Darwinism: Natural selection acting on random mutation. Biophysics: The physical constraints on life forms.
From ScienceDaily:
Régis Chirat and his team have developed a model that explains the morphogenesis of these shells. By using mathematical equations to describe how the shell is secreted by ammonite and grows, they have demonstrated the existence of mechanical forces specific to developing mollusks. These forces depend on the physical properties of the biological tissues and on the geometry of the shell. They cause mechanical oscillations at the edge of the shell that generate ribs, a sort of ornamental pattern on the spiral.
By examining various fossil specimens in light of the simulations produced by the model, the researchers observed that the latter can predict the number and shape of ribs in several ammonites. The model shows that the ornamentation of the shell evolves as a function of variables such as tissue elasticity and shell expansion rate (the rate at which the diameter of the opening increases with each spiral coil).
By providing a biophysical explanation for how these ornamentations form, this theoretical approach explains the diversity existing within and between species. It thus opens new perspectives for the study of the morphological evolution of ammonites, which seems to be largely governed by mechanical and geometric constraints. This new tool also sheds light on an old mystery. For almost 200 million years, the shells of nautili, distant “cousins” of ammonites, have remained essentially smooth and free of distinctive ornamentation. The model shows that having maintained this shell shape does not mean that nautili — wrongly referred to as “living fossils” — have not evolved, but is due to a high expansion rate, leading to the formation of smooth shells that are difficult to distinguish from one another.
More generally, this work highlights the value of studying the physical bases of biological development: understanding the “construction rules” underlying the morphological diversity of organisms makes it possible to partially predict how their shape evolves.
Here’s the abstract:
Ammonites are a group of extinct cephalopods that garner tremendous interest over a range of scientific fields and have been a paradigm for biochronology, palaeobiology, and evolutionary theories. Their defining feature is the spiral geometry and ribbing pattern through which palaeontologists infer phylogenetic relationships and evolutionary trends. Here, we develop a morpho-mechanical model for ammonite morphogenesis. While a wealth of observations have been compiled on ammonite form, and several functional interpretations may be found, this study presents the first quantitative model to explain rib formation. Our approach, based on fundamental principles of growth and mechanics, gives a natural explanation for the morphogenesis and diversity of ribs, uncovers intrinsic laws linking ribbing and shell geometry, and provides new opportunities to interpret ammonites’ and other mollusks’ evolution. (paywall)
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