Researchers have discovered a 165-million-year-old mammal relative in China’s Inner Mongolia region named Megacauda sungei. Measuring roughly 20 inches long and weighing up to 4.4 pounds, the Chihuahua-sized creature possessed a beaver-like tail, platypus-like hind limbs, and specialized throat bones indicating it could chew, swallow, and suckle.
An international team of scientists has uncovered a remarkably complete fossil from the Middle Jurassic Period that pushes back the timeline of core mammalian traits by millions of years. Unearthed in rock formed from ancient lake sediment, the specimen preserves not just skeletal structure but delicate soft-tissue impressions, offering an unprecedented look at an animal that hunted salamanders and fish in aquatic habitats while dinosaurs roamed the land.
Megacauda Sungei Combines Traits of Modern Animals
Referred to as a chimera of features by researchers, the newly described species was named Megacauda sungei, meaning big tail
in reference to its broad, flattened appendage. The animal measured roughly 50 to 52 centimeters from nose to tail and weighed between 1 and 2 kilograms. While most mammaliaforms of the Jurassic era were tiny, shrew-sized creatures hiding beneath the feet of giant predators, Megacauda was twice as big as the next-largest member of its lineage at the time.
The creature’s physical traits combined aspects of several modern animals. Paleontologists noted that it sported a broad tail similar to a river otter or beaver, short hind legs with long feet resembling a platypus, and canine teeth alongside shearing cheek-teeth designed to cut and chew meat. Its fossilized stomach contents even contained tiny vertebrate bones, indicating it preyed upon the abundant salamanders inhabiting the same freshwater lakes and rivers.
The large fossil slab required separation into smaller pieces for scanning, which allowed April Neander, a scientific artist and expert on CT imaging at UChicago, to visualize its internal structures in fine detail. The fossil is preserved with the pelvis, hind limbs, and tail that distinguished it as a powerful swimmer, featuring wide, H-shaped tail vertebrae and a broad outline of a tail to propel itself through water using both paddling movements and undulating motions.
Skull Anatomy Reveals Ancient Swallowing Mechanics
Beyond its swimming adaptations, the fossil’s skull and throat anatomy provide critical clues about the evolution of feeding behaviors. CT scans of the underside of the skull revealed a delicate roof of the mouth and a series of bone hooks and ridges forming jointed hyoid bones in the throat region.
“All modern mammals, including humans, have hyoid bones that link the mouth and throat passage down to the esophagus and the larynx. They are arranged in a U-shaped saddle, suspended by a chain of bony segments to the skull. This helps adult mammals to use throat muscles to swallow chewed food one lump at a time, instead of gulping down huge bites or whole prey like an alligator.”
Zhe-Xi Luo, University of Chicago
This complex arrangement allowed the animal to coordinate swallowing and sucking long before the appearance of modern mammals. Researchers explained that while reptiles like crocodiles rely on gravity and jerky head movements to force unchewed prey down their throats, Megacauda possessed the machinery for controlled chewing, swallowing, and infant suckling.
Scientists note that this evidence pushes the history of neonatal suckling behavior back before the split of modern marsupials and placental eutherian mammals. According to Sue Hand, a vertebrate paleontologist who was not involved in the work, suckling behavior was previously thought of as a hallmark of advanced mammals rather than something shared in the common ancestor of docodonts and mammals at least 165 million years ago.
Docodonts Flourished During the Mesozoic Era
Megacauda sungei belonged to the docodonts, an extinct group of early mammal relatives that first appeared roughly 200 million years ago and flourished for about 90 million years during the Mesozoic Era while leaving no living descendants. Rather than occupying just a single niche, docodonts adapted to a wide variety of environments, with various members burrowing, living on the ground or in trees, or becoming semiaquatic. Previous discoveries in China include Microdocodon gracilis, a tiny docodont described in 2019 that also featured mammal-like hyoid bones, and Castorocauda lutrasimilis, a semiaquatic swimmer that lived roughly 164 million years ago and was discovered by Luo’s team in 2006.
| Fossil Species | Estimated Age | Ecomorphotype / Lifestyle |
|---|---|---|
| Megacauda sungei | ~165 million years ago | Semiaquatic swimmer (otter/platypus-like) |
| Castorocauda lutrasimilis | ~164 million years ago | Semiaquatic swimmer (beaver-like) |
| Microdocodon gracilis | Middle Jurassic | Docodont |
While Megacauda shared physical similarities with modern platypuses, scientists emphasize that it was not a direct ancestor. Platypuses and echidnas split far later, and modern monotremes likely lost elements of the ancestral suckling apparatus during their evolutionary history while developing an alternative method of milk intake. Independent researchers such as Tim Flannery, a mammalogist at the Australian Museum Research Institute, noted that the discovery highlights how an aquatic lifestyle arose repeatedly among early mammal-like species.
Authors Detail the Discovery in Nature
The detailed description of the fossil was published on October 7, 2026, in the journal Nature under the citation A Jurassic Mammaliaform Swimmer and Transformation of the Mammalian Pharynx
by Li et al. The international research team behind the discovery included Peishu Li, assistant professor in the Department of Biomedical Sciences at the Ohio University Heritage College of Osteopathic Medicine; April Neander and Zhe-Xi Luo of the University of Chicago; Yikun Li and Honggang Zhang of Shenyang Normal University in China; Chang-Fu Zhou of Shandong University of Science and Technology in Qingdao, China; and Thomas Martin of the University of Bonn in Germany.
Funding for the research was provided by the National Science Foundation USA, the Taishan Scholar Program of Shandong, China, Deutsche Forschungsgemeinschaft, and Ohio University. As researchers continue analyzing the exceptionally complete skeletal remains, the specimen helps show how early mammal relatives diversified long before the asteroid impact toppled the dinosaurs.