Shell-closing systems likely promoted evolutionary transitions from aquatic to terrestrial ecological niches in turtles

Background/Question/Methods

During 220 million years of evolution, turtles have radiated across seven continents and three oceans where they now occupy freshwater, marine, and terrestrial ecological niches. In spite of these adaptive radiations, the basic turtle body plan consisting of dorsal (carapace) and ventral (plastron) shell components is presumed to be highly conserved. Upon close examination, however, intricate modifications to the turtle's shell become apparent. For example, the shell-closing system phenotype enables the absolute concealment of all body parts in turtle species with terrestrial ecological niches. By enhancing anti-predator defense, this complex trait likely promoted the invasion and successful colonization of terrestrial environments. I used a comparative phylogenetic statistical framework to test the hypothesis that the presence/absence of shell-closing systems is correlated with aquatic and terrestrial ecological niches of turtles.

Results/Conclusions

Pagel's test for correlated character evolution supported the hypothesis that the evolution of shell-closing systems is correlated with ecological niche. Logistic regression analysis (uncorrected for phylogeny) suggested that small semi-aquatic and terrestrial turtles are more likely to feature some form of shell-closing system. Parsimony and maximum likelihood ancestral state reconstructions revealed that this trait has evolved independently at least 10 times across the turtle tree of life. I am currently employing Bayesian ancestral state reconstruction methods to more robustly support these conclusions. My findings suggest that the turtle’s unusual body plan is not an “evolutionary straitjacket” and has potential to evolve in response to shifts in ecological niche.