Monotremes are a group of mammals that form the order Monotremata. Monotremes are the only mammals that lay eggs, but they also feed their babies with milk. The word 'monotreme' refers to their common rear opening, the cloaca. In amphibia, reptiles, birds and probably all early tetrapods, there is a common opening for urine, reproduction and faeces. Evolution Monotremes are derived from earlier mammals than the marsupials and eutherians, but their fossil record is poor. The time at which the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago. This is in the Upper Triassic. Fossils of a jaw fragment 110 million years old were found at Lightning Ridge, New South Wales. This means they originated up to 100 million years before the Metatheria or Eutheria, so they are definitely 'living fossils'. Mosaic features The egg-laying and the cloaca are primitive ('basal') features inherited from earlier synapsid tetrapods. But the monotremes...