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type of bird may have looked like at this stage. Coincidentally it’s appearance is somewhat similar to that of a penguin - which begs the question could all penguins have been derived from these intermediates? The fact that penguins are distributed in the lower Gondwanan fragments only - seems to support this notion. In other words, they are only found in Australia, Antarctica, South America and the Southern tip of Africa - being absent from the rest of the World.

 

One notable point is that some penguins tuck their eggs into a fold of abdominal skin to keep them warm. This skin fold is like the rudiments of a pouch exhibited by the echidna and marsupials. The feathers of penguins are considerably reduced in their form as one would expect in the abandonment of flight capability.

 

Another Change of Habitat

 

 

It is likely that monotremes evolved because some of these penguin/duck like birds eventually changed their habitat and behaviour once more. Overpopulation and competition for food resources could have triggered these changes. Some individuals may have started looking for worms and insects in the earth of river banks and this would have entailed excavation or tunnelling into the soil. Alternatively, this tunnelling could have been provoked by the need to nest safely underground and be concealed from predators like crocodiles and large lizards.

 

Paddle-like wings covered in fine feathers would not be the ideal implements for this purpose and most of the digging would perhaps be achieved initially by using their beaks.

 

But the chance reversion to reptilian arms and five digit hands would have provided a massive advantage for this way of life. This reversion may have taken thousands or even hundreds of thousands of years to appear as it is no doubt the type of macro-reversion which occurs very infrequently.

 

The fragile feathery covering would still present a problem at this stage if these animals were constantly burrowing into earth. Mutation which expressed baldness could have occurred - and would have been an advantage over feathers - but perhaps ambient temperatures were often low and some form of insulation was necessary for these small animals. The porcupine-like quills of the echidna could have been the first alternative form of insulation. These quills are much more robust than feathers and appear to be a simplification of feather structures in that they are composed of the central shaft of the feather only, lacking lateral structures.

 

True hair, which offered better insulation qualities than quills may have occurred some time later. Hair like structures are present on some birds where they form eyelashes or spikes around their beaks and so a mutation where all feather or quills are replaced by these spicules could have happened spontaneously. It would just mean a shift of gene expression in their epidermal cells.

 

Quills have served their purpose in short beaked echidnas all these years, being more terrestrial in their habits, but the platypus has evolved true fur and this may have been more appropriate for its aquatic and earth burrowing way of life.

 

The evolution of milk glands.

 

Many birds (particularly waterbirds like anserids) have a uropygial gland on the dorsal side of their tails. This gland secretes waxes and lipids from a small ‘grease nipple’. The bird uses it to coat its feathers when preening.

 

It is feasible that lactatory glands of mammals and monotremes are derived from these. A repositioning - from the dorsal position to the underside and selection for improved  nutritive value may have come about at the monotreme stage.

 

 

 

 

 

 

 

                                                                   The end................