Fossil evidence of monotremes?

Yes, in a later series, Attenborough showed some platypus fossils that had teeth in Australia. Monotremes almost certainly evolved before 45 million years ago.

http://www.ucmp.berkeley.edu/mammal/monotremefr.ht...

Some scientists suggest that the monotremes are close but more primitive relatives of the marsupials and placental mammals. As evidence, they point out that monotremes have the same middle ear bones (the incus and malleus) as placental and marsupial mammals, and the same mammalian jaw joint. the incus and malleus were originally small bones (quadrate and articular) found in the jaw joints of reptiles, but they have migrated to become middle ear bones in mammals, because mammals have a different jaw joint than reptiles that did not include these two small bones. Mammals have a dentary-squamosal jaw joint instead of the quadroarticular jaw joint found in reptiles.

http://www.mun.ca/biology/scarr/QA_vs_DS_jaw.html

At our present state of knowledge we can distinguish a marsupial skeleton from a placental skeleton, but can't distinguish it from a monotreme. We presume that the very earliest mammals were egg layers, but have no evidence. However, as they descended from the mammal-like reptiles it is a safe assumption.

That is an excellent question, the problem is we can never tell. Just looking at the skeletal structure that a fossil provides, we have no way of telling if they were placental, marsupial or montreme. You would have to see the organs to know for sure.