Unitary associations method: use of graph theory and computer algorithm

Two species are defined as compatible if their chronologic cooccurrence has been observed (= real association) or can be deduced from biostratigraphic data (= virtual association). An unitary association (U.A.) is a maximal set of compatible species. Each U.A. is characterized by a set of species and/or by a set of species pairs: these characteristic elements are used to identify the U.A. in fossiliferous beds. The U.A.'s which can be identified in a large geographical area are said to be reproducible. A biochronological scale is an ordered sequence of reproducible U.A.'s. The problem of constructing such a discrete “time” scale is approached from a graph-theoretic point of view.