In the human skull, a sagittal keel, or sagittal torus, is a thickening of part or all of the midline of the

frontal bone In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...

, or

parietal bone The parietal bones ( ) are two bones in the skull which, when joined at a fibrous joint known as a cranial suture, form the sides and roof of the neurocranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four bord ...

s where they meet along the

sagittal suture The sagittal suture, also known as the interparietal suture and the ''sutura interparietalis'', is a dense, fibrous connective tissue joint between the two parietal bones of the skull. The term is derived from the Latin word ''sagitta'', meaning ...

, or on both bones. Sagittal keels differ from

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are excepti ...

s, which are found in some earlier

hominins The Hominini (hominins) form a taxonomic tribe of the subfamily Homininae (hominines). They comprise two extant genera: ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos), and in standard usage exclude the genus ''Gorilla'' (gorillas), ...

(notably the genus ''

Paranthropus ''Paranthropus'' is a genus of extinct hominin which contains two widely accepted species: ''Paranthropus robustus, P. robustus'' and ''P. boisei''. However, the validity of ''Paranthropus'' is contested, and it is sometimes considered to be sy ...

'') and in a range of other mammals. While a proper crest functions in anchoring the

muscles of mastication The four classical muscles of mastication elevate the mandible (closing the jaw) and move it forward/backward and laterally, facilitating biting and chewing. Other muscles are responsible for opening the jaw, namely the geniohyoid, mylohyoid, an ...

to the cranium, the keel is lower and rounded in cross-section, and the jaw muscles do not attach to it. Sagittal keels occur in several early human species, most noticeably in ''

Homo erectus ''Homo erectus'' ( ) is an extinction, extinct species of Homo, archaic human from the Pleistocene, spanning nearly 2 million years. It is the first human species to evolve a humanlike body plan and human gait, gait, to early expansions of h ...

'', occasionally in ''

Homo heidelbergensis ''Homo heidelbergensis'' is a species of archaic human from the Middle Pleistocene of Europe and Africa, as well as potentially Asia depending on the taxonomic convention used. The species-level classification of ''Homo'' during the Middle Pleis ...

'' and in some

Upper Paleolithic The Upper Paleolithic (or Upper Palaeolithic) is the third and last subdivision of the Paleolithic or Old Stone Age. Very broadly, it dates to between 50,000 and 12,000 years ago (the beginning of the Holocene), according to some theories ...

Homo sapiens Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

'' specimens. Most modern ''Homo sapiens'' groups have lost them, likely as part of the general trend toward thinning of the cranial bones to make room for larger brains during the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

. However, there is a small portion of modern humans who have the feature, but its function and

etiology Etiology (; alternatively spelled aetiology or ætiology) is the study of causation or origination. The word is derived from the Greek word ''()'', meaning "giving a reason for" (). More completely, etiology is the study of the causes, origins ...

are unknown. Actor

Patrick Stewart Sir Patrick Stewart (born 13 July 1940) is an English actor. With a career spanning over seven decades of Patrick Stewart on stage and screen, stage and screen, he has received List of awards and nominations received by Patrick Stewart, variou ...

and the martial artist

Shi Yan Ming Shi Yan Ming (born Duan Gen Shan; February 13, 1964) is a 34th generation Shaolin warrior monk, teacher and actor, best known as the founder of the USA Shaolin Temple. Trained at the Shaolin Temple in Henan, People's Republic of China (PRC) sin ...

present good examples of modern humans (''

Homo sapiens sapiens Human taxonomy is the classification of the human species within zoological taxonomy. The systematic genus, ''Homo'', is designed to include both anatomically modern humans and extinct varieties of archaic humans. Current humans are classified ...

'') with this feature. The keel appears to be tied to general cranial robustness and is more common in adult men than women and absent in children.

References

{{Reflist Skull

See also

References