Jugal bone

See also: Evolution of mammalian auditory ossicles

The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species.

Diagram showing homologous bones of the skulls of a monitor lizard and a crocodile. Jugal bone labelled Ju, in pale green, at centre left.

Anatomy

The jugal bone is located on either side of the skull in the circumorbital region. It is the origin of several masticatory muscles in the skull. The jugal and lacrimal bones are the only two remaining from the ancestral circumorbital series: the prefrontal, postfrontal, postorbital, jugal, and lacrimal bones.

During development, the jugal bone originates from dermal bone.

In dinosaurs

This bone is considered key in the determination of general traits in cases in which the entire skull has not been found intact (for instance, as with dinosaurs in paleontology). In some dinosaur genera the jugal also forms part of the lower margin of either the antorbital fenestra or the infratemporal fenestra, or both. Most commonly, this bone articulates with the quadratojugal, the postorbital, the lacrimal, and the maxilla. In horned dinosaurs, like Pentaceratops, the jugal bone is thick and comes to a point, which has led paleontologists to refer to it as the "jugal horn".

Function

Skull of a California condor (Gymnogyps californianus)

In reptiles

The earliest reptiles primitively had a lower temporal bar (also referred to as a temporal arcade) where the posterior of the jugal bone contacts the quadratojugal. This structure has been repeatedly lost and regained in various groups.

In birds

While the jugal bone is thick and straplike in most other reptiles, the jugal bone is thin and strutlike in birds. This is thought to reduce the weight of the skull and facilitate cranial kinesis.

In mammals

Skull of stem-mammal Morganucodon, with jugal labelled

In mammals, including humans, the jugal bone is more commonly referred to as the zygoma. It assists in constructing the facial contour, protecting the eye from damage, and providing attachment sites for facial muscles. The zygoma provides important functions as the origin of the masseter muscle and as a point of resistance for masticatory forces. Preliminary studies also indicate that variation in zygomatic structure may be useful in determining ancestral origins of modern human populations.