Where does milk - an essential part of the viviparity equation - come into it?
Henrik Kaessman from the University of Lausanne says it was originally used for keeping the egg moist. He says mammalian eggs were originally soft, parchment-like, as seen in reptiles today.
This from PLoS Biology [Public Library of Science], reported today in New Scientist.
The argument revolves around a calcium-rich protein, casein, which is in mammalian milk. Monotremes - the only living egg-laying mammals - also produce milk. It was confirmed through genetic analysis that platypus milk had casein-like proteins, which leads Kaessman to suggest milk arose in a common ancestor to mammals, up to 310 million years ago.
Further, he analysed genes for a protein called vitellogenin, which sends nutrients into egg yolk. These are active in chickens, and inactive in mammals, yet one is active in the platypus. Thus, according to Kaessman lactation came long before eggs disappeared in (eutherian) mammals.
This is of interest particularly in describing the transition from eggs to viviparity as a process that went through a series of stages - exactly as one should expect in such an evolutionary process.