Environmental Causation of Turtle Scute Anomalies in ovo and in silico.

The turtle shell is often described as an evolutionary novelty that facilitated the radiation of the clade Testudines. The scutes, or keratinous plates, of the turtle shell are hypothesized to be patterned by reaction-diffusion dynamics, and this property of their development provides explanatory power to mechanisms of anomalous variation. A mathematical model of scute development predicts that anomalous variation in the phylogenetically stable pattern of scutes is achieved by environmental influence on the developmental program. We test this prediction with data on patterns of scute variation from natural nests and controlled incubation of sea turtle eggs in Florida and Western Australia. We find that high temperatures are sufficient to produce anomalous patterns in turtle scutes, and that this correlation is even stronger when conditions are dry. Furthermore, we find that the patterns of variation are not random; greater anomalous variation is found in the midline vertebral scutes and during a critical period of turtle development.