A simple method for inferring habitats of extinct turtles

We present a simple method of inferring extinct turtle habitats based on four measurements of the shell (two ratios). These ratios are the maximum width of the carapace to the width of the abdominal-femoral sulcus of the plastron, and the ratio of the carapace length to the total height of the shell. Based on an extensive database, extant terrestrial turtles do not have a carapace-width-to-plastron-width ratio that exceeds 1.76. This carapace-width-to-plastron-width ratio in the Triassic turtle Proganochelys is ∼2.1, and the length-to-height ratio of Proganochelys is ∼2, which suggest an aquatic habitus. Triassic Proterochersis has a carapace-to-plastron-width ratio of 1.89, and a length-to-height ratio of 1.82, which are in the terrestrial range based on discriminant analysis. Inferences based on forelimb proportions and shell bone histology indicate a terrestrial paleoecology for Proganochelys. Conversely, femur morphology and shell cross sections indicate a semi-terrestrial to aquatic paleoecology of Proganochelys. The new method presented here indicates the early turtle Proganochelys was aquatic and Proterochersis was terrestrial in habitus.