Rating Newest Oldest. Best Answer: The K–T boundary is a geological signature, usually a thin band, dated to 65.5 million years ago. K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period. The boundary marks the end of the Mesozoic era and the beginning of the Cenozoic era, and is associated with the Cretaceous–Tertiary extinction event, a mass extinction. See the link below for more info. Source(s): http://en.wikipedia.org/wiki/K-t_boundar...