Bone Sutures Protect and Flex

brown, yellow and black turtle on dried autumn leaves

Biological Strategy

Vertebrates (Mammals, Fish, Birds, Reptiles)

Eleanor Banwell

Image: JamesDeMers / Pixabay / Some rights reserved

Manage Impact

An impact is a high force or mechanical shock that happens over a short period of time, such as a hammer hitting a nail rather than a hand pushing slowly against a wall. Because of their speed and force, impacts don’t allow materials to slowly adjust to the force, which can lead to cracks, ruptures, and complete breakage. Therefore, living systems have strategies that can absorb, dissipate, or otherwise survive that force without the need to add large amounts of material. For example, the Toco toucan’s large beak is very lightweight, yet can withstand impacts because it’s made of a composite material with rigid foam inside and layers of a hard, fibrous material outside.

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Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Vertebrates (Mammals, Fish, Birds, Reptiles)

Subphylum Vertebrata (“jointed”): Mammals, fish, birds, reptiles

Vertebrates are a subgroup of chordates, which all have a flexible, rod-shaped structure that supports the body called a notochord. Non-vertebrate chordates include tunicates, hagfish, and lancelets. In vertebrates the notochord ultimately becomes part of the spine, usually encased in bony joints. All chordates also have a dorsal hollow nerve cord that forms the nervous system and pharyngeal slits that open outside the body during development (and persist to form gills in aquatic animals). Lastly, chordates have a tail at the back of the body––it’s just that sometimes you need an x-ray to see it.

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Sutures between bony plates protect from impact and provide flexibility

Plate shaped bones are found in the crania (skulls) of vertebrates as well as in the shells of turtles and tortoises. In skulls as well as in shells, these bones protect the inner organs from impact, but they must also be able to accommodate movement and growth.

At birth, there are gaps between the plates of a baby’s skull. As the baby develops, the plates grow and fuse so that by around 18 months of age, the gaps have disappeared. However, the bones do not completely fuse and the joins between them, called cranial sutures, are packed with soft connective tissue and function as very stiff joints. These joints provide a mixture of protection and flexibility that enable the cranium to accommodate the growing brain. Cranial sutures do finally ossify (become solid bone), but not until we are around 30 years of age, when the brain has finally ceased to grow. In turtles, the shell is formed when bony plates that grow between the ribs fuse. Fusion occurs after the turtle has hatched and remains a stiff but slightly flexible suture joint for its entire life.

Sutures have a unique structure: they form wavy interdigitations, with bone from each adjoining plate inserting finger-like projections into the plate next to it. This forms the very wiggly line that gives the feature its name.

Like vertebrate skulls, turtle shell sutures are also highly wavy. In their case, however, the interdigitations form in three dimensions, meaning the sutures are wavy through the shell as well as along it. In a turtle shell, impact protection is the primary function, and so resistance is critical. However, various internal organs required for movement and respiration are attached to the inside of the shell and these can be more efficiently carried out if the structure has some flexibility. When impacted, the finger-like projections of sutures “jam,” increasing toughness. The higher the degree of interdigitation, the better the impact resistance of the bone.

Bone sutures are less tolerant to bending than non-sutured flat bone; however, the sutures have greater impact resistance. In this way, turtle shells and our skulls combine the flexibility needed to grow and move with protection from hard knocks.