Did the latitudinal diversity gradient and temperature-size rule hold during the Late Palaeozoic Ice Age?

The Late Palaeozoic Ice Age (LPIA) represents one of the most significant climatic transitions in Earth's history, characterized by dramatic fluctuations in atmospheric CO2, O2, and global temperatures. This study investigates the influence of these global climatic shifts and latitudinal gradients on the diversity, complexity, and size of trace fossils during the Carboniferous and Permian periods in continental deposits. By analysing an extensive ichnological database of ichnodiversity and ichnodisparity (architectural designs) through statistical methods, palaeogeographic reconstructions, and multiple palaeotemperature and palaeo-O2 model reconstructions, we evaluated how the Latitudinal Diversity Gradient (LDG) operated under glacial and interglacial conditions of the LPIA. Our results reveal clear latitudinal and climatic gradients in ichnodiversity (tropical > temperate > polar > arid), with consistently higher diversity in coastal and transitional settings relative to lacustrine and alluvial/subaerial deposits, supporting the hypothesis that the LDG operated during the LPIA. The exception was lacustrine environments, where no significant latitudinal pattern was detected, likely reflecting the weaker expression of the LDG in isolated freshwater ecosystems. We also tested whether arthropod gigantism was influenced by palaeolatitude and climatic conditions, using trace fossil external width as a proxy for arthropod body size. The relationship between size and environmental variables was highly dependent on depositional context. In the global dataset, size exhibited a negative correlation with palaeotemperature and atmospheric O2. However, when partitioned by environment, non-subaqueous (terrestrial) traces showed a positive correlation with temperature and a strong negative correlation with atmospheric O2, whereas coastal subaqueous traces exhibited a negative correlation with seawater temperature and a positive correlation with marine O2. These contrasting patterns are consistent with the Temperature-Size Rule (TSR), where aquatic and terrestrial arthropods respond differently to temperature and O2. The predominance of small traces throughout the LPIA indicates that giant arthropods were exceptional rather than typical, with gigantism largely confined to the Carboniferous and disappearing after the Carboniferous–Permian transition. We interpret these patterns as resulting from the interplay of primary productivity, habitat heterogeneity, and ecological constraints (such as lack of predators), modulated by taphonomic and sampling biases that preferentially record larger traces and iconic body fossils.