Kai Caspar (@nomascus.bsky.social)

Obligate faunivorous megatheropod size class patterns across the Jurassic-Cretaceous Periods Allosauroidea, Ceratosauria, Megalosauroidea, Megaraptora, and Tyrannosauroidea are five clades containing obligate faunivorous megatheropods. These clades included apex predators from the Early Jurassic until the end of the Cretaceous Period. Studying the timeline of median size class change for ceratosaurians, tyrannosauroids, and megaraptorans compared to the extinction of the incumbent apex predator clades, allosauroids and megalosauroids, is important regarding megatheropod guild structure. This study used the median size classes exhibited by these clades throughout the Jurassic and Cretaceous Periods, along with the relationship of the median number of missing size classes under different apex predatory regimes. We calculated size class medians for each clade during six discrete time bins. Statistical tests on the median size class data were run to identify potential significant differences and test if increases in median size class occurred after the Cretaceous Thermal Maximum (KTM). Statistical tests were run on the number of missing size classes for each type of apex predator regime to determine if previously proposed hypotheses may explain potential differences. Statistical differences were found between four pairs of clades and their respective median size classes. Median size class increased after the Cretaceous Thermal Maximum for tyrannosauroids and potentially megaraptorans, but increased before the Cretaceous Thermal Maximum for ceratosaurians and did not change afterwards. The median number of missing size classes was found to be higher in the abelisauroid, abelisauroid/megaraptoran, and tyrannosauroid ecosystems compared to allosauroid/megalosauroid ecosystems. The median number of missing size classes between allosauroid/megalosauroid environments and tyrannosauroid-dominated environments was found to be significantly different, with a higher median number of missing size classes in tyrannosauroid-dominated environments. The analysis provides support for hypotheses, such as intraclade niche partitioning or niche shifting, to explain the differences in the median number of missing size classes between abelisauroid, abelisauroid/megaraptoran, and tyrannosauroid ecosystems and allosauroid/megalosauroid ecosystems. This study implies a complex history regarding the timing of the increase in the median size class for clades that survived the Cretaceous Thermal Maximum, which requires further study. https://peerj.com/articles/21007/