Ground-truthing Late Ordovician climate models using the paleobiogeography of graptolites

The delta(18) O records obtained from conodonts suggest that during the Mid-Ordovician, equatorial temperatures stabilized at close to the present day, a hypothesis broadly supported by published climate models. However, the degree to which equatorial temperatures represent global climate state and varied between different climatic modes (greenhouse/icehouse, interglacial/glacial) and to what extent Ordovician delta(18) O and climate models truly reflect the global climate remain to be tested. Here we present paleobiogeographical data for the planktonic graptolites, from the gracilis time slice (i.e., early Sandbian Stage) that just postdates the postulated onset of climate stabilization. TWINSPAN analysis and constrained seriation provide robust ecological groupings in paleobiogeographical presence/absence data. The highest-level groups reflect tropical-subtropical assemblages and a high-southerly latitude assemblage. Constrained seriation defines latitude-distinct biotopes that are considered equivalent to modern zooplanktonic provinces. The distribution pattern of graptolite biotopes in the gracilis time slice show (1) that models explaining local graptolite ecological assemblages using lateral differentiation (e. g., upwelling) are to be favored above those using depth stratification and (2) a steep faunal gradient from equator to pole, which is typical of a cool, nongreenhouse world and comparable to the modern situation. We therefore broadly support the climate stabilization hypothesis. Paradoxically the climate of the early Late Ordovician appears similar to that of the present day despite the higher atmospheric pCO(2) levels. Graptolite species were indicative of temperature-controlled ocean water masses, in much the same way as the modern zooplankton