LATITUDINAL BIODIVERSITY GRADIENTS FOR LATE-PALEOZOIC AND EARLY MESOZOIC SYNAPSIDS

Although latitudinal biodiversity gradients (LBGs) today are recognized to coincide with strong climatic gradients and seasonality, during times of weak climatic gradients, LBGs have been difficult to recognize in several terrestrial reptilian clades (e.g., Mesozoic dinosaurs and crocodylomorphs). A total of 1192 geo-referenced occurrences of 334 synapsid “reptile” genera from the late Carboniferous (Moscovian) through late Triassic (Rhaetian) accessed from the Paleobiology Database to examine LBGs late Paleozoic ice house to greenhouse transition. Through the interval, synapsid generic richness increased from 22 genera in the late Carboniferous to approximately 75 in the late Permian. The end-Permian mass extinction was particularly severe in which ca. 80% of all synapsid genera (and 100% of non-therapsid genera) became extinct. LBGs were constructed using latitudinal bins of 15 degrees and five time bins of between 16 –27 Ma. A weak correlation exists between proportional land area and richness in each latitudinal bin suggesting sampling bias effects cannot be overlooked. The data show a sharp unimodal peak in tropical generic richness in the late Carboniferous and early Permian, creating a strong LBG during the ice house conditions. From the middle Permian, across the end-Permian event, and through the late Triassic, the unimodal LBG transitions to dichotomous, temperate peaks in synapsid generic richness, resulting in weak LBGs coinciding with the development of greenhouse conditions. The shift in strong unimodal LBG to bimodal and weaker LBG pattern is the same for therapsids, the ancestors of modern mammals, and non-therapsid synapsids for all time bins examined. This suggests that therapsids had not yet evolved the mammalian heat-conserving traits that allow expansion into temperate latitudes during the ice house period of the Permian.