Scientists examined a 165-million-year-old fossil from China and found a broad swimming tail plus bony throat structures; the Jurassic mammal predecessor could feed and swallow in ways seen in modern mammals
Scientists have examined a 165-million-year-old fossil found in China and identified a mammal predecessor that had a broad tail for swimming and throat bones similar to those found in modern mammals. The discovery suggests that some early relatives of mammals had already developed ways of chewing and swallowing food millions of years before modern mammals evolved.The newly identified species, named Megacauda sungei, lived during the Jurassic period. Researchers from the University of Chicago studied its well-preserved fossil and found that the animal had strong hind limbs, a flattened tail and specialised throat structures. Their findings were published in the journal Natureon on Wednesday.The animal belonged to an extinct group called docodonts, which were close relatives of mammals during the Mesozoic Era. It measured around 40 to 50 centimetres in length and weighed between one and two kilograms. Its physical features suggest that it spent time both on land and in water, much like modern semi-aquatic mammals.What makes the discovery important is the combination of its swimming adaptations and feeding structures. The fossil contains delicate bones that helped support muscles involved in swallowing. According to researchers, these structures provide some of the earliest evidence of a feeding mechanism that later became important in the evolution of mammals with different diets.
Ancient animal swallowed like modern mammals
One of the most important findings concerns the hyoid bones, which help connect the mouth and throat in mammals. In modern mammals, including humans, these bones form part of a structure that supports the muscles used during swallowing.The bones are arranged in a U-shaped structure and connected to the skull through smaller bony segments. This arrangement allows mammals to chew their food and then swallow it in smaller portions, rather than swallowing large pieces or entire prey as animals such as alligators do. These structures also play a role in allowing young mammals to suckle their mother’s milk.Researchers found that Megacaudahad similar structures beneath its skull. The preservation was detailed enough for scientists to examine the arrangement of the bones and understand how they supported the muscles involved in feeding.Zhe-Xi Luo, professor of organismal biology and anatomy at the University of Chicago and senior author of the study, explained why this development mattered.“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,” Luo said.“Once early mammals had this apparatus, each group could develop specialized teeth and diversify into a tremendous range of food resources. They could be carnivores, insectivores, herbivores, omnivores, or anywhere in between.”According to study co-author Peishu Li, the animal’s throat anatomy can be compared with the throat muscles of the modern opossum.
Neander looks at CT images of Megacauda.
How Megacauda moved through water
Apart from its throat structures, the fossil also provided evidence of how the animal moved and hunted. Its hind limbs had long outer digits similar to those of a platypus. The tail contained wide, H-shaped vertebrae and had a broad outline that would have helped propel the animal through water.The skull also contained strong canine teeth and sharp cheek teeth capable of cutting and chewing meat. However, the arrangement of these teeth differed from those of modern carnivorous mammals.Researchers also found tiny pieces of vertebrate bones preserved in the animal’s stomach. This suggests that it fed on small animals, possibly including salamanders, whose fossils have also been found in the same geological deposits.Based on these features, scientists believe Megacaudafollowed a semi-aquatic lifestyle, entering water to hunt while also spending time on land.The discovery adds to earlier evidence that docodonts occupied different habitats. In 2006, Luo’s team identified another swimming docodont called Castorocauda. The newly studied Megacaudafossil is older and preserves more of the skeleton.“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,” said April I. Neander, a scientific artist and CT imaging specialist at the University of Chicago.
How scientists examined fossil
The fossil was discovered in China’s Inner Mongolia region. During the COVID-19 pandemic, study co-author Chang-Fu Zhou from Shandong University of Science and Technology transported the specimen to different research laboratories for examination.Scientists used CT scanning and X-ray fluorescence photography to study the fossil. Because the original slab was large, it had to be divided into smaller pieces before scanning. Neander then used the scans to reconstruct and examine the preserved anatomical structures in detail.The researchers also compared the specimen with Microdocodon gracilis, a much smaller docodont identified by Luo and colleagues in 2019. That animal, which lived around the same geological period, also possessed mammal-like hyoid bones.The new fossil provides further evidence that complex throat structures had already appeared among early mammal relatives during the Jurassic period.Luo said the nearly complete specimen also helps researchers understand how these animals lived in ecosystems dominated by dinosaurs.The findings suggest that important changes in swallowing and feeding anatomy were taking place long before modern mammals separated from their evolutionary predecessors. These developments later helped mammals evolve specialised teeth and adapt to a wide range of food sources.
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