 A single large opening on either side of the back of the skull links Dimetrodon with mammals and distinguishes it from most of the earliest sauropsids , which either lack openings or have two openings . Features such as ridges on the inside of the nasal cavity and a ridge at the back of the lower jaw are thought to be part of an evolutionary progression from early tetrapods ( four @-@ limbed vertebrates ) to mammals . 
