Almirall, Esteve, and Ramon Casadesus-Masanell. "Open vs. Closed Innovation: A Model of Discovery and Divergence." Academy of Management Review 35, no. 1 (January 2010): 27–47.

There are currently two major types of relaxed-clock models. The first type assumes ... one must know the absolute age of some evolutionary divergence event, such as the split between mammals ...