The duck-billed platypus is a monotreme - a mammal that lays eggs. For many years it has been seen as more primitive than both marsupial (pouched) and placental mammals, and has been placed on an evolutionary lineage which pre-dates the divergence of marsupials and placentals. Analyses of long sequences of mitochondrial DNA, however, show that monotremes lie squarely on the marsupial branch of the evolutionary tree.
- David Penny
- Masami Hasegawa
