The missing link between reptiles and mammals, the only creatures to have survived the cataclysm which ended the reign of the dinosaurs some 65 million years ago.

Key innovations (happenstance mutations that happened to have been advantageous in the new environments they found themselves thrust into) included a softening of scales (precursor to skin/fur), expanded jaw-joint system (precursor to an improved hearing system) and warmer blood.

(From: vague recollections of a biology lecture years ago.)