What creature were we during the period of Dinosaur dominance here on Earth?

Question

I’m talking about the common ancestor of all mammals on Earth… Assuming you believe in Evolution.

Answer 1

In the Paleozoic era (before the great Permian extinction) a kind of lobe finned fish called a coelecanthe came onto land and produced the earliest amphibia. These evolved into reptiles called cotylosaurs which were the original "stem" reptiles. These reptiles proliferated and deverged into a variety of types, among which were the synapsid reptiles, which were mammal-like, and the diapsid reptiles, which eventually evolved into the two main types of dinosaurs, as well as other kinds of reptiles. The Mammal-like reptiles were very successful, initially, but they began losing ground to the diapsids, and only a few very small and very mammal like synapsids escaped extinction. The great Permian extinction came and wiped out over 90% of all species, and after this, in the Mesozoic era, the dinosaurs quickly became the dominant life forms, and the mammal like synapsid reptiles remained small and nocturnal and gradually became more and more mammalian, and lost their color vision, due to being so dependent on the nocturnal likestyle. If you could visit the Mesozoic, you would find our ancestors as very small mice-like creatures, existing in the protection of the dark of night, and most likely egg laying. The mammalian placenta is thought to have arisen ~70,000,000 years ago, just before the the great K/T extinction of 65,000,000 years ago, which wiped out the dinosaurs (except for some birds). The small nocturnal mice-like placental mammals were then free to radiate into every conceivable evolutionary direction, including a bunch that took to the trees sort of like squirrils, and then became more like monkeys, who regained color vision, and who eventually became us. If you are interested in reading more about this, I highly recommend the book "The Ancestor's Tale" by Richard Dawkins, available on Amazon and in many local book stores.

Answer 2

Mostly small, shrew (rodent) like animals, who mostly fed on insects. Curiously, some of the first mammals were like the modern platypus and laid eggs to bear young, but still breast fed their young with milk, the true calling card of a mammal.