Mechanics unlocks the morphogenetic puzzle of interlocking bivalved shells

Area of Science:

• Developmental Biology
• Paleontology
• Biophysics

Background:

• Brachiopods and mollusks, ancient shell-bearing phyla, exhibit convergent evolution of bivalved shells.
• A key feature is the seamless commissure where shell valve edges meet without gaps or overlaps, despite potential antisymmetric patterns.
• This precise interlocking persists throughout development, even with environmental irregularities, suggesting a dynamic physical regulation.

Purpose of the Study:

• To develop a mathematical framework explaining the formation and regulation of the interlocking shell commissure.
• To investigate the role of mechanical instabilities and mantle lobe mechanics in shell pattern formation.
• To account for the diversity of shell forms and the intricate edge patterns observed in brachiopods.

Main Methods:

• Derivation of a mathematical model based on the physics of shell growth.
• Analysis of the geometry and mechanics of mantle lobes constrained by shell secretion.
• Modeling of mechanical instabilities to explain pattern formation and regulation.

Main Results:

• The model successfully explains the seamless interlocking of shell valves through mechanical instabilities.
• It accounts for the regulation of the commissure despite antisymmetric ornamental patterns on individual valves.
• The framework reproduces diverse shell forms and explains multiscale edge patterns in brachiopods as secondary instabilities.

Conclusions:

• The interlocking shell commissure is governed by physical processes and mechanical instabilities, not solely genetic specification.
• Mantle lobe mechanics and shell constraints provide the mechanistic basis for this precise developmental pattern.
• Parametric variations in growth dynamics lead to morphological diversity, offering insights into evolutionary patterns.