Phylogenetic and Spatial Distribution of Evolutionary Isolation and Threat in Turtles and Crocodilians (Non-Avian Archosauromorphs)

The origin of turtles and crocodiles and their easily recognized body
forms dates to the Triassic. 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 machine
learning algorithms to present a phylogenetically-informed, comprehensive
assessment of diversification, threat status, and evolutionary
distinctiveness of all extant species. In contrast to other terrestrial
vertebrates and their own diversity in the fossil record, extant turtles
and crocodilians have not experienced any mass extinctions or shifts in
diversification rate, or any significant jumps in rates of body-size
evolution over time. We predict threat for 114 as-yet unassessed or
data-deficient species and identify a concentration of threatened
crocodile and turtle species 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.