. . . e v o l u t i o n e m . . .

 

         taking a fresh look at evolution

From a Bird to a Monotreme

This article is about the evolution of birds from a possible candidate reptile. The requirement for sustained flight has meant that birds have evolved many derived characteristics essential for mastering this process. The most important legacy of this adaptation was the emergence of endothermy (bodily heat generation from within). This feature alone allowed survivability in changing climates and also a wider distribution of species across the planet. Perhaps without this spin-off benefit for the requirement for flight, land animals would still be at the reptile stage - and no mammals (including) ourselves would have appeared on this Earth. This was, perhaps, another one of those tenuous, fluke happenings which went along to determine our very existence here today.

 

Fossils of the proto-bird, Archaeopteryx, discovered in Germany, have been dated to around 160 million years old. The fossils clearly show an array of feathers, many of which resemble those of modern birds. Archaeopteryx, however, does  not appear to have a developed keel bone, which in modern birds, is used to attach the powerful muscles needed for sustained flight. So we assume from this that this bird could only glide or soar. The fossil remains also show a significant bony tail and what appears to be a heavy skull and mandible with teeth - not the lightweight features associated with true birds.

 

In evolutionary form, Archaeopteryx appears to have been just one stage on from another proto-bird known as Microraptor, fossils of which have been discovered only recently in China. Their fossils show flight like feathers as in Archaeopteryx but in addition they also have long feathers on their legs, giving a kind of double wing configuration. Microraptor had an even longer bony tail and it seems apparent these animals only used their feathers to enhance their gliding capabilities.

 

If we extrapolate further, we may want to consider what type of reptile could have been the forerunner of Microraptor? Heterodontosaurus is a possible example of an ancestral type of reptile. It was a small bipedal ‘dinosaur’ and fossils discovered in South Africa show it had powerful legs. These could have enabled it to jump up into trees in search of food resources or escape predators.

 

South Africa, in Jurassic times, was part of the Super-continent of Gondwanaland and climatic conditions could have varied  considerably. Climate change may have brought about cooler temperatures which may have resulted in selection for greater size (in the case of the huge dinosaurs) for the benefits of reduced heat loss. Alternatively, for reptiles needing to remain small in relation to their habitat, downy scales which provide insulating value could have been selected for. For a small creature like Heterodontosaurus there would be advantages in being able to hop up into a tree and from there move up onto successively higher branches. Being able to leap considerable distances back down to ground without injury would be also have been a big advantage and these downy feathers may have provided incremental improvements in these actions.

 

We have to bear in mind the type of trees which were common in Jurassic times. These were tree ferns, conifers, gingkos and palms. The branching systems were different to modern species but these trees will have been accessible for Heterodontosaurus.

 

If longer downy scales which enhanced gliding and reduced fall rates were selected for over many thousands of generations we can also imagine how flight assisting feathers could have come about. The feathers on the bony tails of these early creatures would initially serve as a sort of rudder, so that dangerous obstacles such as branches or rocks outcrops could be avoided during descent. The glide path to another tree could be also be navigated.

 

There would naturally come a stage where gliding or soaring on thermals could be assisted by the occasional arm movements. This in turn would lead, by natural selection, to longer and longer flight time. This would be a reflection of stronger arm muscles and an increased metabolic rate to fuel them.

 

So, perhaps by about 120 million years ago, true birds existed which could achieve an almost effortless sustained flight.  

 

Some modern looking birds, therefore will have co-existed with dinosaurs in warmer climes but their warm-bloodedness, an adaptation for the power of flight, held them in good stead for the colder regions too. Flight has the enormous benefit of allowing the bird to cover considerable distances, often over great expanses of water or difficult terrain, during migration. Migration allows access to seasonal food resources in remote areas and the escape from changing weather patterns.

 

It is considered today that all extant birds  are derived from the same line of flight capable birds - even the flightless rattites like the ostrich. This is believed to be the case because they all have keel bones which are used to attach the large flight muscles. The flightless birds have greatly diminished keels. It is likely, however that these large flightless species like the African Ostrich, The Australian Emu and Cassowary, and the extinct Giant Moa of New Zealand abandoned this flight capability something like 100 million years ago. This was at a time when the Gondwanan segments had not fully separated from each other. I believe this accounts for the similarity between these birds. All these birds have the archaic cranial architecture wherein they are unable to articulate the upper beak.

 

Bird Beaks

 

The bird’s beak is derived from reptile gums which have developed outside the original mouth area. Beaks are composed of a less dense collagen fibre material (keratin) than gum tissue which in turn is less calcified than the bone tissue of the skull. The gene clusters which determine teeth development could be ‘clamped’ permanently to avoid the progression of the developmental  sequence which determines tooth structures.

 

Many birds have loose wattles which are the equivalent of lips of skin over protruding gums.

