Trait‐based range expansion aided in the global radiation of Crocodylidae

Abstract

Aim Almost half of the extant species of Crocodylia (nine genera, sensu Benton & Clark) belong to the genus Crocodylus, which originated in the Miocene. Today, this genus has a circumtropical distribution, with representatives found in Asia, Oceania, Africa and the Neotropics. However, its geographical origin and the historical events behind its rapid diversification and global radiation are still debated. Here, we inferred the evolution of a set of life traits that aid in dispersal and how they influenced the biogeography of Crocodylus. Location Global. Time period Cretaceous to the present. Major taxa studied Crocodylidae. Methods We estimated biogeographical history on a published phylogeny using probabilistic biogeography models. Next, we identified four life traits likely to promote range expansion and used a trait‐based dispersal model jointly to infer evolution of these traits and their influence on the dispersal of crocodilians. Finally, we used diversification analyses to identify shifts in diversification rates. Results An Asian origin was reconstructed for Crocodylus. Despite the small size of the phylogeny, statistical model comparison reports substantially improved the model fit of trait‐dependent over trait‐independent dispersal models. In individual tests, the four life traits considered (salt tolerance, large size, large clutches and habitat generalism) appear to be correlated statistically with higher dispersal probabilities. However, the traits are likely to be correlated with each other, and we hypothesize that they all reflect the gradual evolution of a range‐expansion phenotype in early Crocodylus that resulted in increased diversification of the clade. Main conclusions Increased dispersal in the genus Crocodylus was associated with the gradual evolution of a range‐expansion phenotype. Interestingly, the evolution of the range‐expansion phenotype was also associated with the diversification of the genus in a period of global crocodilian extinction, indicating that range expansion might have served as a potential driver of speciation. This suggests that the concept of “spatial sorting”, normally applied at the population genetic level at the leading edge of an expanding population, might also act on a macroevolutionary scale.

Old African fossils provide new evidence for the origin of the American crocodiles

“…by Moreno-Bernal et al 27 . None of them is older than the early Pliocene and, in fact, Nicolai and Mazke 3 considered as Pliocene the dispersal of Crocodylus from Africa to America.…”

“…The late Miocene age of the African Crocodylus checchiai slightly predates previous estimates and is however fully congruent with a westward dispersal towards America and the appearance of Crocodylus in America in the early Pliocene fossil record. It seems likely that at the same time Crocodylus dispersed northward from Africa to Europe across the Tethys (or alternatively from Asia to Europe 3 ) as testified by several remains coming from different late Miocene southern European localities. At least the remains from the fissures fillings of Gargano promontory 28 can be confidently referred to Crocodylus sp., but also the remains from Montebamboli 29 and Scontrone 30 have been tentatively referred to the same taxon.…”

“…Remarkably, thanks to the evolution of range-expansion-promoting traits (salt tolerance 32 , but not only, see Nicolai and Mazke 3 ) Crocodylus started to disperse across the globe very quickly around the end of the Miocene and successfully replaced the local taxa that went extinct because of climatic changes and related environmental remodeling 6 , 19 , 27 , 30 , 33 , 34 , 35 .…”

“…The most speciose and widespread genus is Crocodylus Laurenti, 1768 that hosts 12 species inhabiting a longitudinally very broad intertropical belt ranging from Australia to South Asia, to Africa and then America. The origin of Crocodylus was placed in the late Miocene (about 13.6–8.3 Ma) of Asia by Oaks 2 , on the basis of a time-calibrated species tree stemming from a DNA analysis, and recently reconstructed as Asian by Nicolai and Mazke 3 on the basis of historical biogeography models. Both the molecular 2 , 4 , 5 and the morphological/palaeontological 6 – 9 approaches, as well as the shared presence of parasites 10 concur in indicating that the origin of the American clade of Crocodylus should be sought in Africa.…”

Biotic and abiotic factors driving the diversification dynamics of Crocodylia

“…; Godoy et al . ; Nicolaï & Matzke ). Crocodylian speciation is a diversity‐dependent process (also known as density‐dependent processes), a phenomenon interpreted as the result of biotic interactions involving ecologically similar species (Alroy ; Rabosky ; Foote et al .…”