Complex sentence: In 1937, her filly Mars Shield won the Kentucky Oaks then on May 4,1940 Ethel Mars had her greatest single achievement in racing with her colt Gallahadion.
Split into simpler sentences: In 1937, her filly Mars Shield won the Kentucky Oaks. Then on May 4,1940, Ethel Mars had her greatest single achievement in racing with her colt Gallahadion.