THE JURASSIC SEAWAY OF THE U.S. WESTERN INTERIOR AS A LABORATORY FOR EVOLUTIONARY PALEOECOLOGY

The epicontinental seaway that covered the western United States during the Middle and Late Jurassic provides an interesting comparison with its European counterparts. The US seaway was quite shallow (probably not deeper than 100 m), encompassed a large latitudinal gradient, and had only one northern opening. There were about six major marine incursions and marine intervals are associated with many marginal marine and hypersaline deposits.

Paleoecological studies were conducted on both trace and body fossils and in both soft and hard substrate (including hardgrounds, cobbles, and skeletal material) settings. In general, bivalves dominate the fauna although locally, crinoids can form encrinites. While the level of endemism was previously thought to be very high, more recent taxonomic work suggests that some European cosmopolitan taxa did colonize the American seaway. For both ichnofossils and body fossils, gamma (regional) diversity is lower than in comparable geographic areas elsewhere in the world as is both alpha (within paleocommunities) and beta (between paleocommunities) diversities.

Paleocommunity structure as indicated by trace fossils and interpretations of life habits on soft and hard substrates suggests low levels of complexity: low levels of infaunal or epifaunal tiering, abundance of monospecific assemblages, and predominance of few guilds. Little evidence of predators or predation has been found—even ammonites are uncommon; the presence of encrinites also suggests a lower level of predation in the Jurassic American seaway than elsewhere in the marine realm.

Even accounting for taphonomic processes, these lower levels of diversity and paleocommunity complexity appear to reflect underlying differences in paleoenvironments, disturbance regimes, and biogeographic factors. Many of the paleoecological characteristics of the Jurassic American faunas are more reminiscent of the Paleozoic rather than contemporaneous Jurassic or Mesozoic faunas even while the dominant higher taxa (i.e., bivalves) remain the same. This large biogeographic region exhibits patterns unlike the global Jurassic one and indicates the environmental context for marine faunas greatly affects the diversity, structure and complexity of their paleocommunities.