A venomous mammal has been found in fossil beds from 60 million years ago in Alberta. The interesting part is that it had fangs like a snake, or rather a Gila monster: “The fossilized remains of two curved canines, found in the Canadian province of Alberta, shows a groove from which the creature, Bisonalveus browni, probably shot venom into its prey”.
Now I don’t know about “shot”– Gila monsters drip and chew. But specialized venom teeth in a mammal are still news.
It may be that venom was much more common then, though. In addition to the solenodon mentioned in the article, and the platypus’s heel spurs, many shrews have enough venom to give a nasty toxic bite (experience speaking: Blarina brevicauda in Massachusetts, if I remember right). All these are very old families.
Thanks to David at Cronaca, which everyone should read.