Background The origin of turtles and crocodiles and their easily recognized body forms dates to the Triassic and Jurassic. Despite their long-term success, extant species diversity is low, and endangerment is extremely high compared to other terrestrial vertebrate groups, with ~ 65% of ~ 25 crocodilian and ~ 360 turtle species now threatened by exploitation and habitat loss. Here, we combine available molecular and morphological evidence with statistical and machine learning algorithms to present a phylogenetically informed, comprehensive assessment of diversification, threat status, and evolutionary distinctiveness of all extant species. Results In contrast to other terrestrial vertebrates and their own diversity in the fossil record, the recent extant lineages of turtles and crocodilians have not experienced any global mass extinctions or lineage-wide shifts in diversification rate or body-size evolution over time. We predict threat statuses for 114 as-yet unassessed or data-deficient species and identify a concentration of threatened turtles and crocodilians in South and Southeast Asia, western Africa, and the eastern Amazon. We find that unlike other terrestrial vertebrate groups, extinction risk increases with evolutionary distinctiveness: a disproportionate amount of phylogenetic diversity is concentrated in evolutionarily isolated, at-risk taxa, particularly those with small geographic ranges. Our findings highlight the important role of geographic determinants of extinction risk, particularly those resulting from anthropogenic habitat-disturbance, which affect species across body sizes and ecologies. Conclusions Extant turtles and crocodilians maintain unique, conserved morphologies which make them globally recognizable. Many species are threatened due to exploitation and global change. We use taxonomically complete, dated molecular phylogenies and various approaches to produce a comprehensive assessment of threat status and evolutionary distinctiveness of both groups. Neither group exhibits significant overall shifts in diversification rate or body-size evolution, or any signature of global mass extinctions in recent, extant lineages. However, the most evolutionarily distinct species tend to be the most threatened, and species richness and extinction risk are centered in areas of high anthropogenic disturbance, particularly South and Southeast Asia. Range size is the strongest predictor of threat, and a disproportionate amount of evolutionary diversity is at risk of imminent extinction.

12The origin of turtles and crocodiles and their easily recognized body forms dates to the Triassic. 13Despite their long-term success, extant species diversity is low, and endangerment is extremely 14 high compared to other terrestrial vertebrate groups, with ~ 65% of ~25 crocodilian and ~360 15 turtle species now threatened by exploitation and habitat loss. Here, we combine available 16 molecular and morphological evidence with machine learning algorithms to present a 17 phylogenetically-informed, comprehensive assessment of diversification, threat status, and 18 evolutionary distinctiveness of all extant species. In contrast to other terrestrial vertebrates and 19 their own diversity in the fossil record, extant turtles and crocodilians have not experienced any 20 mass extinctions or shifts in diversification rate, or any significant jumps in rates of body-size 21 evolution over time. We predict threat for 114 as-yet unassessed or data-deficient species and 22 identify a concentration of threatened crocodile and turtle species in South and Southeast Asia, 23 western Africa, and the eastern Amazon. We find that unlike other terrestrial vertebrate 24 groups, extinction risk increases with evolutionary distinctiveness: a disproportionate amount 25 of phylogenetic diversity is concentrated in evolutionarily isolated, at-risk taxa, particularly 26 those with small geographic ranges. Our findings highlight the important role of geographic 27 determinants of extinction risk, particularly those resulting from anthropogenic habitat-28 disturbance, which affect species across body sizes and ecologies. 29

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