DO LAURENTIAN TRILOBITES EXHIBIT THE LILLIPUT EFFECT ACROSS THE LATE ORDOVICIAN MASS EXTINCTION?

The Late Ordovician mass extinction resulted in the loss of 85% of species. This sharp drop in biodiversity was likely caused by a cocktail of factors, where global cooling led to glaciation followed by eustatic sea level fall, and changes in ocean chemistry caused anoxia. Past studies of the Late Ordovician mass extinction have found evidence for selective extinction of brachiopods along bathymetric gradients and of trilobites based on larval type (benthic vs pelagic). Certain groups of brachiopods, crinoids, and trilobites also exhibit a “Lilliput Effect,” a statically significant size decrease during a time of high stress. However, all prior studies of the Late Ordovician Lilliput Effect have been restricted to individual paleocontinents. For example, the only Lilliput Effect that has been described in trilobites is restricted to taxa from Norway, part of the paleocontinent of Baltica. Therefore, it remains unclear whether the Lilliput Effect phenomenon in trilobites is global or paleogeographically restricted over the Ordovician mass extinction.

In this study we evaluated whether the Lilliput Effect is present in trilobites from the paleocontinent of Laurentia during the Late Ordovician extinction. Estimated body size using log areas was calculated from the length and width measurements of trilobite cranidia. To maximize sample size for the study, cranidia were used as proxies for body length because many specimens did not have full exoskeletons preserved. Only specimens preserving at least half of the cranidium, as well as the free cheeks were measured. Over 600 Middle Ordovician to Middle Silurian trilobites were measured from museum specimens and literature that represent more than 80 trilobite genera from Laurentia. Results show no evidence for a Lilliput Effect in trilobites through the Ordovician extinction on Laurentia. A general decrease in size prior to the Late Ordovician extinction and an increase in size directly after the extinction are observed. No statistically significant evidence suggests a decline in mean body size across the extinction event. These results contrast with those of a prior study that found significant body size decrease in Baltica and suggest paleogeographic heterogeneity in the expression of the Lilliput Effect in trilobites over the Late Ordovician mass extinction.