Beaks have been fashioned by natural selection for a wide variety of purposes such as flesh cutting, nut cracking, for poking out insects in crevices, for eating juicy fruits or for sifting through mud in river beds.

 

 

Continuing the Evolutionary Pathway to the Monotreme

 

Now that we have established a scenario for the evolutionary pathway of true birds from a reptile, we can continue to speculate on how monotremes, like the platypus, could have evolved from birds.

 

Monotremes are curious intermediates between birds and mammals, sharing some features from both types of animals. Montremes are represented by only three species living today, the Duck-billed Platypus (Ornithorynchus anatinus), and two species of Echidna.

All are found in Australia and New Guinea.

 

Monotreme Biology

 

Bird Like Characteristics

 

Monotremes are all egg laying. They have a single exit for the urinary, reproductive and excretory systems.

The male has an inverted phallus, held in the preputal sack, which can be erected for mating. This phallus is very similar to those found on some birds of the Anseridae (Ducks, Geese and Swans).

 

In their reproductive systems - only the left ovary of the female platypus is functional and this is similar to birds where we find that the right ovary regresses.

 

The platypus has a duck-like bill of a keratin composition.

 

It has webbed rear feet.

 

The opening for the ears lies at the base of the jaw.

 

The echidna has quills which resemble feather shafts.

 

The eyes of the platypus have retinal structures with both single and double cones, oil droplet, and slender rods in a similar arrangement to birds.

 

Mammal Like Characteristics

 

Monotremes like the platypus have true mammalian type hair. The long beaked echidna has a mixture of hair and quills while the short beaked echidna has a predominance of quills.

 

All monotremes have milk producing glands. There are no teats but milk exudes from pores and fills little grooves in the skin.

 

The male reproductive tract is more mammal-like than bird-like.

 

The bones of the ear are more mammal-like in that they are incorporated into the skull.

 

Monotreme males have a bifid phallus similar to marsupials.

 

Intermediate Characteristics

 

The platypus has an interclavicle bone in its pectoral girdle.

An interclavicle does not feature in true mammals but in birds it forms part of the ‘wishbone’ - the fused clavicular structure.

 

Quills found in both echidna species appear to be intermediate between feathers and true hair.

 

The chromosomes of monotremes are considered to be intermediate between birds and mammals. Gene sequences of the Protamine P1 genes of the platypus and echidna were analysed and the gene sequences appear to indicate that monotremes occupy a position halfway between eutherian animals and birds (Retief, Winkstein and Dixon 1993).

 

The Envisaged Scenario for Bird to Monotreme Evolution

 

As mentioned elsewhere, significant evolutionary change comes about through adaptation to a new habitat and new ‘way of life’. The greatest degree of change to the way of life brings about  the greatest change in form of the animal.

An ‘uncomfortable period’ has to be passed through where the pioneer species and their many descendents are not really adequately equipped for the new habitat or way of life and it may take tens of millions of years of natural selection before their biology is best suited to that new environment.

 

The change from bird to monotreme is a good example of an extreme change. Much of the change was, however, likely brought about by the mechanism of reversion, described elsewhere, which involved the abandonment of many derived characteristics and a return to more archaic forms.

 

Location

 

To set the scene, let us imagine the continent of Australia about a 100 million years ago. By this time the Super-continent of Gondwanaland had become fragmented. The Australian fragment was still attached to the tip of South America in its eastern side which in turn was attached to Antarctica. The Indian triangular fragment had departed from the West side of Australia.

 

Player

 

At around this time existed the Presbyornithids, the ancestors of the Anseriforms - geese, ducks and swans. (see Professor Paul’s Guide to Birds

www.ppne.co.uk - for illustration of Presbyornis)

Presbyornis was a bird with many of the characteristic features of modern geese and was believed to have lived in colonies around lakes where its duck-like bill was perhaps used to filter food from water. Like many modern anseriforms it may have migrated to other lakes at different times of the year. An important point to mention here is that Anseriforms, possibly like their ancestors, the presbyornithids are among the most intelligent of birds and this is perhaps due to the demands on navigational abilities needed in successful migration.

 

Motive

 

During migration from one area to another, some Presbyornids may have been blown eastwards off course and reached the Continent of Australia.

Finding abundant food resources and no competition from other birds could have induced them to stay there.

 

As their food resources were in rivers and lakes, they may have decided to spend all their time on the water or diving down into it, and the ability to fly would no longer be necessary.

 

Dedication to Life in Water.

 

If these birds and their successive generations became dedicated to life in water, wings and flight feathers would be trimmed down by natural selection and they would eventually resemble the paddle like wings of penguins. On the right is my impression of what this new type of bird may have looked like at this stage. Coincidentally its 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 even happen 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 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 protruding ‘gums’  of this vulture have been  adapted into sharp cutting instruments. Note also the ‘lips’ at the base of  the beak.

Heterodontosaurus

Microraptor

Archaeopteryx

Click on for larger image

Duck-billed platypus