Professor Peishu Li coauthors Nature study uncovering ancient origins of mammalian suckling and swallowing

An international team of researchers, including Ohio University’s Peishu Li, has uncovered new evidence illuminating the evolutionary origins of one of the defining features of mammals: the ability to suckle and consume milk during infancy. Their findings, published in Nature, suggest that key anatomical structures required for mammalian suckling, swallowing and breastfeeding behaviors evolved nearly 165 million years ago, long before the appearance of modern mammals.

The findings describe a newly discovered species called Megacauda sungei, a large mammaliaform that lived during the Middle Jurassic Period in what is now Inner Mongolia, China. The research was led by scientists from the University of Chicago, Shenyang Normal University, Shandong University of Science and Technology, the University of Bonn and Ohio University.

Using high-resolution microCT imaging, the team reconstructed the skull and skeletal anatomy of the fossil. They discovered an unexpected combination of traits. Megacauda had specialized blade-like cheek teeth resembling those of modern seals, a platypus-like pelvis, and an otter-like tail, indicating it was likely a semiaquatic predator that hunted small vertebrates and invertebrates in freshwater environments.

More significantly, the researchers identified a pair of small bony projections on the roof of the mouth that closely resemble features found in living therian mammals, a group that includes marsupials (such as kangaroos and opossums) and placental mammals, (such as elephants and humans). In modern mammals, these structures help support the soft palate and muscles involved in suckling, swallowing and protecting the airway. 

Because soft tissues are rarely preserved in fossils, scientists have long struggled to determine when these important feeding structures first evolved.

“The anatomy of Megacauda suggests that structures involved in coordinating swallowing and sucking had already begun to evolve by the Middle Jurassic. This discovery helps us better understand the biological foundations of one of the defining characteristics of mammals: feeding young with milk,” said Zhe-Xi Luo, Ph.D., professor of organismal biology and anatomy at the University of Chicago and senior author of the study.

The researchers also found highly specialized hyoid bones in the throat region. In living mammals, these bones help anchor muscles for swallowing. Together, the evidence suggests that Megacauda likely possessed an early version of the coordinated muscular system used by modern mammals to move milk and food safely from the mouth to the esophagus.

The discovery challenges a longstanding assumption about how these structures evolved. Researchers found that the newly identified therian-like bony hooks existed alongside more primitive bony ridges known from earlier mammaliaforms and mammalian ancestors. Their coexistence in the same animal suggests the modern structures did not evolve directly from the primitive ridges as previously believed.

“This fossil captures an important stage in mammalian evolution,” said Luo. “For decades, scientists have wanted to know when the soft-tissue structures necessary for suckling and complex swallowing first appeared. Megacauda provides some of the clearest evidence yet that many of these innovations emerged well before the rise of modern mammals.”

The research also has important implications for understanding living mammals. While therian mammals use a sophisticated suckling mechanism supported by soft palate and constrictor muscles, monotremes such as platypuses and echidnas use a different method for obtaining milk and lack some of these anatomical features. The new evidence suggests that monotremes likely lost elements of the ancestral suckling apparatus during their evolutionary history and developed an alternative method of milk intake.

Beyond feeding biology, Megacauda highlights the remarkable ecological diversity of mammaliaforms living alongside dinosaurs. At approximately the size of a modern otter, it is among the largest known Jurassic mammaliaforms and represents one of the earliest examples of adaptation to a semiaquatic lifestyle.

“These discoveries continue to reveal that the Age of Dinosaurs was also a time of extraordinary evolutionary innovation among early mammals,” said Li, assistant professor in the Department of Biomedical Sciences at the Ohio University Heritage College of Osteopathic Medicine and a co-author of the study.

The study’s authors include April I. Neander and Zhe-Xi Luo, Ph.D., (University of Chicago); Yikun Li and Honggang Zhang, Ph.D., (Shenyang Normal University, China); Chang-Fu Zhou, Ph.D., (Shandong University of Science and Technology, Qingdao, China); Thomas Martin, Ph.D., (University of Bonn, Germany); and Peishu Li, Ph.D., (Ohio University Heritage College of Osteopathic Medicine).