An overlapping role with the works of the Western Australian Museum was eventually altered to a full time position of curator of the palaeontology, mineral and meteorite collections.
[1] Amongst the works reporting the findings of excavations at fossil sites and study of the museum's specimens, Merrilees published a thesis on the impact of human practices introduced to the environment.
[1] In his proposed model of ecological changes after the first arrival of humans, the disappearance of megafauna as a direct consequence of their activities in Australia is comparable to Quaternary extinction events on other continents.
[2] Merrilees presented his research and conclusions, conducted under the supervision of W. D. L. Ride, in his presidential address to the Royal Society of Western Australia in 1967.
His conservation activities were associated with a personal interest in farming, and in overseeing an official program of semi-cleared land purchase that sought to both enhance the agricultural potential and rehabilitation of bushland to its previous state.