Constraining the global niche suitability of the Eusuchia clade across the Cretaceous-Paleogene boundary

Abstract

The crocodiles and their close relatives, the alligators and gharials, have a compelling evolutionary history. They are a clade of great antiquity, with their most recent common ancestor emerging within the Mesozoic. However, unlike many groups of such a great age, the crocodilians have an extensive crown-group, with around two dozen extant examples. They have a limited ecomorphology, which has varied little since their inception, and their biogeography has been shown to interact closely with climate. The biogeography of crocodilians in deep time remains an outstanding question, which is complicated further by the limitations of the fossil record. The fossil record is fundamentally incomplete yet represents the most common method used to infer biogeography of organisms. The scarcity of fossil remains makes apparent absences difficult to confirm. Preservation bias will promote fossil occurrences in areas with a high sedimentation rate, which may not be the true ecological niche for a given taxon. This study uses species distribution models of extant crocodilians to infer the ecological niche of related taxa in the Maastrichtian and Danian. Models indicate a much wider latitudinal range than is observed among extant examples, and the invasion of new ecospace following the end-Cretaceous mass extinction. In addition, we find that while temperature is of significance to crocodilian biogeography, it is precipitation that is the most influential climatic variable.

Approaches to Species Distribution Modelling in Dinosaurs and Other Fossil Organisms

“…The model parameters are documented in Cox et al (2001) and Gordon et al (2000). This model has been used in previous efforts to reconstruct species distribution in deep time (Waterson et al 2016, Chiarenza et al 2019, Harper et al 2022), and therefore offers meaningful comparison. The model output palaeoclimate variables were temperature and precipitation in a monthly resolution.…”

“…Previous research has used occurrences of extant organisms, together with phylogenetic bracketing, to extrapolate species distribution models in deep time. Analysis by Watterson et al (2016) and Harper et al (2022) trained species distribution models using occurrences of modern reptiles and modelled representations of the current climate. These models were then fitted to palaeoclimate variables derived from a general circulation model, and validated using fossil data.…”

“…However, it makes a significant assumption that prehistoric examples of a given taxon will occupy the same fundamental niche as their modern counterparts. In addition, this methodology is also limited to taxa which have extant members such as turtles (Watterson et al 2016) and crocodiles (Harper et al 2022).…”

“…However, it makes a significant assumption that prehistoric examples of a given taxon will occupy the same fundamental niche as their modern counterparts. In addition, this methodology is also limited to taxa which have extant members such as turtles (Watterson et al 2016) and crocodiles (Harper et al 2022).Species distribution models assume that occurrences of a subject organism will be most concentrated in areas where the environment is most amenable to that organism. Fossil data does not fulfil this assumption, because fossilisation takes place under specific environmental conditions.…”