Agterberg, F. P., F. M. Gradstein, and A.S. Gale. “Estimation of the Ages of Devonian and Cretaceous Stage Boundaries in the Geologic Time Scale”. Boletín Geológico y Minero 131, no. 2 (June 30, 2020): 309–323. Accessed May 3, 2025. https://bgm.revistas.csic.es/index.php/bgm/article/view/134.