P. senegalus has a protective armor coat of scales, but this armor is not simply a single material. The scales are interlocking, and constructed with four layers of nanocomposite materials. In all the armor coat is about 400 micrometers (a bit less than half a millimeter) thick. On the outside is a thin layer (about 8 micrometers) of the very strong and hard ganoine. This is followed by layers of the softer dentine and isopedine. These layers are each about 50 micrometers, and they are followed by a 300 micrometer layer of bone.
The objective of structural designs is often to meet the requirement while minimizing weight. In this case, the armor should provide protection against attacks, such as sharp teeth bites, without adding more weight than necessary to the fish. This unique multilayered composite design does just that.
The researchers modeled P. senegalus’ armor using the finite element method and calibrated the mechanical properties of the four layers to experimental observations (such as from atomic force microscopy). They could then use their model to make design conclusions.
Perhaps the main conclusion is that the unique four-layer approach works well. For instance, it provides a 20% weight reduction compared to two-layer models the researchers tested. One reason for this is that the different layers work together to absorb attacks:
In conclusion, here we report on the fascinating, complex and multiscale materials design principles of ancient fish armour in the context of its specific primary environmental threat (penetrating biting attacks) and mechanically protective function. One overarching mechanical design strategy is the juxtaposition of multiple distinct reinforcing layers, each of which has its own unique deformation and energy dissipation mechanisms. As the stiff ganoine transfers load through the ganoine–dentine junction, the underlying softer, more compliant dentine layer dissipates energy via plasticity (at high enough loads). The ganoine thickness was selected (1) to simultaneously access the advantageous mechanical properties of the ganoine (hardness, stiffness) and underlying dentine (energy dissipation), (2) to reduce weight while maintaining the required mechanical properties and (3) to promote the advantageous circumferential cracking mechanism (S22 > S11), rather than disadvantageous radial cracking.
But there is more to the design than the mere use of these four different materials together. For instance, the order is important:
The material layer sequence is also critical; for example, reversing the ganoine and dentine layers in a virtual microindentation leads to magnified interfacial tensile normal and shear stresses, promoting delamination
Also, the ganoine and dentine layers are not uniform, but have mechanical properties that very across the cross section. Furthermore, there are geometrically corrugated junctions between the layers, that provide transitions between the mechanical properties of the different layers. These junctions are a crucial part of the design:
The junctions between material layers are clearly ‘functionally graded’, that is, they possess a gradual spatial change in properties motivated by the performance requirements and are able to promote load transfer and stress redistribution, thereby suppressing plasticity, arresting cracks, improving adhesion and preventing delamination between dissimilar material layers. Last, the corrugated junction between the layers is expected to lead to spatially heterogeneous stresses and a higher net interfacial compression, also resisting delamination. Such multiscale materials principles may be incorporated into the design of improved engineered biomimetic structural materials.
There is, of course, no explanation from evolutionists for such designs. That is not surprising because random events don’t create exquisitely designed multi-layered nanocomposite structures. Nor do they create the manufacturing facilities required to construct such designs. Nor do they create the instructions required to create such facilities. Evolutionists are left only with empty rebuttals. Religion drives science, and it matters.