There were reptiles(Cynodonts) that heavily resembled dogs in the triassic.
Even the earliest cynodonts, the Procynosuchidae of the Late Permian, show many advanced mammalian characteristics, such as a reduced number of bones in the lower jaw, a secondary bony palate and a complex pattern of the crowns of their cheek teeth. It is likely that Cynodonts were at least partially if not completely warm-blooded, covered with hair, which would have insulated them and helped to maintain a high body temperature.
By Early Triassic times, cynodonts had diverged into large predaceous carnivores such as Cynognathus and moderate large omnivorous and herbivorous types such as Trirachodon and Diademodon. The Middle Triassic saw a major radiation of herbivorous forms included in the family Traversodontidae. From this family evolved the highly specialized and extremely mammal-like Tritylodontidae of the Late Triassic to Middle Jurassic, the "rodents" of the early Mesozoic and culmination of the herbivorous cynodont radiation. At the same time, the descendents of Cynognathus evolved into medium-sized to small carnivorous and insectivorous forms. It is interesting that as the archosaurian reptiles were becoming larger, the cynodonts became smaller, perhaps nocturnal. The hot arid Triassic conditions favored the ectothermic reptilian metabolism of the archosaurs over the warm-blooded mammalian organization of the cynodonts. (In his Dinosaur Heresies, Bob Bakker has clamed that even the early thecodont archosaurs like Erythosuchus were warm-blooded, and out-competed the cynodonts for this reason, but this position is almost never held nowadays
Found that here:http://www.palaeos.com/Vertebrates/Units/4...ia/410.000.html