Ephemeral species in the fossil record? Synchronous coupling of macroevolutionary dynamics in mid-Paleozoic zooplankton

We document a positive and strong correlation between speciation and extinction rates in the Paleozoic zooplankton graptoloid clade, between 481 and 419 Ma. This correlation has a magnitude of similar to 0.35-0.45 and manifests at a temporal resolution of <50 kyr and, for part of our data set, <25 kyr. It cannot be explained as an artifact of the method used to measure rates, sampling bias, bias resulting from construction of the time series, autocorrelation, underestimation of species durations, or undetected phyletic evolution. Correlations are approximately equal during the Ordovician and Silurian, despite the very different speciation and extinction regimes prevailing during these two periods, and correlation is strongest in the shortest-lived cohorts of species. We infer that this correlation reflects approximately synchronous coupling of speciation and extinction in the graptoloids on timescales of a few tens of thousands of years. Almost half of graptoloid species in our data set have durations of <0.5 Myr, and previous studies have demonstrated that, during times of background extinction, short-lived species were selectively targeted by extinction. These observations may be consistent with the model of ephemeral speciation, whereby new species are inferred to form constantly and at high rate, but most of them disappear rapidly through extinction or reabsorption into the ancestral lineage. Diversity dependence with a lag of similar to 1 Myr, also documented elsewhere, may reflect a subsequent and relatively slow, competitive dynamic that governed those species that dispersed beyond their originating water mass and escaped the ephemeral species filter.