LATITUDINAL DIVERSITY GRADIENTS IN MESOZOIC NON-MARINE TURTLES

Latitudinal gradients in diversity and temporal changes in those gradients are considered a first-order pattern of richness and diversification through time, especially in the evolutionary history of climate-sensitive ectothermic organisms like reptiles. Here we explore the importance of the latitudinal diversity gradient in shaping the early diversification of turtles, which first appear in the Triassic and are represented by at least 17 families and 75 genera by the end of the Cretaceous.

Data on turtle genus- and family-level richness from the Triassic through Cretaceous, grouped into latitudinal bands of 15 degrees each, have been compiled based on more than 1400 non-marine occurrences. To allow comparison with modern turtle distribution, we reanalyzed a recently published data set of Recent occurrences, subsetted to match the resolution of the fossil data set. Corrections for land area were applied to both fossil and extant data, with paleo-land areas output from the Hadley Centre general circulation model (HadCM3L) for successive time periods. The fossil data were additionally corrected for sampling by comparison with the geographic spread of the entire tetrapod record for the same latitudinal bands and time bins and using shareholder quorum subsampling (SQS) of richness.

Recent area-corrected species richness is highest at 25°N, but area-corrected genus richness is even from 30°N to 5°S, with a notable drop in richness outside the tropics and subtropics at ca. 30°N and 30°S. In the Mesozoic, raw genus richness is greatest at 30-45°N and is lowest in the tropics and above 60°N and 60°S. This is partly a reflection of sampling bias, as tropical regions are under-represented in the tetrapod fossil record. However, the absence of turtles at well-sampled high latitude tetrapod localities suggests that low richness above 60°N/S is genuine and the SQS subsampled data bear out this general pattern. Area-corrected genus richness in the Late Cretaceous is not demonstrably greater than that in the modern turtle fauna.