Original: The Short-beaked Echidna has the largest prefrontal cortex with respect to body size of any mammal, it shows rapid eye movement during sleep, and its brain has been shown to contain a claustrum that is similar to placental mammals, linking this structure to their common ancestor.
Simplified: The Short-beaked Echidna has the largest prefrontal cortex with respect to body size of any mammal. It shows rapid eye movement during sleep, and its brain has been shown to contain a claustrum that is similar to placental mammals, linking this structure to their common ancestor.