tremes – includes two species of
             echidna, or 'spiny anteater'. All three species are confined to Australasia.
             Monotremes lay eggs, like birds and reptiles, but unlike all other mammals.
             They also have a range of other reptile-like anatomical features, features that
             have been lost in more 'advanced' mammals. Researchers think that
             monotremes have been distinct as a group for at least 80 million years, long
             before the dinosaurs became extinct.
             Monotremes have taken a cameo role in studies on the evolution of
             mammalian brain function. A study in 1972 suggested that the echidna
             Tachyglossus had no REM sleep. This was important, because it implied that
             REM sleep must have evolved in higher mammals. Subsequent research made
             this result look anomalous, as REM-like sleep phenomena have since been
             observed in birds and some reptiles: in which case, the echidna may have lost
             the capacity somewhere in its evolution.
             This is the conundrum that Siegel and colleagues have been investigating. First,
             it turns out that the term 'REM' is a misnomer: animals
             ...