The platypus, native to Australia, is so odd that when the first specimens were sent to Europe in the 19th century, scientists suspected a hoax. It was classified as a mammal, one of only two monotremes (echidna is the other) living today that are offshoots of the main mammalian lineage. The divergence occurred some 166 million years ago from primitive ancestors combining features of both mammals and reptiles.

Image A platypus baby, or puggle, being held before being transferred back to its burrow at Taronga Zoo in Sydney, Australia. Credit... Greg Wood/Agence France-Presse — Getty Images

“What is unique about the platypus is that it has retained a large overlap between two very different classifications, while later mammals lost the features of reptiles,” Dr. Warren said in an interview.

In their investigation of the platypus genetic blueprint, the scientists found that its genome contains about 18,500 genes, similar to other vertebrates and about two-thirds the size of the human genome. The platypus shares 82 percent of its genes with the human, mouse, dog, opossum and chicken. Some repeated elements in the genome, the scientists noted, hold hints as to the chronology of changes in the platypus.

Of particular interest, the researchers reported, the analysis identified families of genes that link the platypus to reptiles (like those for egg-laying, vision and venom production), as well as to mammals (antibacterial proteins and lactation). The platypus lacks nipples; the young nurse through the abdominal skin.

One surprise was finding genes responsible for sensitive odor receptors. As a primarily aquatic animal, the platypus was already known to rely on electrosensory receptors in its bill to detect faint electric fields emitted by underwater prey. So why the considerable ability to sense odors? The scientists speculate that it may involve sexual communication or the use of water-soluble odorants in navigating and hunting underwater.