With NBA legend Gregg Popovich stepping away from coaching duties, Mike Tomlin of the Pittsburgh Steelers now holds the title of the longest-tenured head coach across the four major North American sports leagues.

It's the end of an era in San Antonio as head coach Gregg Popovich steps down from the role after 29 seasons leading the Spurs. While this is a seismic development in the NBA, it also has ramifications that bleed into the NFL as well.

Gregg Popovich stepped down. Mike Tomlin moved up. The decision by Popovich to end his run as coach of the San Antonio Spurs means that the longest active tenure by any coach in any of the four U.S. major pro sports had ended.