DID ECOLOGY DRIVE BODY SIZE EVOLUTION OVER THE LATE ORDOVICIAN MASS EXTINCTION? EVIDENCE FROM BRACHIOPODS AND TRILOBITES

The Late Ordovician extinction was first of the “Big Five” mass extinctions, driven by cooling, glaciation, eustatic sea level fall, and changes in ocean chemistry. The “Lilliput Effect,” a reduction in body size, occurred in several groups of marine invertebrates over the Ordovician extinction, but whether these size decreases were the result of environmental stressors, selective extinction of larger taxa, and/or other drivers remains unclear. Previous studies have found evidence for ecologically-driven selective extinction during the Late Ordovician mass extinction, including changes in extinction intensity across bathymetric gradients in brachiopods and by larval type (benthic vs. pelagic) in trilobites. These drivers of extinction intensity may have also been responsible for decreases in body size over the event, but this potential relationship has not been explored.

Here, we test whether changes in body size over the Ordovician extinction correspond to the same ecological factors driving extinction intensity (i.e., depth preference in brachiopods and larval ecology in trilobites). Body size, estimated as log area from length and width measurements, was collected for Middle Ordovician to middle Silurian orthids, strophomenids, and trilobites using museum collections and published literature. Depth preference was assigned to brachiopods as benthic assemblage zones, and pelagic vs. benthic larval types were assigned to trilobites. Statistical analyses were conducted to identify trends in body size for 1) strophomenid and orthid brachiopods with regards to water depth preference and 2) trilobites with regards to larval ecology. Preliminary results suggest these ecological factors are not significant controls on body size reduction, although further investigation is merited. Water depth does not correspond to significant body size change in either orthid or strophomenid brachiopods. Although trilobites with pelagic larvae exhibit greater body size decreases than those with benthic larvae, results are not statistically significant. However, trilobite sample sizes are small over the extinction boundary, so collection of additional data is warranted to more definitively establish whether larval ecology affected body size change over the Late Ordovician mass extinction.