A sagittal crest is a ridge of bone that runs lengthwise along the top of the skull. It is located along the sagittal suture, the midline joint connecting the two parietal bones of the skull. Its presence is an indicator of powerful jaw muscles and is specific to certain species with particular dietary and behavioral adaptations.
The Purpose of a Sagittal Crest
The primary function of a sagittal crest is to provide an expanded surface area for the attachment of the temporalis muscles. These large, fan-shaped muscles on the sides of the head are responsible for closing the jaw. A larger crest allows for a greater volume of muscle fibers to anchor to the skull, which directly translates to a more powerful bite.
This is a direct adaptation for diets that require immense chewing force. Animals with this feature process tough materials, such as leaves, stems, or hard-shelled nuts. The crest’s development is connected to the growth of the temporalis muscle, often forming during the juvenile stage as these muscles grow and converge at the top of the skull. The size of the crest reflects the demands of an animal’s diet.
Which Animals Have a Sagittal Crest
A variety of animals, both living and extinct, exhibit a sagittal crest. Among modern primates, prominent examples are found in adult male gorillas and orangutans, where the crest is linked to both diet and social structure. Other mammals, including carnivores like lions and dogs, also possess this crest to support the strong jaw muscles needed for hunting and processing prey.
The fossil record also provides examples within the early hominin lineage. Paranthropus boisei, an extinct hominin, is known for its pronounced sagittal crest and large cheek teeth, which were adaptations for a diet of tough plant matter. The crest can also be an indicator of sexual dimorphism, where it is larger or only present in males. This is evident in gorillas, where the larger crest in males corresponds to greater muscle mass for group defense and dominance displays.
The Disappearance in Human Evolution
The absence of a sagittal crest in modern humans is a result of evolutionary changes in our lineage. A primary factor was the shift in diet and food processing techniques. As early humans began to incorporate softer, cooked foods and use tools to cut their food, the need for powerful jaw muscles diminished. This dietary transition meant the large temporalis muscles, and the crest needed to anchor them, were no longer necessary.
This reduction in jaw musculature coincided with the expansion of the brain. As the human brain grew larger, the skull vault became more rounded to accommodate it. In apes and early hominins with smaller brains and larger chewing muscles, the temporalis muscles extend high up on the skull. In humans, the combination of a large cranium and smaller temporalis muscles meant there was no anatomical need or space for a crest to form.