Ontogenetic change in shell morphology associated with an evolutionary transition from an aquatic to a terrestrial niche in box turtles
Novel phenotypes are usually a response to selective pressures in the environment. Thus, these traits may allow for organisms to occupy new ecological niches. In turtles, a key innovation that likely promoted the invasion of terrestrial environments is their shell closing systems. Shell closing systems are an adaptation that improves defense from terrestrial predators. In order for this adaptation to become fully functional, the ventral shell (plastron) has to undergo major morphological changes that render the plastron moveable. This is a unique example of a complex trait that requires post-natal developmental change. We conducted an observational study to quantify these changes. We sampled a population of 152 museum specimens of the Eastern Box Turtle (Terrapene carolina) representing three age classes: hatchling, juvenile, and adult. We quantified shape variation of the plastron for each age class using geometric morphometrics. We then used these data to test the hypothesis that the plastron undergoes a major transformation when transitioning from the hatchling to juvenile age class.
Our preliminary results support the hypothesis of major morphological change in the plastrons of box turtles as they transition from hatchling to juvenile life stages. Although shape variation was statistically significant, the morphological change was not as pronounced when comparing plastrons of juveniles and adults. These observations are consistent with bone tissue remodeling associated with hinge formation. We are currently using histological assays to further support this conclusion as well as comparing observed ontogenetic patterns in the Eastern Box Turtle to an aquatic species that does not feature a shell closing system. We have shown that ecologically relevant traits often require long developmental periods.