Researchers have unspooled the anatomy of a 165-million-year-old mammaliaform named Megacauda sungei, unearthed in Inner Mongolia, China. Described in Nature, the Chihuahua-sized creature combines a platypus-like body and otter-like tail with delicate throat bones, offering some of the oldest known evidence of mammalian chewing, swallowing, and suckling.
Deep inside a thick slab of dark stone from the Middle Jurassic, a remarkably intact fossil has handed paleontologists a rare window into the early evolution of mammalian biology. Discovered originally by local farmers in late 2015 in an abandoned quarry in the Xia Yingzi area, the specimen eventually made its way to the Paleontological Museum of Liaoning. There, University of Chicago paleontologist Zhe-Xi Luo and Thomas Martin of the University of Bonn spotted the remains while visiting the facility to examine new finds.
The animal weighed between 1 and 2 kilograms, measuring roughly 40 to 50 centimeters long. That scale makes it the largest known mammaliaform from the entire Jurassic period, a time when most contemporaries weighed only a few grams. Named Megacauda sungei — with the genus name translating to “big tail” — the creature lived approximately 165 million to 168 million years ago alongside dinosaurs, hunting prey in freshwater lakes and rivers across what is now Inner Mongolia. Collaborating on the study alongside others are Yikun Li and Honggang Zhang, representing Shenyang Normal University in China, as well as Thomas Martin from Universität Bonn in Germany.
Anatomy of a Jurassic Swimmer and Predator
Megacauda possessed a striking suite of physical adaptations that earned it comparisons to a chimeric mix of modern water-dwellers. Researchers used X-ray fluorescence to make the dark bones pop out against the dark stone slab, rendering the skeleton a bright yellow and exposing a bizarre combination of traits. The species possessed a short-haired coat and feet built for paddling, leading the team to conclude it lived a semi-aquatic lifestyle. It's like a chimera,
study co-author Zhe-Xi Luo, a paleontologist at the University of Chicago, told Live Science.
The back half of the skeleton featured a broad, flattened tail with enlarged bones alongside short hind limbs and long outer digits. According to Sue Hand, a vertebrate paleontologist who was not involved in the work, the animal shared foot, ankle, and pelvis morphology, expanded ribs, and a flattened tail with living platypuses. But its skull told a different story. Equipped with strong canines and sharp, shearing cheek teeth shaped like cutting blades, the predator hunted live prey, leaving behind tiny bits of vertebrate bone in its stomach beds that indicate a diet of salamanders, fish, and aquatic invertebrates. Jessica Theodor, a paleontologist who wasn’t involved in the research, calls it a fascinating specimen.
Inside the Throat Bones of Megacauda sungei
Beyond its swimming gear, the fossil preserved an intricate and delicate set of pharyngeal remains that changed how researchers view the timeline of mammalian traits. Luo immediately zeroed in on tiny hyoid bones in the throat, which the team later analyzed using computed tomography scanning of the underside of the skull.
That scan exposed a delicate roof of the mouth and a series of bone hooks and ridges that supported the backside of the tongue. In modern vertebrates like crocodiles, animals bite and gulp large prey whole, relying on gravity and jerky head movements to slide unchewed food down. Mammals process food differently, chewing it down into a single bolus before swallowing through an esophagus equipped with a soft palate and complex hyoid machinery. After mammals chew their food, be it plants or meat, they all swallow it in the same efficient and effortless way,
said Zhe-Xi Luo, a paleontologist at the University of Chicago and senior author of a new study describing the fossil in Nature.
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.
Dr. Thomas Martin, University of Bonn
That internal architecture made safe feeding on nutrient-rich milk possible without fluid entering the nasal passages. Although soft tissues like mammary glands do not survive in the fossil record, the presence of these throat structures in an adult specimen indicates that the biological machinery required for suckling and mammalian swallowing had already evolved by the Middle Jurassic. While hoping to examine certain image details more closely, Theodor points out that this specimen illuminates lactation’s evolutionary path—describing it as a dietary and hormonal dialogue between mother and offspring that traces back as far as the Jurassic and Cretaceous eras within the Mesozoic.
Where Docodonts Fit on the Mammaliaform Tree
Megacauda belonged to the docodonts, an extinct branch of early relatives to mammals. These cousins showed remarkable ecological diversity long before modern mammals diversified. The fossil record includes a tiny docodont from around the same geological age called Microdocodon gracilis, which also featured early mammal-like hyoid bones.
In the same Chinese region, researchers previously unearthed Castorocauda lutrasimilis, a prehistoric beaver-like swimming mammaliaform that lived between 164 million and 159 million years ago. Megacauda now claims the crown as the earliest-known semiaquatic mammaliaform, pushing the timeline of that specialized lifestyle back further. For Luo, this discovery stands as the most recent proof of how novel feeding mechanisms enabled early mammals and their precursors to thrive and diversify, even during epochs when they occupied the underdog position.
| Fossil Species | Estimated Age | Key Traits |
|---|---|---|
| Megacauda sungei | 168 to 163 million years ago | Largest Jurassic mammaliaform, otter-like tail, platypus-like swimming limbs, advanced hyoid bones |
| Castorocauda lutrasimilis | 164 to 159 million years ago | Beaver-like prehistoric swimmer from the same Inner Mongolia region |
| Microdocodon gracilis | Middle Jurassic | Tiny docodont with early mammal-like hyoids |