In 1874, he went on his first field fossil hunting expedition for Othniel Charles Marsh at Yale University under the mentorship of Benjamin Franklin Mudge, and led his first expedition in 1877. With Mudge, Williston discovered the first fossils of the dinosaurs "Allosaurus" and "Diplodocus". He was noted for painstakingly illustrating the finds. In 1880, he matriculated to Yale University, for several years was a post-graduate student and faculty member. Around this time, he proposed the first explicit model for the terrestrial origin of bird flight (i.e., that dinosaurs developed flight by running along the ground rather than jumping from trees).