When the first platypus furs were brought to Europe, people thought that duck bills had been glued to the skins.
The platypuses and the four current echidna species are the only living "monotremes," mammals with reptilian traits as they lay eggs and have a cloaca and three bones in the shoulder girdle (the other mammals have only two). It has been believed that platypuses and echidnas separated from a common ancestor sometime between 17 to 65 million years ago (when dinosaurs went extinct - the K-T boundary).

But the X-ray analysis of the jawbone of a newfound fossil shows that these primitive mammals are much older than that. It was believed that Teinolophos, discovered in southeast Australia, was a common forebear of both the platypus and the echidna; now, the 112-122 million-year-old fossil is the oldest known platypus. This means that the echidna-platypus separation is much older than 122 million years. The research was made using a specially modified CT scanner delivering high-resolution images of the internal structure of the fossil bones, and it was published in the journal "Proceedings of the National Academy of Sciences".
The team found an unique trait of the platypus in Teinolophos: the electro-sensitive "bill" for detecting aquatic prey.

"This pushes the platypus back across the K-T boundary. Now it looks like [platypuses] crossed the boundary without any problem," said lead researcher Timothy Rowe, of the University of Texas, Austin.

Platypus bills are filled with electrical receptors and, in the murky waters, the platypus detects its prey due to the weak electrical fields generated by living organisms.

"All mammals have some type of (jaw) canal that conducts nerve fibers to the teeth," Rowe noted.

Teinolophos had a broad canal in the lower jaw, while the platypus has a very large canal to accommodate a massive network of nerves collecting sensory information from the bill.

"Nothing but the platypus has this huge canal," Rowe said.

Others say that an early platypus-echidna ancestor could have had wide jaw canals, a trait maintained by platypuses but not by echidnas, this meaning that the two species have split more recently.

"Evidence for a more recent divergence is weak. It makes more sense to assume the wide canals are a unique feature of the platypus lineage. Because platypus and echidna fossils are rare, most previous estimates of the strange animals' antiquity were based on molecular rather than fossil evidence," responded Rowe.

Scientists use the "molecular clock", an estimation of DNA changes in related species, for assessing the moment of the evolutionary split of one species from the other. But DNA mutational speed varies amongst animal groups, and the clock must be fixed combining data offered by other elements, like 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. 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. None [of the molecular studies] predicted we'd find a platypus this old," said Rowe.

"The picture now emerging is that the monotremes are 'slow' in many respects. 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," added Rowe.