What do a snail shell, climbing vines and ram’s horns have in common? All of these biological structures have a spiral shape. To understand the formation of this geometry, a new study has just put into equation the morphogenesis of the shells of molluscs, and more particularly of ammonites.
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The asymmetrical winding of the shells of some gastropods, like snails or some ammonites, has long fascinated collectors and scientists. This helicospiral shape is indeed very present in the world of molluscs and this for 540 million years. In addition, it is not characteristic of an environment, since it is found in all environments, marine and terrestrial, abyssal plains to our gardens.
A team of researchers from the geology de Lyon and the Institute of Mathematics at the University of Oxford have attempted to equate this biological form and to explain the formation of these shells.
The intriguing asymmetry of the shells of certain ammonites
While gastropods often have a symmetrical shell, some cash, like snails, however, have an asymmetric helicospiral shell, with the opening usually placed on the right side. This asymmetry has long puzzled biologists, who have made snails the model organisms to understand the basics genetics developmental aspects of this break in symmetry in organisms bilateral.
Unlike snails and like most gastropods, the vast majority of ammonites have a symmetrically coiled shell on a plane. These mollusks cephalopods have colonized the oceans for 340 million years and went out during the biological crisis from the end of Cretaceous, There are 66 million years. It seems that the ammonites had the same mechanisms of construction of their shell than current gastropods. However, 1% of the 3,000 genera of ammonites listed still have an asymmetric helicospiral shell, like snails. These specimens, long considered aberrant, are intriguing since it appears that the body of these ammonites is nevertheless symmetrical. But how can a symmetrical body therefore secrete an unsymmetrical shell?
The body of the mollusk shapes the shape of the shell
Previous studies explained the geometry of asymmetric shells by the growth processes occurring at the base of the shell. The results of the article published in the journal PNAS show, however, that the position and mechanical forces of the body within the shell largely control the growth and shape of the shell.
The mathematical model developed by the authors of the study thus takes into account the interaction between the soft body of the mollusk and its rigid shell during its development. In gastropods, the body is thus characterized by an intrinsic coiling, linked to an asymmetry of muscle development. The physical constraints generated by this winding of the body inside the shell would therefore naturally favor the helicospiral shape. It would even seem that it is impossible for these organisms with asymmetric bodies to produce a symmetrical shell. Conversely, for ammonites, which have a symmetrical body, anything seems possible. The mathematical model shows that the helicospiral shape sometimes observed is linked to a growth lag between the shell tube and the body in some species, thus generating an imbalance of the mechanical forces counterbalanced by a coiling of the body and leading to a rupture of the body. the symmetry of the shell.
The study therefore highlights the importance of physical forces in biological development and in the generation of sometimes extravagant forms.
The shell of ammonites governed by the laws of physics
A biomechanical model developed by French and English researchers explains the formation and diversity of ammonite shells. This group of cephalopods fossils, close to the current nautilus, had shells in the form of wavy spirals.
CNRS article published on October 14, 2014
The shape of living organisms evolved over time and the questions raised by these transformations have favored theemergence from evolution theories. To understand how biological forms change over geological timescale, researchers have recently been interested in how they are generated during the development and growth of an individual: we speak of morphogenesis. The group of ammonites, given the exceptional diversity of the shells shape and the patterns (in particular the ribs) which adorn them, is very studied from an evolutionary point of view but the mechanisms at the origin of these wavy spirals were until now unknown. The researchers were therefore trying to understand the evolution of these forms without knowing how they had been generated.
In an article published on the journal’s website Journal of Theoretical Biology, Régis Chirat, researcher at the Lyon Geology Laboratory: Earth, planets and environment (CNRS / University Claude bernard Lyon 1 / ENS de Lyon), and two collaborators from the Institute of Mathematics of the University of Oxford present a model that explains the morphogenesis of these shells. By describing by mathematical equations how the shell is secreted by ammonite and grows, they demonstrated the existence of mechanical forces specific to mollusks in development. These forces depend on the physical properties of biological tissues and the geometry of the shell. They are at the origin of mechanical oscillations at the end of the shell which generate the ribs, kinds of undulations adorning the spiral.
The biomechanical model describes the evolution of ammonites and nautiluses
By comparing different specimens fossils in simulations produced by the model, the researchers observed that it can predict the number and shape of ribs for different ammonites. The model shows that the ornamentation of the shell changes as a function of variables such as the elasticity of the tissues and the rate of expansion of the shell, the rate at which the diameter of the opening increases with each turn of the coil.
By offering a biophysical explanation for the formation of these ornamentations, this theoretical approach makes it possible to explain the diversity existing within and between species. It thus opens up new perspectives in the study ofmorphological evolution of ammonites, an evolution which appears to be strongly channeled by mechanical and geometric constraints. This new tool also sheds light on an old enigma. For nearly 200 million years, shells nautiluses, distant cousins of the ammonites that still inhabit the waters of the Indian and Pacific Oceans, have remained essentially smooth and without distinctive ornamentation. The model shows that maintaining this shell shape does not reflect an absence of evolution as suggested by the – bad – qualifier of living fossils attributed to the current nautilus. It is due to a high rate of expansion which leads to the formation of smooth shells which are difficult to distinguish from one another.
More generally, this work underlines the interest of the study of physical basis of biological development : by understanding the construction rules at the origin of the morphological diversity of organisms, the evolution of their shape becomes partly predictable.
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