Conjunction among taxonomic distributions and the Miocene mammalian biochronology of the Great Plains

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Abstract
This paper presents a new means of interpreting the distribution of taxa among taxonomic lists. Traditionally, “similarity” indices have been used to compare lists, and “association” measures have been used to compare taxonomic distributions. It is argued that when sampling regimes are poorly understood, all similarity and association indices are unjustifiable. However, observations that taxa have overlapping (conjunct) or nonoverlapping (disjunct) distributions are universally meaningful. Because greater sampling can only increase the number of known conjunctions and because long lists serve as conjunctional Rosetta Stones, conjunction data sets can be far more reliable than the lists that generate them. One way to account for patterns of conjunction and disjunction is to create theoretical spaces composed of two distributional boundaries or edges for each taxon in each dimension. A parsimonious arrangement of edges implies distributions that always overlap when taxa are conjunct and infrequently overlap when taxa are disjunct. A procedure involving correspondence analysis is shown to minimize the number of implied disjunctions. The method is used to analyze a set of 271 Miocene large-mammal genus lists from the Great Plains region. Biostratigraphic and radiometric data demonstrate that the “best” arrangement of distributional edges is temporal and corresponds to an age-range chart. The edge sequence is calibrated to the radiometric time scale and used to compute a detailed genus-level diversity curve and to redefine the boundaries between Miocene Land-Mammal Ages and Subages in the Great Plains. Each of the Miocene Ages can be divided into two or three Subages. Diversity levels are apparently low throughout the Early Miocene and rise sharply during the Middle Miocene. Significant drops in diversity occurred at about 17.4, 11.6, 8.7, and 6.5 Ma. The base of the Barstovian corresponds to the base of the Middle Miocene, and the Clarendonian straddles the Middle-Late Miocene boundary.