The End-Cretaceous Extinction and Ecosystem Change

Examination of fossil plant–insect associations in the continental realm and trace fossils in the marine realm provide considerable data for understanding organismic response to major ecological crises, such as the Cretaceous–Paleogene (K-Pg) event. For the continental realm, terrestrial data from the Williston Basin of North Dakota records plant–insect interactions representing 183 m of strata, 2.2 million years, 106 distinctive stratigraphic levels, 143 localities (floras), and 13,441 specimens that were collected, analyzed, and interpreted. Williston data indicate a major ecological restructuring of plant–insect interactions after the K-Pg event. There was general continuity of generalized interactions but a major decline in host-specialized feeding diversity and abundance across the K-Pg boundary, with recovery occurring ca. 107 years later. These results were not captured in earlier diversity studies of insect body fossils.

In the marine realm, four intensively documented stratigraphic sections representing open-water shelf habitats were examined from the K-Pg boundary interval of Spain and France. Substrate–ichnofossil relationships were assessed to document the extent and timing of tracemaker colonization of macrobenthic environments immediately after the K-Pg event. The results indicate a modest effect of the K-Pg event on macrobenthic trace-making organisms and minor effects in the environment-wide disruption of habitats inhabited by deposit-feeders. A quick recovery is manifested by the record of iron oxide spherules in Thalassinoides burrow infillings, by the highly bioturbated fabric of the K-Pg boundary layer, and by multiple substrate colonization events immediately after the crisis. This rapid recovery is associated with an almost instantaneous return on the order of 102–103 years to pre-impact environmental conditions.

From both studies we identify eight benefits of using ichnologic approaches. Methodological, empirical, and theoretical advances include: (1) application of previously unused methods to quantify ichnologic data; (2) assembly of new matrices with abundant, and rich data; (3) access to novel ecological data, particularly associational and behavioral data; and (4) because of developments 1–3, generation of new paleoecological hypotheses. More phenomenologically based accomplishments are: (5) use of both the preserved and nonpreserved aspects of the fossil record to infer patterns of colonization and recovery; (6) generation of ichnologic data with increased biostratigraphic resolution; (7) capture of ichnologically reworked sediments to provide important, highly resolved data to address catastrophic vs. gradualistic responses accompanying the K-Pg event; and (8) employing effects of the K-Pg event to better understand organismic response to major, future ecologic transformation.