Comparative limb bone scaling in turtles: Phylogenetic transitions with changes in functional demands?

Area of Science:

• * Evolutionary biology
• * Comparative anatomy
• * Biomechanics

Background:

• * Many terrestrial vertebrates have evolved aquatic lifestyles, often accompanied by limb modifications for swimming.
• * Limb bone loading and shape changes are crucial for adapting to aquatic locomotion, particularly the transition from terrestrial walking to aquatic swimming via flapping.
• * Turtles provide a model system to study these transitions, with diverse lineages exhibiting walking, rowing, and flapping locomotion.

Purpose of the Study:

• * To investigate how limb bone shape and scaling patterns differ across turtle clades with distinct locomotor habits (walking, rowing, flapping).
• * To test the hypothesis that rowing species exhibit intermediate limb bone flattening compared to walkers and flappers.
• * To explore functional correlations between limb bone morphology, body size, and ecological niche.

Main Methods:

• * Comparative analysis of humerus and femur measurements from four turtle clades: sea turtles (flappers), softshells (freshwater rowers), emydids (semiaquatic rowers), and tortoises (walkers).
• * Application of phylogenetic comparative methods to analyze limb bone scaling patterns with respect to body mass.
• * Examination of limb bone diameter allometry relative to body size across different functional groups.

Main Results:

• * Rowing turtle taxa did not display the predicted intermediate limb bone flattening patterns.
• * Sea turtle humeri showed positive allometry in diameter perpendicular to the elbow's flexion-extension plane, correlating with flattened flippers.
• * Softshell turtles exhibited positive allometry in femoral diameters, potentially aiding benthic stability.
• * Tortoise limb bones showed positive allometry in diameters and elongated humeri, possibly related to digging adaptations.

Conclusions:

• * Limb bone scaling patterns in turtles are diverse and reflect specific locomotor behaviors and ecological adaptations beyond a simple walker-rower-flapper continuum.
• * Positive allometry in limb bone diameters can serve various functions, including hydrodynamic shaping, buoyancy control, and load resistance.
• * The study highlights the complex interplay between morphology, function, and evolution in vertebrate adaptation to aquatic environments.