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The otter-like animal was Megacauda sungei, an extinct docodont mammaliaform that lived about 165 million years ago in what is now Inner Mongolia, China. It was not an otter and it was not a dinosaur. The comparison to an otter describes its body plan and water-based way of life, not its ancestry. University of Chicago researchers announced the fossil on October 7, 2026.
What kind of animal it was
Docodonts were an extinct group of mammaliaforms, a lineage closely related to mammals. Megacauda belonged to that group, so it sat outside the living mammal lineage that includes otters. Its resemblance to an otter comes from a broad, paddling build and an aquatic lifestyle, which evolved independently of any otter ancestry.
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Age, location and size
- Age: about 165 million years old, from the Middle Jurassic.
- Location: Inner Mongolia, China.
- Size: 40 to 50 centimeters long and one to two kilograms. These are the estimates given in the University of Chicago report, not measurements of a complete living animal.
Evidence that it swam
The fossil preserves the pelvis, hind limbs and tail, which together explain how the animal moved in water.
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- Wide, H-shaped tail vertebrae outline a broad tail, the kind of structure used for propulsion.
- Interpretation: the researchers read this anatomy as evidence of powerful swimming and a semi-aquatic lifestyle.
The strength of this conclusion rests on skeletal shape. No swimming speed or performance measurement was reported.
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What it ate
The teeth point toward meat. Megacauda had robust canines and shearing cheek teeth suited to cutting and chewing flesh. Tiny vertebrate bone fragments found in its stomach support the idea that it hunted small animals. The researchers suggest salamanders as one possible prey type, because they were abundant in the same fossil beds. That is a plausible inference from the stomach contents and local fauna, not a confirmed menu.
Throat structures and swallowing
The fossil preserves delicate throat structures related to the hyoid apparatus, the bony support that moves the tongue and throat during feeding and swallowing. The University of Chicago account describes these as early evidence of bony throat structures that enabled a mammal-like way of feeding and swallowing. Zhe-Xi Luo, Mila Pierce-Rhoads Professor of Organismal Biology and Anatomy at the University of Chicago, said: “This is an incredibly important evolutionary innovation. After mammals chew their food, be it plants or meat, they all swallow it in the same efficient and effortless way.”
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How it compares with Castorocauda
Castorocauda is another Middle Jurassic docodont from China. A 2006 study in Science described it with swimming and burrowing adaptations and dental features suited to aquatic feeding. The two are related comparative examples, not alternative names for one animal.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minute| Feature | Megacauda sungei | Castorocauda |
|---|---|---|
| Age | About 165 million years (Middle Jurassic) | Middle Jurassic; exact age not stated in the sources reviewed |
| Location | Inner Mongolia, China | China |
| Preservation | Pelvis, hind limbs, tail, throat structures and stomach contents | Described in the 2006 Science study; the University of Chicago account calls Megacauda more complete |
| Aquatic adaptations | Long outer back-leg digits, broad H-shaped tail vertebrae | Swimming and burrowing adaptations |
| Feeding evidence | Robust canines, shearing cheek teeth, small vertebrate bone fragments in the stomach | Dental features for aquatic feeding |
| Size | 40 to 50 cm; 1 to 2 kg (University of Chicago estimates) | Not stated in the sources reviewed |
Paleontologist April Neander, a UChicago scientific artist and CT imaging expert, put the difference this way: “The new fossil is particularly interesting because it’s older than Castorocauda. Although both docodonts occupied the same niche in the water, the new fossil is more complete.” The sources do not support a detailed ranking of which animal swam better.
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Lakes, rivers, or both?
The phrase “lakes and rivers” in the headline goes beyond the evidence. The fossil supports a semi-aquatic lifestyle, with swimming and hunting in water. The published account does not identify whether the animal lived in lakes, rivers, or both, so the accurate description is simply that it lived in water-associated environments in what is now northern China. That is also the limit of what the fossil establishes about its habitat, and it is worth keeping in mind when reading secondhand summaries.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the evidence does not establish
- A complete diet. Small vertebrates, possibly salamanders, are inferred from stomach fragments and fossil-bed abundance.
- Swimming speed or a ranking against Castorocauda.
- How common Megacauda was. No population or frequency estimate has been reported.
These gaps do not weaken the core finding. The animal was a meat-eating, water-adapted mammaliaform whose throat anatomy gives scientists an early look at the bony structures behind mammal-style swallowing.
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- Learn & play: Gemini&Genius animal figures are modeled in the finest details which enables children to learn about animals from all over the world, from ancient to present, to make children love nature and history.
- Excellent workmanship and durability: Through the use of high-quality materials, combined with the latest production methods. We use every scrap of animal hair to bring animal lovers back to the wilderness of ancient times. Bring life back to the most amazing animals once inhabited on Earth.
- Quality and safety: Gemini&Genius figures are eco-friendly paint and material make it with no smell and safe for children. The Action Figures exquisite workmanship and the whole length is 7-10 inches from the head to the tail which is the perfect size for whatever kids toy or home decoration.
- Welcome to the finest collection series: This is a collection series of very delicate, very beautiful prehistoric animals that are both toys and the best collection on display and gift.
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- Learn & play: Gemini&Genius animal figures are modeled in the finest details which enables children to learn about animals from all over the world, from ancient to present, to make children love nature and history.
- Excellent workmanship and durability: Through the use of high-quality materials, combined with the latest production methods. We use every scrap of animal hair to bring animal lovers back to the wilderness of ancient times. Bring life back to the most amazing animals once inhabited on Earth.
- Quality and safety: Gemini&Genius figures are eco-friendly paint and material make it with no smell and safe for children. The Action Figures exquisite workmanship and the whole length is 6-10 inches from the head to the tail which is the perfect size for whatever kids toy or home decoration.
- Welcome to the finest collection series: This is a collection series of very delicate, very beautiful prehistoric animals that are both toys and the best collection on display and gift.
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