How Small Did Things Get? The ‘Lilliput Effect' in Ammonites across the Cenomanian-Turonian (Late Cretaceous) Mass Extinction

One of the primary issues surrounding mass extinction events is the presence of the so-called ‘Lilliput Effect': size reduction associated with the environmental perturbations that characterize mass extinction events. The ubiquity and duration of this size diminution remains controversial both from taxonomic perspectives as well as for individual extinction events. Here, we examine the record of ammonite size across the Cenomanian-Turonian (C-T) mass extinction boundary using a literature compilation. This database currently represents measurements of the maximum diameter (D_max) from 3488 individual ammonite specimens taken from 72 references that examined ammonite material from outcrops primarily in North America, northwestern Europe, Africa, and Japan. To investigate the general trends, the data were analyzed at the substage level. As the vast majority of genera had durations of only a single substage, this study was focused on families and subfamilies. With one exception, all of the families showed a marked reduction in average Dmax during the Late Cenomanian extinction interval followed by a rapid increase in average D_max during the Early Turonian to dimensions equivalent to or exceeding those of the Middle Cenomanian. This same pattern is mirrored in the subfamily data. The size trends during the transitions between the other intervals appear to be much less consistent. Furthermore, a selective analysis of the data ‘binned' to the more temporally refined ammonite biozones suggests a much more pronounced ‘Lilliput Effect' with ammonite size reduction dominating during the late Late Cenomanian and early Early Turonian. These results suggest that for the ammonites, the environmental deterioration associated with the C-T stepwise extinction resulted in a pronounced size reduction, but this outcome appears to have dominantly had short-term ecological consequences rather than long-term evolutionary impacts.