Teinolophos had an electro-sensitive bill, the scientists concluded after imaging revealed a broad canal running through the bone of the lower jaw.
All mammals have some type of canal that conducts nerve fibers to the teeth, Rowe noted.
But in the platypus, this canal is greatly enlarged to accommodate a massive network of fibers that carry sensory information from the bill. The claim that Teinolophos is a very ancient platypus rests largely on this feature.
"Nothing but the platypus has this huge canal," Rowe said.
But Matt Phillips, of the Australian National University in Canberra, said more evidence may be needed.
The research "does not confirm that the platypuses and echidnas diverged more than 112 million years ago," Phillips said.
Phillips offered an alternative explanation for the new findings—that an early platypus-echidna ancestor had wide jaw canals, and this feature was retained by platypuses but reduced during subsequent echidna evolution.
In such a scenario, the split of the two species could still have been relatively recent, Phillips said.
Lead author Rowe counters that evidence for a more recent divergence is weak. He says it makes more sense to assume the wide canals are a unique feature of the platypus lineage.
Resetting the Molecular Clock
Because platypus and echidna fossils are rare, Rowe noted, most previous estimates of the strange animals' antiquity were based on molecular rather than fossil evidence.
The gradual accumulation of changes in the DNA of closely related species provides a kind of "molecular clock" that biologists can use to estimate when the species branched apart from one another.
DNA changes, however, don't occur at the same rate in different kinds of animals. The clock must be calibrated using other evidence, such as fossils.
Studies suggesting a more recent platypus origin have used a molecular clock calibrated with fossil information from marsupials and other mammals, not platypuses and echidnas, Rowe said.
The newfound early days of the platypus suggest that molecular evolution in platypuses and echidnas has proceeded at a far slower pace than in other mammal groups, the researchers say.
"None [of the molecular studies] predicted we'd find a platypus this old," Rowe said.
"The picture now emerging is that the monotremes are 'slow' in many respects," he continued.
Platypuses and echidnas are the only extant "monotremes," or mammals that lay eggs.
"Their metabolic and respiration rates are slower, their body temperature is lower, and it's possible that the monotreme lineage evolved at really slow rates," he said.
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