Jurassic Fossil Reveals 165-Million-Year-Old Roots of Mammalian Suckling

One of the most intimate and defining behaviors in the animal kingdom — a mother feeding her young with milk — may have far deeper evolutionary roots than scientists once believed. An international team of researchers has announced the discovery of a remarkable new fossil species from the Middle Jurassic of China that preserves some of the clearest anatomical evidence yet for when the machinery of mammalian suckling and swallowing first began to take shape. The species, named Megacauda sungei, lived roughly 165 million years ago in what is now Inner Mongolia, and its exquisitely preserved skull and skeleton are forcing paleontologists to rethink how one of mammals’ signature traits evolved.

The study, published in the journal Nature, 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. The team’s findings center not on milk itself, which almost never fossilizes, but on the bony architecture that makes nursing possible. Because the soft tissues involved in suckling — the soft palate, the constrictor muscles of the throat and the coordinated reflexes that move liquid from mouth to esophagus — are rarely preserved in the fossil record, researchers have long lacked direct evidence for when this feeding apparatus appeared. Megacauda offers a rare window into that hidden history through the skeletal structures that once anchored these soft tissues.

Using high-resolution microCT imaging, the researchers digitally reconstructed the skull and skeletal anatomy of the fossil in extraordinary detail. The scans revealed an unexpected and almost whimsical mosaic of traits. Megacauda possessed specialized blade-like cheek teeth resembling those of modern seals, a pelvis reminiscent of a platypus, and a tail like that of an otter. Taken together, these features indicate that the animal was likely a semiaquatic predator, hunting small vertebrates and invertebrates in freshwater environments during a time when dinosaurs dominated the land. At approximately the size of a modern otter, Megacauda ranks among the largest known Jurassic mammaliaforms and stands as one of the earliest documented examples of adaptation to a semiaquatic lifestyle in this lineage.

But the most consequential discovery was hiding in the roof of the animal’s mouth. The team identified a pair of small bony projections that closely resemble structures found in living therian mammals — the group that includes both 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 the muscles involved in suckling, swallowing and protecting the airway. Their presence in a Middle Jurassic mammaliaform suggests that the anatomical foundations of nursing were already emerging tens of millions of years before the rise of modern mammals, pushing back the inferred origins of this sophisticated feeding system dramatically.

The throat region yielded further surprises. Megacauda also possessed highly specialized hyoid bones, the slender elements in the neck that, in living mammals, anchor the muscles responsible for swallowing. In combination with the palatal projections, this evidence suggests that Megacauda likely possessed an early version of the coordinated muscular system that modern mammals use to move milk and food safely from the mouth to the esophagus — a system that must perform a delicate balancing act, directing liquids and solids into the digestive tract while keeping the airway clear. The anatomy of the fossil, the researchers conclude, indicates that structures involved in coordinating swallowing and sucking had already begun to evolve by the Middle Jurassic.

Perhaps the most provocative implication of the discovery is that it overturns a longstanding assumption about how these structures came to be. In the same animal, the newly identified therian-like bony hooks coexist with more primitive bony ridges known from earlier mammaliaforms and mammalian ancestors. If the modern structures had evolved directly from those primitive ridges, as previously believed, one would expect the ridges to be replaced rather than retained alongside their supposed descendants. Their coexistence in a single skull suggests instead that the evolutionary pathway was more complex — that the modern suckling apparatus did not arise by simple transformation of the older anatomy, but through a more intricate sequence of innovations layered onto an existing framework.

Zhe-Xi Luo, professor of organismal biology and anatomy at the University of Chicago and senior author of the study, emphasized the significance of the find. The anatomy of Megacauda, he noted, suggests that structures involved in coordinating swallowing and sucking had already begun to evolve by the Middle Jurassic, and the discovery helps scientists better understand the biological foundations of one of the defining characteristics of mammals: feeding young with milk. For decades, he added, researchers have wanted to know when the soft-tissue structures necessary for suckling and complex swallowing first appeared, and Megacauda provides some of the clearest evidence yet that many of these innovations emerged well before the rise of modern mammals.

The findings also carry important implications for understanding the diversity of living mammals. Today’s therian mammals nurse their young using a sophisticated suckling mechanism supported by the soft palate and constrictor muscles, while monotremes — the egg-laying mammals such as platypuses and echidnas — use a distinctly different method for obtaining milk and lack some of the anatomical features that therians rely on. The new evidence suggests a surprising twist: rather than monotremes retaining a primitive condition, they likely lost elements of the ancestral suckling apparatus during their evolutionary history and independently developed an alternative means of milk intake. In other words, the common ancestor of all living mammals may have been equipped with a more complete version of the nursing toolkit than any of its modern descendants now possess in full.

Beyond the story of feeding, Megacauda adds to a growing appreciation of just how ecologically inventive early mammaliaforms were at a time when they are often imagined as small, shrew-like creatures scurrying beneath the feet of dinosaurs. A semiaquatic predator the size of an otter, armed with seal-like teeth and propelled by a platypus-like pelvis and otter-like tail, paints a very different picture of the Jurassic world. Peishu 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, observed that these discoveries continue to reveal that the Age of Dinosaurs was also a time of extraordinary evolutionary innovation among early mammals.

The research team included April I. Neander and Zhe-Xi Luo of the University of Chicago; Yikun Li and Honggang Zhang of Shenyang Normal University in China; Chang-Fu Zhou of Shandong University of Science and Technology in Qingdao, China; Thomas Martin of the University of Bonn in Germany; and Peishu Li of the Ohio University Heritage College of Osteopathic Medicine. Together, their work transforms a single fossil from Inner Mongolia into a milestone in the study of mammalian origins — evidence that the gentle, life-sustaining act of a mother nursing her infant was being assembled, piece by piece, in the freshwater waterways of the Jurassic, millions of years before the first modern mammals ever drew breath.

Subject of Research: Evolutionary origins of mammalian suckling and swallowing anatomy revealed by a Jurassic mammaliaform fossil

Article Title: Ancient origins of mammalian suckling and swallowing uncovered

Article References: Ancient origins of mammalian suckling and swallowing uncovered. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: mammalian evolution, suckling, swallowing, Jurassic, fossil, Megacauda sungei, mammaliaforms, hyoid bones, therian mammals, monotremes, microCT imaging, Inner Mongolia