A novel phylogenetic comparative model reveals strong signals for early high disparity in ichthyosaurs

Early high disparity, where clades reach maximum morphological diversity early in their history, is often observed in the fossil record. Since there are currently no suitable methods to model these Early High Disparity (EHD) processes accurately, we introduce a new phylogenetic comparative method (PCM) to address this gap by combining two evolutionary modes--Ornstein-Uhlenbeck (OU) and Early Burst (EB)--leading to an initial surge in disparity until an early maximum, driven by the EB component, after which disparity declines due to OU constraints. We tested the EHD model on ichthyosaur skull length data and found it to be the best fit in nearly all cases compared to other modes. The median time of peak disparity (TEHD) is about 231.1 million years ago, during the Triassic, roughly 12.4% into the ichthyosaur lineage. To explore how environmental and geological factors might influence peak disparity, we performed changepoint analyses on sea level, sea surface temperature, rock area, and sedimentary sequence ages. While changepoints in temperature at 237.2 Ma and sequence boundaries at 238 Ma were closest to TEHD, no significant changepoints appeared to match TEHD.