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.)