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Asagittal crest is a ridge of bone running lengthwise along the midline of the top of theskull (at thesagittal suture) of manymammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are exceptionally strongjaw muscles. The sagittal crest serves primarily for attachment of thetemporalis muscle, which is one of the main chewing muscles. Development of the sagittal crest is thought to be connected to the development of this muscle. A sagittal crest usually develops during the juvenile stage of an animal in conjunction with the growth of the temporalis muscle, as a result of convergence and gradual heightening of thetemporal lines.[citation needed]
A sagittal crest tends to be present on the skulls of adult animals that rely on powerful biting and clenching of their teeth, usually as a part of their hunting strategy. Skulls of some dinosaur species, includingtyrannosaurs, possessed well developed sagittal crests. Among mammals, dogs, cats, lions, and many other carnivores have sagittal crests, as do some leaf eaters, including tapirs and someapes.[citation needed]
Sagittal crests are found in robustgreat apes, and some early hominins (Paranthropus). Prominent sagittal crests are found among malegorillas andorangutans, but only rarely occur in malechimpanzees such asBili apes.
The largest sagittal crest ever discovered in thehuman lineage belongs to the "Black Skull",Paranthropus aethiopicus field numberKNM WT 17000, the earliest known robust hominid ancestor and the oldest robust australopithecine discovered to date. The prominence of the crest appears to have been an adaptation for theP. aethiopicus' heavy chewing, and the Black Skull's cheek teeth are correspondingly large. Smaller sagittal crests are also present on the skulls of other Paranthropines, includingParanthropus boisei andParanthropus robustus.[1]
The shrinking of the sagittal crest in human ancestors was widely attributed to a growing brain and shrinking teeth. However, it was discovered in 2004 by a group of researchers led by Dr. Hansell Stedman,[2] that aframeshift mutation shrank the individual muscle fibers of the temporalis muscle, which attached to the sagittal crest. This was believed to have allowed brain size to increase, since the crest was no longer strictly necessary, but a later paper from 2017, led by researchers from George Washington University,[3] found that the increase in brain size and reduction of tooth size were not linked, as originally hypothesized.