What might have it been like way back in that prehistoric time when the world's first mammals were only just beginning to evolve?

"Dude, like, have you seen those weird growths on Agatha's belly? The heck are those anyways? Some sort of cyst or something? And they squirt out some sort of weird white goop too. Gross... You know what's even worse though? She feeds that stuff to her kids! Disgusting!"