A 165-million-year-old swimming mammal relative already had a throat built for suckling

Crocodiles show the alternative. They lack a soft palate and have only a simple hyoid apparatus, so they raise their heads and jerk unchewed prey down their throats ^[17]. "This is achieved with the aid of gravity," said Thomas Martin, a palaeontologist at the University of Bonn ^[18]. Mammals have a more complex hyoid structure in the pharynx and can swallow against gravity ^[19]. The soft palate separates the mouth from the pharynx so an animal can swallow and breathe at once, and the Bonn team calls it an important requirement for suckling ^[2]. Zhe-Xi Luo of the University of Chicago told New Scientist that a mammal's throat can form "a suction vacuum for milk and squeeze the chewed food balls and liquid down their throats" ^[10].

For Megacauda, the case rests on bone. New Scientist reports that the hyoids are preserved in three dimensions, as are the structures beneath the skull that form the roof of the mouth and the throat passage ^[8]. Every feature that enables suction and swallowing is present, according to the same account ^[11]. "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," Luo said ^[12].

Oliver Griffith at Macquarie University in Sydney described the finding in terms of traits. "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 ^[13]. Martin went further. "It was this adaptation that made brain development, eventually leading to humans, possible," he said ^[26]. In my view that second sentence is an argument about the lineage's later history. A single Jurassic skeleton can date the anatomy, but it cannot test that argument.

The thing this doesn't tell you is whether Megacauda sits on the line that led to living mammals. The Bonn release calls it a "furry ancestor of today's mammals" ^[23]. New Scientist places it before the last common ancestor of monotremes, marsupials and placentals ^[6], and Griffith called it a "sister lineage to today's mammals" ^[14]. I think Griffith's wording is the more precise of the two. Megacauda is a docodont, a member of an extinct group of early mammaliforms that lived mostly in the Jurassic ^[5].

The rest of the animal fits a life in water. "It is a chimera," Luo said. "The back half of Megacauda is more similar to the platypus, of all mammals. But the skull and teeth are analogous to river otters." ^[15] Its tooth cusps curve backward, which the Bonn team says helped it catch slippery prey, and its canines have three roots ^[20]. Martin called it "the earliest evidence of an aquatic lifestyle in mammals" ^[4].