How ubiquitous is the Lilliput effect? A global, cross-clade investigation of body size over the Late Ordovician mass extinction

Body size reduction following extinction events, known as the “Lilliput Effect,” has been identified in many groups of organisms throughout the history of life. However, the prevalence of this phenomenon over any given extinction remains unclear, in part due to publication bias (e.g., instances where the Lilliput Effect does not occur are not commonly reported), and because most studies have focused on a single taxonomic group. To better understand the ubiquity of the Lilliput effect and the mechanisms driving this phenomenon, we constructed an extensive, global body size database for Middle Ordovician to middle Silurian marine invertebrates and analyzed patterns of size change over the Late Ordovician mass extinction. Most major groups of solitary, skeletonized marine invertebrates are represented in the dataset, including bivalves, gastropods, strophomenids, orthids, crinoids, and rugose corals. Data are based primarily on museum collections, which avoids potential biases tied to specimen publishing (e.g., publications are more likely to figure or report larger specimens), are temporally standardized for cross-clade comparisons, and include multiple measurements for taxa to permit investigation of within-lineage body size trends.

Preliminary results recover significant decreases in body size over the Ordovician extinction in roughly half the groups investigated (crinoids, bivalves, and orthid brachiopods). Among groups exhibiting a Lilliput Effect, all decreased in size over the first pulse of the Ordovician extinction (Katian–Hirnantian) but not the second pulse (Hirnantian–Llandovery). Post-extinction recovery time is variable, with some clades recovering by the early Silurian and others not returning to pre-extinction sizes until the middle–late Silurian. Although further investigation is needed to disentangle the mechanisms responsible for driving the Lilliput Effect, there is some evidence that selective extinction of larger taxa drove body size reduction, as opposed to within-lineage dwarfing and/or the evolution of smaller taxa post-extinction. Overall, results indicate that the Lilliput Effect is a common but not ubiquitous phenomenon over the Late Ordovician mass extinction, and multiple processes were responsible for driving body size reduction patterns across clades.