Abstract: Evolution of Western Interior Cretaceous Paleocommunities and Faunal Associations

E. G. Kauffman

During the 40-m.y. history of the Western Interior Cretaceous seaway, eustatic sea-level changes produced four major depositional cycles. The resultant cyclothems contain a broad variety of marine-marginal marine facies, specificially ordered during transgression or regression, and repetitive in ordering and lithology within and between cyclothems. The great variety of benthic environments of this mid- to warm-temperate seaway, and their long duration and recurrent nature, enhanced development of diverse paleocommunities. Broad mixing of northern and southern biotas had a profound effect on community composition, as did high levels of endemism within the seaway. More than 50 common recurrent benthonic assemblages of organisms, molluscan-dominated, have been defined in the Western Interior Cretaceous. Most are of low diversity, reflecting abnormal marine conditions within the basin during much of its history. Diversity decreases offshore for taxa and ecologic units, reflecting poor substrate conditions in clay and carbonate-mud facies. Spatial distribution of ecological units is patchy throughout the basin. No single lithofacies is characterized by a single "paleocommunity;" rarely is a single paleocommunity restricted to a single, narrowly defined facies.

Assemblages, paleocommunities, and associations (ecologic units) clearly evolved through the Cretaceous in a variety of modes and at varied rates. Evolution may be so fast that adaptive morphotypes must be used in place of taxa to characterize ecologic units, or so slow that a single set of lineages may be used throughout the Cretaceous. Rate of community evolution relates specifically to the collective ecologic adaptability of the component organisms (stenotopy versus eurytopy) and to type and duration of stress gradients affecting them through time. Four modes of "community" evolution are defined, with examples: (a) evolution through taxonomic replacement by similar and related organisms; (b) evolution through functional replacement by similarly adapted but unrelated homeomorphs; (c evolution by addition of new taxa and adaptive types, and niche partitioning; (d) evolution by radiation into previously unoccupied niches and establishment of new community types. Within these broad trends, data in many areas are sufficient to recognize seres and details of succession in paleocommunities.

AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico