A well-preserved fossil of a 165-million-year-old mammal ancestor from China with hind limbs like a platypus and a broad, flattened tail like a river otter appears to be one of the oldest such creatures adapted for swimming and hunting in the water.
Dubbed Megacauda sungei (meaning “big tail”), the Jurassic mammaliaform also shows the earliest evidence yet of bony structures of the throat, including hyoids, that gave this mammal predecessor the ability to feed and swallow like modern mammals, leading to the diversification of mammal species with a wide variety of diets.
“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,” said Zhe-Xi Luo, PhD, Mila Pierce-Rhoads Professor of Organismal Biology and Anatomy and senior author of a new study describing the fossil in Nature. “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.”
A modern way of feeding
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. These structures also give babies the ability to suckle mother’s milk.
Megacauda is from an extinct lineage of docodonts, which are near relatives of mammals from the Mesozoic Era. In 2019, Luo and his colleagues discovered a tiny docodont from around the same geological age called Microdocodon gracilis, which also had mammal-like hyoid bones. Megacauda is a much bigger animal, though—40 to 50 centimeters long and weighing one to two kilograms. It also has better preservation than other fossils of the delicate bony structures underneath the skull for suspending the palatal and pharyngeal muscles. This anatomy is comparable to the throat muscles of the modern opossum, according to co-author Peishu Li, who recently earned his PhD in Integrative Biology at UChicago and is now on the faculty at Ohio University.
The fossil was discovered in the Inner Mongolia region of China. During the difficult years of the COVID-19 pandemic, study co-author Chang-Fu Zhou from Shandong University of Science and Technology took the specimen to several research labs around the country to have it imaged with CT scanning and X-ray fluorescence photography. The large fossil slab had to be separated into smaller pieces for scanning, but the scans allowed April Neander, a scientific artist and expert on CT imaging at UChicago, to visualize its structures in fine detail.
A semi-aquatic kind of life
In addition to the intricate hyoid bones and throat structure, the fossil is also preserved with the pelvis, hind limbs, and tail that distinguished it as a powerful swimmer. The back legs had long outer digits like a platypus with wide, H-shaped tail vertebrae and a broad outline of a tail to propel itself in the water. The skull had robust canines and shearing cheek-teeth to cut and chew meat in a different way from modern carnivorous mammals.
In 2006, Luo’s team also discovered another swimming docodont called Castorocauda. “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,” Neander said. “In addition to the exciting features of the skull and the hyoid bones, we have this fantastic pelvis, which is more complete than all other specimens from this time.”
The researchers believe that Megacauda had a semi-aquatic lifestyle, dipping into the water to hunt long before modern mammals like platypuses, otters, and minks. The fossil even has some tiny bits of vertebrate bones in its stomach, suggesting that it likely preyed on small animals like the abundant salamanders also found in the same fossil beds.
Like modern mammals, docodonts were extraordinarily diverse, living in a variety of habitats, from the tree-dwelling Microdocodon to swimmers like Megacauda. Luo said this nearly complete specimen helps researchers to interpret how such mammals fit in the dinosaur-dominated Mesozoic ecosystems, well before the later diversification of modern mammals.
“Docodonts are the most speciose group of the stem mammals across North America, Europe, and China, and this is probably one of the two most complete specimens for the group,” Luo said. “This fossil shows that the intricate structures of the throat are starting to come together, long before modern mammals split off from their predecessors, and they are very important to the evolutionary innovation that underlies later adaptive radiation.”
The study, “A Jurassic Mammaliaform Swimmer and Transformation of the Mammalian Pharynx,” was supported by the National Science Foundation USA, the Taishan Scholar Program of Shandong, China, Deutsche Forschungsgemeinschaft, and Ohio University. Additional authors include Yikun Li and Honggang Zhang from Shenyang Normal University, China; and Thomas Martin from Universität Bonn, Germany.
