When do mammals emerge in the fossil record?

What parts of skull structure differentiate mammals?

The early mammal skull was very heavy. How was this solved?

Different skull hole patterns?

What are monotremes?

What are marsupials?

What are Afrotheria?

What are proboscidea?

What are Sirenia?

What are tubuildents?

What are hyracoidea?

What are Xenarthra?

What are the rodents/lagomorphs?

What are carnivores?

What are Artiodactyls?

What are perrisodactyls?

What are cetaceans?

What are chiroptera?

What are eulipotyphla?

What does make a mammal?