DO NEOGENE COMMUNITIES MARCH TO A CLIMATIC DRUMBEAT?

Repeating, hierarchical climatic cycles may form a predictive framework for comprehending basic patterns in the geologic record, such as sedimentary cycles, biostratigraphic bundles, radiations, extinctions, rates of morphological change, and convergent evolution. A recent paper by van Dam and others extends Milankovitch orbital variations to include a 2.4 my climatic cycle, which is roughly equivalent to a marine zone's duration. It also provides a mechanism for a pattern of climatically forced biologic change first noted by van der Hammen in the 1950s that bundled smaller cycles (~2.3 my) into triplets (~7 my). A similar pattern was later described from the North American mammal record by Martin. We later used radiometric dates relating to the North American land mammal biostratigraphy to recalculate the duration of the small van der Hammen cycle as 2.41 my, making his triplet cycle 7.23 my. This 2.41 my interval is equivalent to the duration of the astronomical forcing cycle of van Dam and others.

The iterative turnover in pollen assemblages in South America, mammal assemblages in North America, and Spanish rodent faunas march to the same drummer and are all claimed to result from change to cooler, more arid conditions at cycle terminations. van der Hammen concluded that repeating climatic cycles of various scales are the ultimate basis for global biostratigraphic correlation, and we think these cycles influence the patterns found in the fossil record. The triplet cycles are not explained by astronomical forcing as presently known, but are probably reflected in the concept of chronofaunas as it is sometimes used. In our scheme, communities reflect repeating climatic conditions that contain specific evolutionary opportunities which select for particular adaptive types (ecomorphs). In the geologic record, ecomorphs are often not phylogenetically close, but independently evolve tremendous morphological similarity. Because ecomorphs are integral parts of a complex community structure, successive communities resemble each other, but have different phylogenetic compositions. The extinction and re-evolution of ecomorphs occurs in synchrony with the community as a whole, giving a repeating pattern of community structure on a time scale concordant with van der Hammen triplet cycles.