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The evolutionary history of monotremes is ancient, with recent molecular phylogeny suggesting that monotremes and marsupials diverged from the line leading to placentals (eutherian mammals) more than 100 million years ago. Recognisable platypus fossils have been found in Cretaceous deposits (>65 Mya) in South America and Australia, while echidnas seem to be a more recent branch from the platypus-like line that have become adapted to drier terrestrial conditions.
The "X-ray style" of painting is commonly used in Australian Aboriginal art and reflects the intimate knowledge that the indigenous peoples have developed for Australian wildlife. Echidnas are also a prized food source to many indigenous tribes.
A much-quoted study (Allison 197?) failed to find any evidence of REM sleep in echidnas. This finding fueled a new set of speculations about the mysterious function of REM sleep, including Crick and Mitchison's (198?) theory that REM sleep allows the elimination of "parasitic" synaptic modifications that were not set up directly by experience.
Recent work shows that it may have been premature to conclude that echidnas lack REM sleep. If one uses single-unit recording criteria, the brain stem changes in sleeping echidnas look very similar to those that occur during REM sleep in humans and other placental mammals. The major difference lies in the absence of a cortical desynchronisation (the EEG state that also occurs in wakefulness) and in a reduced amount of synchrony between adjacent bursting brain stem neurons in echidna when compared with a placental.
Since prenatal and neonatal placentals (including humans) also fail to show evidence of cortical desynchronisation during REM sleep, it could be argued that echidna REM sleep is present (as shown by the brain stem neuron pattern of bursty firing) but represents a more undifferentiated sleep pattern from which both Slow wave sleep and REM sleep later emerge, either in phylogeny (after the placentals split from the marsupial-monotreme line) or during ontogeny (since their is only a single prenatal sleep state, like REM sleep without desynchronisation, from which REM and SWS later emerge).
Platypus REM:
The above interpretation, that monotremes have REM after all, was supported recently by new work on sleep in another monotreme, the platypus. The technical difficulties of studying sleep in platypus were overcome in an elaborate "platypussary" with an articificial tunnel system connected to a pond, all electrically-shielded to protect the 40,000 sensitive electroreceptors in the platypus' bill from stray electrical fields...and by using a radiotelemetry system to record EEG, ECG, EMG while the platypus dived and foraged in the pond and slept in its nest tunnel.
Video in the tunnel showed unequivocal evidence that pltypus sleep a lot (around nine hours/day) and that most of this sleep is accompanied by twitching movements that are very suggestive that the platypus is dreaming about foraging, attacking and eating its prey with vigorous side-to-side oscillatory movements of its head, ......and about swimming, with fictive forelimb movements. This modern work is in line with very old observations, largely ignored in the hoopla surrounding the early electrophysiological work that was thought to indicate that monotremes had no REM sleep. These old observations on very young, orphan platypus in captivity, clearly indicated sleep was accompanied by the fictive twitching that we now associate with REM sleep.
EEG recording shows a synchronised EEG throughout sleep, as in the echidna.
Platypus and Echidna Sleep Research Summary:
Taken all together, the recent monotreme sleep research shows that REM sleep may be a primitive condition in amniotes. It may not been necessary to postulate that birds have independently evolved this kind of sleep, a common conclusion that has been drawn from the apparent absence of REM sleep in monotremes.
Why Don't Echidnas Twitch in REM Sleep?
Unlike playpus, safe in its tunnel, an echidna is relatively vulnerable to predation. Aborigines and dingos eat them regulalry, as may the now-extinct marsupial predators like the thylacine and the sabre-toothed marsupials.
Since the twitching of the large spines would be highly visible during REM sleep, to both the visual and auditory system of predators, I propose that the relative lack of synchrony in the bursting of neighbouring brainstem neurons during REM is a derived character that has been selected since the echidna anestor left the water to deal with the new problems posed by a terrestrial existence.
This speculation is not completely idle, since the long-billed echidna of New Guinea lives in wetter conditions in rainforest and has longer fur and shorter spines than its Australian cousin. Perhaps long-billed echidnas, living in less danger on account of large conspicuous spines, will have a more platypus-like REM sleep, with more prominent twitches and more synchrony between adjacent brain stem neurons during REM.