The loss of teeth
The Echidna and Platypus are of great antiquity, but we have no hard evidence to indicate which fossil reptiles were their ancestors. Our knowledge of many of the candidates is based to a considerable degree on its teeth, one of the most durable parts of any animal's anatomy. Fossilized teeth provide information about an animal's diet and habits. They are also highly characteristic of a species and similarities between teeth are strong evidence of genealogical relationships. Both Platypus and Echidna became highly specialised for underwater foraging and ant eating respectively and consequently lost their teeth (although young Platypuses still produce three tiny ones soon after birth which are lost in a very short time). We therefore have virtually nothing to help us link these creatures to any group of fossilreptiles. This is further complicated since the features that characterize mammals are hair, warm bloodedness and milk producing glands which cannot be easily deduced from fossils.