The period that the mammal-like reptiles were most common is the Permian, from about 280 million years ago to 250 million years ago. We can tell more about how mammal-like reptiles lived by the shape of their bones and the length of them. Long bones are usually from fast runners, while short bones are usually from slow movers. We can tell the skin of some extinct animals if we find impressions on rock of their skins. We have not yet found any skin impressions for mammal-like reptiles so we are not sure what their skin looked like.
Modern mammals have fur or hair, modern reptiles have scales.
The mammal-like reptiles were around for about 120 million years, while the dinosaurs were around for 180 million years. The climate began to get warmer, which favoured the dinosaurs while not being as good for the mammal-like reptiles. Fossil mammals have been found in Australia and Victoria since the first geological surveys were undertaken in the 1820s, but the aboriginal people of Central Australia knew of them long before this, they called them the Kadimakara, and thought they had fallen from the great sky land. People are mammals, and mammals evolved from mammal-like reptiles over the past 220 million years. We can tell what the animals looked like by carefully studying their bones and reconstructing where their muscles would go, and then putting skin and fur over the muscles.