Stunning early mammal fossil sheds light on the evolution of suckling
A predecessor of modern mammals that lived in the Jurassic has been found in China with its throat anatomy preserved, allowing researchers to examine the earliest evidence for the evolution of suckling
A semi-aquatic forebear of living mammals that lived alongside the dinosaurs was also the first known animal capable of suckling.
“The mammalian ways of swallowing and suckling are unique,” says Zhe-Xi Luo at the University of Chicago. “It is one of several biological adaptations of mammals that stand out from all other vertebrates.”
Luo and his colleagues have discovered a new genus of mammaliform, a creature that lived before the last common ancestor of the living mammalian groups – monotremes, marsupials and placentals – that would have been able to chew and suck.
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It is also the earliest known example of a mammal that evolved for a semi-aquatic lifestyle. The animal has been dubbed Megacauda sungei, referencing its broad tail and palaeontologist Sun Ge.
The fossil, which is dated to between 163 million and 168 million years ago, was found by local farmers in the Daohugou area of Inner Mongolia, China.
“It is a chimera,” says Luo. “The back half of Megacauda is more similar to the platypus, of all mammals. But the skull and teeth are analogous to river otters.”
With an estimated weight of 1 kilogram, the creature is the largest known mammaliform from the Middle-Late Jurassic, when the planet was dominated by dinosaurs.
The pelvis is preserved in 3D, as are its hyoids, or throat bones. The bone structures under the skull that form the roof of the mouth and throat passage are also visible.
This meant that, for the first time in any mammaliform, scientists were able to reconstruct the animal’s entire throat anatomy.
Mammals’ throats can form “a suction vacuum for milk and squeeze the chewed food balls and liquid down their throats”, says Luo. All the features that enable this are present in Megacauda.
“We used to know very little about the evolutionary pattern of pharyngeal and hyoid structure. Now we know their evolutionary origins went back to before the rise of the modern mammals,” he says.
The researchers found tiny limb fragments in the fossil’s stomach area that resembled the bones of salamanders, which would have lived in the same environment and may have been the creature’s last meal.
Oliver Griffith at Macquarie University in Sydney says this 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.
“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,” says Griffith.
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