Complex sentence: Simpson was the most influential paleontologist of the twentieth century and a major participant in the Modern synthesis, contribting "Tempo and Mode in Evolution" (1944) and "Principles of Classification and a Classification of Mammals" (1945).
Split into simpler sentences: Simpson was the most influential paleontologist of the twentieth century and a major participant in the Modern synthesis. His main publications were "Tempo and Mode in Evolution" (1944) and "Principles of Classification and a Classification of Mammals" (1945).