Page 1 :
Affinities of Monotremes with other groups, Prototheria resemble the reptiles and birds with some advanced characters over them,, establishing mammalian ancestry., Reptilian affinity:, 1. Presence of cloaca., 2. Presence of ectopterygoid in skull., 3. Vertebrae without epiphysis and with cervical ribs., 4. Ribs are single headed., 5. Cervical ribs are present., 6. Thoracic ribs are single headed., 7. A median T-shaped interclavicle present., 8. Acetabulum in echidna is perforated., 9. Body temperature is not constant., 10. Cochlea of internal ear with lagina., 11. Ureters lead into a urinogenital sinus., 12. Corpus callosum is absent and anterior commissure is well developed., 13. Testis abdominal., 14. Oviparous and meroblastic segmentation., Presence of strong reptilian features in Monotremata speaks of its primitiveness. These primitive, mammals have failed to cope up with many of the evolutionary transformations which, culminated in the establishment of better characteristics in higher mammals., Avian affinity:, 1., 2., 3., 4., , Beak of the platypus resembles that of birds., Teeth in adults are absent., Presence of webbed feet., Oil gland is present., , The relationship between monotremes and birds does not stand on a solid ground. The, converging characters seen in them are due more to the fact that both possess common reptilian, ancestry., Mammalian affinity:, The prototherians are essentially mammals as they possess the following typical mammalian, characters:, 1. Presence of hair, mammary glands, oil gland and sweat glands.

Page 2 :
2. Double occipital condyles., 3. Presence of palate., 4. A typical mammalian diaphragm is present in the body cavity., 5. Skull is dicondylic., 6. Sternum is segmented., 7. Lobes of liver typically mammalian., 8. Heart 4-chambered., 9. Only left aortic arch present., 10. Circulatory system is typically mammalian., 11. Presence of large ear ossicles., 12. Cochlea is slightly coiled., 13. Fertilization is internal., 14. A slender caecum demarcates two intestines., 15. Lobes of liver typically mammalian., 16. R.B.C. small, circular and non-nucleated., 17. Presence of 4 optic lobes (corpora quadrigemina)., 18. Presence of milk glands secreting milk, Though monotremes show affinity with non-mammalian groups, the above mentioned characters, strongly speak of close and firm affinity with mammals., Evolutionary status of Monotremes, Monotremes are considered primitive mammals, and platypuses are the remaining members of, the family Ornithorhynchidae. Platypuses, along with four species of echidna, are species of, monotremes, defined by their dual nature of having mammary glands and egg-laying capabilities., Monotremes are position at the earliest offshoot of mammalian lineage albeit their phylogenetic, position has been controversial. Some believe they share a common ancestor with marsupials, while other believe that monotremes are more evolutionarily related to the subclass Theria.