers from the Universities of Shenyang, Chicago, and Bonn are now being published in the journal 'Nature'.
Many species of animal do not have a soft palate and only have a simple hyoid bone apparatus without a swallowing mechanism. Crocodiles, for instance, raise their head up and use jerky movements to make their unchewed prey slide down their throat. "This is achieved with the aid of gravity," explains paleontologist Prof. Dr. Thomas Martin from the University of Bonn. In contrast, mammals, with their complex hyoid structure in the pharynx, are even able to swallow against gravity: As a result, we humans can drink upside down through a straw while breathing.
This ability is a key adaptation that allows mammals to feed on nutrient-rich milk and swallow it safely without it entering their nose. "It was this adaptation that made brain development, eventually leading to humans, possible," stresses Martin. "The soft palate and the shaping of the oral cavity and pharynx are directly linked to the evolution of mammals." This new development was already apparent in Megacauda sungei long before the emergence of modern mammals.
Short coat and flattened tail
At first glance, the short-coated animal with a flattened tail resembles an otter, beaver, or platypus. Like these descendants, the predator from the Jurassic was a good swimmer: Three-dimensional reconstructions from computed tomography images of the fossil suggest that the animal paddled with its front legs and moved its tail in a wave-like motion, using its hind legs to steer, 165 million years ago. "It is the earliest evidence of an aquatic lifestyle in mammals," says the paleontologist from the University of Bonn. The analysis of its teeth also offered surprises: The specialized canines are distinctive in that they have three roots.
As the researchers discovered through extensive comparisons with other fossils and animal species alive today, the fossil found in Xiayingzi, Inner Mongolia, is a docodont – an extinct group of early mammaliaforms that primarily lived in Jurassic times.
"The new species Megacauda sungei is the largest of the mammaliaforms from the Jurassic," says Thomas Martin. Given the large mammals that are alive today, such as rhinoceroses, whales, and elephants, this prehistoric "top predator", measuring just half a meter in length and weighing around two kilos, appears anything but impressive. However, the Mesozoic Era around 230 to 66 million years ago was above all the age of the dinosaurs – and early mammals lived in the shadow of these giants, so to speak. At that time, mammal precursors were relatively small in size, often weighing only a few grams. As a result, the aquatic docodont was something of a heavyweight within its group.
Unique find
As is so often the case, chance also played a part in how the fossil came into the hands of the scientific community years ago. The Paleontological Museum of Liaoning in Shenyang (China) acquired the specimen, discovered in Inner Mongolia, together with other finds. During a joint examination by Prof. Dr. Chang-Fu Zhou from Shandong University (China), Prof. Dr. Zhe-Xi Luo from the University of Chicago (USA), and Prof. Dr. Thomas Martin from the University of Bonn, the unique nature of the find caught the experts' attention. It was soon clear that the new species should be named in honor of the Honorary Director of the Paleontological Museum of Liaoning: Prof. Sun Ge. As a result, "large tail" (Megacauda) was combined with the species name "sungei".
It took years before all of the examinations were complete and the new species was securely classified, described, and documented. "It is the result of a long period of teamwork," says Thomas Martin. The efforts were worthwhile: With the description of the new species, it was possible to uncover countless secrets relating to the origin of mammals. As other finds demonstrate, the docodonts in the Jurassic also lived in trees, underground like moles, and in water like Megacauda sungei. "This shows how widespread docodonts were in the Northern Hemisphere and that this diverse group of animals developed very different lifestyles at an incredibly early stage," summarizes Martin.
Publication: Y. Li, P. Li, A. I. Neander, H. Zhang, C.-F. Zhou, T. Martin and Z.-X. Luo: A Jurassic Mammaliaform Swimmer and Transformation of the Mammalian Pharynx, Nature, DOI: 10.1038/s41586-026-11107-0, URL: https://www.nature.com/articles/s41586-026-11107-0
Video: https://youtu.be/BX17OxaaaK0
