Introduction to Dinosaur Cranial Anatomy

Skull of a dromaeosaur（圖片來源：AS，CC0 1.0 公共領域）

Skull of Spinosaurus（圖片來源：AS，CC0 1.0 公共領域）

Dinosaurs are among the most frequently studied subjects in paleontology, and their skeletal structures constitute some of the most important fossil evidence available to researchers. Among these skeletal elements, the skull is especially informative because it preserves numerous anatomical features that help scientists interpret feeding behavior, sensory abilities, and evolutionary relationships. The following overview introduces several key cranial bones commonly found in theropod dinosaurs. Once these structures become familiar, it becomes much easier to understand anatomical descriptions encountered in paleontological literature.

Premaxilla (premaxillary bone): A pair of bones located at the very front of the upper jaw. These bones usually bear teeth. In humans, the premaxilla has fused with the maxilla during development.

Maxilla (maxillary bone): The principal pair of bones forming the upper portion of the mouth. Most of the upper teeth are anchored in the maxilla.

Nasal bone: A pair of elongated bones forming the bridge of the nose, roughly corresponding to the human nasal bridge.

Lacrimal bone: A pair of bones located behind the nasal bones and along the inner front margin of the eye socket.

Orbit: The cavity that houses the eye.

Prefrontal bone: A pair of bones situated between the lacrimal and frontal bones. Many tetrapods possess prefrontal bones, but these elements are absent in mammals.

Frontal bone: A pair of bones forming the anterior portion of the skull roof. In humans, the frontal bones have fused into a single plate that forms the forehead. In many other vertebrates, the frontal bones remain paired.

Sclerotic ring: A ring of small bones embedded within the sclera of the eye. This structure may consist of a single piece or several bones. Sclerotic rings are known from ichthyosaurs, pterosaurs, and some dinosaurs, whereas mammals lack this structure.

Postorbital bone: A bone located behind the orbit. In birds, the postorbital appears during embryonic development but fuses with neighboring bones before hatching. Mammals do not possess a postorbital bone; instead, they have a different structure known as the postorbital bar.

Jugal bone: A bone situated beneath the orbit. In mammals, this bone is known as the zygomatic bone, commonly referred to as the cheekbone.

Parietal bone: A pair of bones forming part of the skull roof toward the back of the head.

Squamosal bone: A bone located beneath the parietal bone. This element occurs in amphibians, reptiles, birds, and early synapsids.

Quadrate bone: A bone that articulates with the quadratojugal and squamosal bones. It occurs in amphibians, reptiles, birds, and early synapsids.

Quadratojugal bone: A bone located between the jugal and quadrate bones. It appears in certain amphibians and reptiles but is absent in squamates, birds, and mammals.

Subnarial foramen: An opening located between the premaxilla and the maxilla. It is present in saurischian dinosaurs and some basal ornithischians. Mammals lack this opening.

External naris: The external opening of the nasal cavity, serving as the airway for respiration.

Antorbital fenestra: An opening located in front of the orbit and behind the nasal region, typically bordered by the lacrimal bone. This structure is characteristic of archosaurs. Modern birds retain an antorbital fenestra, whereas crocodilians no longer possess it. Mammals lack this feature.

Temporal fenestra: Two pairs of openings located behind the orbit in the temporal region of the skull. These consist of the superior temporal fenestra and the inferior temporal fenestra. Many amniotes possess such openings. Animals with two temporal fenestrae belong to the group known as Diapsida, which includes dinosaurs, pterosaurs, ichthyosaurs, birds, snakes, lizards, turtles, and crocodilians. Synapsids—including mammals—have only a single lower temporal opening.

Dentary bone: The main bone of the lower jaw (mandible), bearing the lower teeth.

Surangular bone: A bone located along the upper rear margin of the lower jaw. This bone does not occur in mammals.

Angular bone: A bone located along the lower rear margin of the lower jaw. In mammals, this element evolved into one of the small bones of the middle ear.

Splenial bone: A small bone positioned along the inner lower portion of the mandible. It occurs in amphibians, reptiles, and birds.

Mandibular fenestra: An opening within the lower jaw that characterizes archosaurs.

Understanding these cranial structures provides an essential foundation for interpreting fossil skulls. Because many paleontological studies rely heavily on anatomical comparison, familiarity with these bones allows readers to follow scientific discussions more easily and to appreciate how researchers reconstruct the evolutionary history of dinosaurs.

Edited by Shui-Ye You