Newer studies seem to suggest that Monotremes, including the Platypus and 4 species of the Spiny Anteater, survived competition from newer arrivals to Australia such as the Marsupials, by evolving to live in water, and giving birth by laying eggs. Over time the Anteater returned to living on land but retained this egg laying characteristic common to Monotremes. More on the Platypus:http://www.livescience.com/27572-platypus.html. Pretty cool animal!:-).
The platypus is a monotreme, which is a mammal that lays eggs. There are a total of 5 monotreme species, all indigenous to Australia and New Zealand. So no, there are 4 other species that have this unique characteristic.