The third lineage refers to the Synapsids, which possess skulls with a single (lower) temporal opening behind the orbit of each eye. These were the first group of reptiles to colonize land during the Permian period and are referred to as the mammal-like reptiles. Within the synapsids two orders have been identified. The primitive Order Pelycosauria was characterized by animals which developed elongated spines from the vertebrae and are commonly referred to as sailbacks. The most spectacular example was Dimetrodon with vertebrae projecting more than a metre above the back at their highest point. These vertebral spines supported a web of skin and probably served as a temperature-regulating device that added a great area of skin surface for warming up and cooling off. The other group of synapsids are classified in the Order Therapsida. The therapsids developed into animals that resembled dog-faced tanks, for their limbs extended beneath their bodies, rather than to the sides, they may have had fur, and exhibited specializations of bone and teeth structure. These mammal-like reptiles suffered at least six distinct mass extinctions during the last eight million years of the Permian. The survivors of each extinction appeared to be more warm-blooded, to have more specialized jaws and teeth and to possess a more efficient respiratory system. Although this line ultimately lead to the evolution of the mammals, they came to dominate only fairly recently during the Tertiary period (starting some 65 million years ago).