Pioneer Natural Resources Co. said Scott D. Sheffield will retire as the company's CEO effective Dec. 31. Pioneer's board approved a transition plan under which President and COO Timothy L. Dove will succeed Sheffield as president CEO. Sheffield will continue as executive chairman through Dec. 31, 2017 at which time he will retire as an executive and employee of the company but remain on the board. Sheffield joined Pioneer’s predecessor company, Parker & Parsley, as a petroleum engineer in 1979. Dove joined Parker & Parsley in 1994 as vice president of business development and was promoted to senior vice president in 1996 where he served until the formation of Pioneer in 1997. In 2004, Dove was named president and COO. He was elected to the board of directors in 2013.