EXTINCTION IN THE ORDOVICIAN-SILURIAN GRAPTOLOID CLADE: RATE, SELECTIVITY, AND DRIVERS

The threat of species extinctions is of concern today, but general processes of extinction can only be understood after the event, by reference to countless observations of past extinction in the fossil record. Here, we use a new, highly resolved, global history of early Paleozoic graptoloids to study controls on extinction rate and selectivity through the life span of this clade. The graptoloids were the major macroscopic zooplankton group during the Silurian and Ordovician and occupied the largest biome on Earth; as such, their extinction history is an important record of biological change in the early Paleozoic marine biosphere. Our dataset has unprecedented temporal resolution of 37 thousand years between observations, on average, through 74 million years.

Extinction rate in the graptolites was controlled ultimately by climate change. Through most of the Ordovician, extinction rates were generally low, whereas the latest Ordovician and Silurian were characterised by repeated spikes in extinction; species durations approximately halved from the Ordovician into the Silurian. These differences apparently do not reflect a simple increase in frequency and severity of extinction spikes, but also may result from an approximate doubling of a characteristic ‘background’ extinction rate. For most of the time, newly evolved species were most at risk of extinction, a pattern that is consistent with the expectation of increasing geographic range and thus extinction resistance with increasing species age, although age contributed to survival in some additional, unknown way(s). In contrast, during most extinction spikes, both short- and long-lived species had equal extinction risk, a pattern that matches the ‘field of bullets’ model. During the most severe Late Ordovician Mass Extinction, extinction selectively targeted the oldest species, a distinct pattern that may reflect a unique process or the prolonged duration of this event (~1.5 my). Importantly, extinction selectivity varied on very short timescales (<< 1 million years) and much more rapidly than general ecosystem recovery. Although we argue that the underlying control of extinction risk in the graptoloids was climatic, a simple biotic/abiotic control dichotomy is clearly overly simplistic and these data still require more nuanced interrogation.