THE EXTRATROPICS AS A CENTER OF DIVERSIFICATION FOR IRREGULAR ECHINOIDS

Three models exist to explain the paradigmatic latitudinal diversity gradient (LDG) of high tropical diversity with decreasing diversity toward the poles (i.e., cradle, museum, out of the tropics models). We assessed whether an important clade of marine invertebrates, irregular echinoids (spatangoids, clypeasteroids, and cassiduloids) had an LDG, and if so, which of the models may explain their evolutionary diversification. Using published modern latitudinal data we found an unusual LDG for this group: a slower decline in diversity with increasing latitude and an upturn in diversity in the Antarctic region. By examining paleobiology databases, we also found that irregular echinoids had higher originations in the extratropics than in the tropics (defined as 25^o N&S latitude) from the Cretaceous to Pleistocene. Nearly 78% of the genera that originated in the extratropics now have some presence in the tropics, indicating that migration into the tropics has contributed to high tropical diversity for this group. We assessed whether sample bias and wider tropical belts in the past affected our results. First, we found that tropical and extratropical irregular echinoids were similarly sampled in the fossil record, suggesting little, if any, sample bias: 87.5% of modern extratropical genera (35/40) have at least one fossil extratropical occurrence; 81.3% of modern tropical genera (39/48) have at least one fossil tropical occurrence. Second, we re-assessed our data for three major periods of global warming (Cretaceous, Paleocene, Eocene) by extending the paleotropics to 30° N&S latitude. Only the Eocene had a change in the number of tropical originations from 5 genera (25^oN&S latitude), increasing to 16 genera if the tropical belts were wider. After testing the three LDG models, none of them accounted for the evolutionary diversification of irregular echinoids. Thus, we propose an out of the extratropics (OTE) model to explain the evolutionary pathway of irregular echinoid diversification. The OTE model may also explain the latitudinal diversification of infaunal foraminifera, terrestrial mammals and other marine invertebrates that are known to have extratropical origins. We posit that the extratropics are just as critical as the tropics in contributing to global biodiversity in modern and fossil ecosystems.