An X-ray photograph of the Megacauda fossil 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 form a U-shaped structure that is attached to the skull by smaller bony segments. This structure helps mammals chew food before swallowing it in smaller pieces, unlike animals such as alligators, which can swallow large chunks of food or even whole prey. It also helps young mammals suckle milk from their mothers.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 remarkable fossil was unearthed in the Inner Mongolia region of China. During the challenging times of the COVID-19 pandemic, one of the study co-authors, Chang-Fu Zhou, who is affiliated with Shandong University of Science and Technology, undertook the complex task of transporting the specimen to a variety of research laboratories for thorough examination and analysis.To delve deeper into the fossil’s characteristics, the scientists employed advanced techniques such as CT scanning and X-ray fluorescence photography. Given the sizable nature of the original slab, it became necessary to break it down into smaller sections to facilitate the scanning process. Subsequently, Neander utilized these scans to meticulously reconstruct and systematically examine the preserved anatomical structures in great detail.Moreover, the researchers conducted a comparative analysis between the new specimen and Microdocodon gracilis, a significantly smaller docodont that had previously been identified by Luo and colleagues back in 2019. This smaller creature, known to have existed during roughly the same geological timeframe, also displayed mammal-like hyoid bones, adding to the intrigue.The newly discovered fossil adds substantial evidence to the growing body of research indicating that complex throat structures were already emerging among the early relatives of mammals during the Jurassic period.Luo highlighted that this nearly complete specimen notably aids researchers in gaining insights into the lifestyles of these ancient animals within ecosystems that were primarily dominated by dinosaurs.The outcomes of these findings indicate that significant evolutions in swallowing and feeding anatomy were already occurring well before the lineage of modern mammals branched off from their evolutionary predecessors. These evolutionary changes later played a crucial role in enabling mammals to develop specialized teeth and adapt to a diverse array of food sources.