It is considered today that all extant birds are derived from the same line of flight
capable birds - even the flightless ratites 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 with the
inability to articulate the upper beak.
The bird’s beak is derived from reptile gum tissue which has extended 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 are switched off permanently
in modern birds to avoid the development of tooth structures.
Many birds have loose wattles which are the equivalent of lips of skin over protruding
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. Monotremes 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
Bird Like Characteristics
Monotremes are egg laying. They have a single exit for the urinary, reproductive
and excretory systems.
The male has an inverted phallus, held in the preputal sac, 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 most birds where we find that the right ovary regresses.
The platypus has a duck-like bill composed of keratin.
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
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