Biology of Australia
It has been suggested that monotremes may have evolved from primitive birds (3). According to this suggestion the monotremes evolved from birds by losing the derived features that in birds were beneficial for flight. They are usually believed to have descended from the first radiation of mammals, which could explain their similarity to reptiles and birds.
A neurotrophin study by Kullander, Carlson and Hallbook published in the Journal of Molecular Evolution, found that monotremes are not more closely related to therians than they are to birds (1). Another study, based on the protamine P1 genes, by Retief, Winkfein and Dixon, concluded that the monotremes were phylogenetically half way between the eutherians and the birds (2).
Bird-like characters include
Link 3 lists a number of characters monotremes share with birds and mammals. Features they share with birds - oviporous,(egg laying), the specialised eggs are somewhat similar to those of reptiles. A cloaca, an inverted penis normally stored in a preputal sack (link 3), similar to that in some Anseridae, that in monotremes is attached to the ventral wall of the cloaca, the tip of which is divided into 2 pairs of penis heads, the pairs on either side functioning alternately at each mating, only one pair at a time functioning to carry semen (link 5) during a particular mating.
In the female platypus the left ovary is the only functional one, as in birds, where the right ovary regresses (link 3). Platypus eyes are similar to those of birds in having single and double cones, oil droplet and slender rods. Their quills are similar to the shaft of a bird feather.
Mammal-like characters include
Mammal characteristics include mammal hair. The long-beaked echidna, now found only in New Guinea, having gone extinct on mainland Australia, has mostly quills but the short-beaked echidna, found in Australia, has a mixture of hair and quills. (link 3) Monotremes produce milk, but not through nipples as in other terrestrial mammals, being excreted directly from the mammary glands onto the skin where it is licked by the young.The male reproductive tract is simple, being closer to that found in mammals. The ear bones are incorporated into the skull as in mammals. The males have a bifid penis as in marsupials.
Intermediate characters according to link 3 include an interclavicle bone, part of the wishbone, the fused clavicular structure, of birds, in the pectoral girdle of the platypus, that is not found in therian mammals. The quills of both species of echidna appear intermediate between true hair and feathers. The monotreme chromosomes are intermediate between those of birds and mammals.
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