Paleontologists have identified the earliest known animal capable of suckling, a semi-aquatic mammaliform that lived alongside the dinosaurs during the Jurassic period. The creature, named Megacauda sungei, pushes the evolutionary origins of the mammalian throat and its unique feeding abilities back to before the last common ancestor of today's mammals.
The fossil, dated to between 163 million and 168 million years ago, was discovered by local farmers in the Daohugou area of Inner Mongolia, China. Weighing an estimated 1 kilogram, it is the largest mammaliform known from the Middle to Late Jurassic, a time when dinosaurs dominated the planet. The find is described in the journal Nature.
«The mammalian ways of swallowing and suckling are unique,» said Zhe-Xi Luo of the University of Chicago, who led the research. «It is one of several biological adaptations of mammals that stand out from all other vertebrates.»
Because the pelvis and delicate throat bones, called hyoids, were preserved in three dimensions, along with the bone structures forming the roof of the mouth and throat passage, the team was able to reconstruct the animal's entire throat anatomy — a first for any mammaliform. Those structures allow mammals to create a suction vacuum for milk and to squeeze chewed food and liquid down the throat. All the features required for this dual function are present in Megacauda.
«We used to know very little about the evolutionary pattern of pharyngeal and hyoid structure,» Luo said. «Now we know their evolutionary origins went back to before the rise of the modern mammals.»
The fossil presents a striking mix of traits. Luo described the animal as «a chimera»: its back half resembles that of a platypus, while its skull and teeth are analogous to those of a river otter. This combination marks it as the earliest known mammal to evolve a semi-aquatic lifestyle, a niche that has since appeared repeatedly across the mammalian family tree.
Researchers also found tiny limb fragments in the stomach area that resemble salamander bones. Salamanders lived in the same environment, and the remains may represent the creature's last meal.
Oliver Griffith of Macquarie University in Sydney, who was not involved in the study, said the fossil sits at a key point in mammalian evolution. «It tells us that some key mammalian traits like sucking, which is required for behaviours like drinking milk from mothers, predate the origin of mammals today,» he said. «It also is cool to see that this sister lineage to today's mammals had evolved an aquatic lifestyle, which is something we have seen multiple times across the mammalian family tree, including in the platypus.»
The discovery fills a gap in knowledge about how throat features in early mammaliforms gave rise to the abilities that modern mammals rely on: swallowing as adults and suckling milk as infants. It also shows that the evolutionary roots of those traits extend back well before monotremes, marsupials and placentals diverged.