Mammalian types are evolved from reptilian ancestors. All modern mammals have certain characteristics not present in their reptilian ancestors, including: a middle-ear with jaw-bones displaced and modified into ear-bones for enhanced hearing; hair (fur) to enhance maintenance of self-regulating body-temperature; milk production from modified sweat-glands [mammary glands along a milk-line]. Evolutionary theory would have primitive mammalian ancestors having initially one of those traits, which evolved into a more advanced form having two of those traits, and finally evolving into a form having all three. Descendants of that most-evolved form are the monotremes [duck-billed platypus, etc.], which evolved a lineage from which are descended the marsupials, from which evolved a lineage from which are descended the placentals. The earliest origins of mammals are murky, and I know of no known fossils for those primitive types that had only one or two mammalian traits, but not all three. What evidence do we have as to which arose first - - fur, better-hearing, or milk production?