Deep-time invention and hydrodynamic convergences through amniote flipper evolution

Area of Science:

• Paleontology
• Functional Morphology
• Biomechanics

Background:

• Diapsid plesiosaurs were marine reptiles from the Triassic to Cretaceous periods.
• Their four wing-like flippers suggest underwater flight, though rowing has also been proposed.
• Efficient underwater flight relies on lift generated by flipper twisting via muscular activity.

Purpose of the Study:

• To investigate the evolution of flipper twisting in plesiosaurs and analogous taxa.
• To reassess distal flipper muscle functions using anatomical network analysis (AnNA).
• To differentiate between lift-based (underwater flight) and drag-based (rowing) swimming mechanisms.

Main Methods:

• Anatomical network analysis (AnNA) was employed.
• Bone-to-bone and muscle-to-bone contacts were coded into matrices for various marine animals.
• The 'igraph' package in R, using a walktrap algorithm, identified morphofunctional modules.

Main Results:

• Muscle-to-bone contacts are crucial for analyzing module contributions to flipper motion; bone-only matrices are insufficient.
• Plesiosaurs, marine turtles, sea lions, and penguins exhibit flipper twisting mechanisms.
• Penguins cannot actively twist the trailing edge of their flippers; pig-nosed turtles and whales do not actively twist their foreflippers.

Conclusions:

• AnNA is effective for studying flipper biomechanics and evolution.
• Plesiosaur swimming likely involved active flipper twisting for lift generation.
• Comparative analysis highlights diverse flipper functionalities across marine vertebrates.