This glossary explains technical terms commonly employed in the description of dinosaur body fossils. Besides dinosaur-specific terms, it covers terms with wider usage, when these are of central importance in the study of dinosaurs or when their discussion in the context of dinosaurs is beneficial. The glossary does not cover ichnological and bone histological terms, nor does it cover measurements.
A
acetabulum
In dinosaurs, the acetabulum (plural: acetabula) or hip socket is an opening in the pelvis formed by the ilium, pubis, and ischium that is visible in lateral and medial views. It accommodates the head of the femur, forming the hip joint. Most tetrapods show a closed acetabulum, in which the socket is completely filled with bone, forming a depression. Dinosaurs are unique in showing a perforate or open acetabulum, where the full extent of the socket is a hole without infilling bone.
acromion
The acromion is a bony ridge on the lower part of the scapula that functions in providing an attachment for the clavicle. Nodosaurids develop a pronounced spur known as the pseudoacromion, which probably formed an attachment site for the Musculus scapulohumeralis anterior, and therefore is analogous with the acromion of mammals.
adductor fossa
The adductor fossa or Meckelian orifice in reptiles and dinosaurs is the major opening into the lower jaw, located between the tooth-bearing region and the jaw articulation. It opens dorsally, and is laterally walled by the surangular and medially by the prearticular; as the latter is usually much lower than the former, the fossa is visible in medial view. The floor of the opening is formed by the angular and houses the posterior part of the Meckelian cartilage. The adductor fossa and its surrounding margins serve as an insertion point for major adductor muscles that close the jaw; it also allows the jaw's main nerve, artery, and vein to enter the interior of the jaw.
air sacs
In modern birds, pulmonary air sacs are thin-walled, translucent air-filled bags connected to the lung. Together with the lung, air sacs form a highly efficient respiratory system, which in birds is capable of extracting up to 160% more oxygen than is possible in mammals. Although only limited gas exchange is taking place within air sacs, they power the ventilation of the lung. Air sacs located both posterior and anterior to the lung allow for a constant airflow through the lung. Smaller and blind air-filled bags extending from the air sacs and the lung are known as pneumatic diverticula (singular: diverticulum); these can be numerous and present in most of the body. Some pneumatic diverticula will resorb and penetrate bones as an animal grows, creating fossae (depressions) on the bone surface as well as internal chambers within the bones, a process known as postcranial skeletal pneumatization (PSP). Although air sacs do not fossilize, their presence at least in saurischian dinosaurs is indicated by distinctive traces of pneumatization in the bones.
alveoli
Dental alveoli (singular: alveolus) or tooth sockets are pits in the jaws containing the roots of teeth. Within the alveoli, teeth are periodically replaced by new teeth growing beneath; shedding of the old tooth crown occurs after resorption of the root. The condition of having deep alveoli is known as thecodont, and is present throughout Archosauria, including dinosaurs. Most squamates, on the other hand, show the pleurodont condition, where the teeth are fused to the inner sides of the jaw bones, or the acrodont dentition, in which the teeth are fused to the top of the jaw bones without sockets. The thecodont condition was historically used to define a clade, the Thecodontia, which is now considered paraphyletic and thus obsolete.
angular
The angular is a dermal bone of the lower jaw. In lateral view, it covers a larger area of the posteroventral region of the jaw, being located behind the dentary and below the surangular. In medial view of the lower jaw, it is visible below the prearticular. It forms the floor of the adductor fossa and supports the posterior portion of the Meckelian cartilage.
antorbital fenestra
The antorbital fenestra is one of the five major openings of the skull, located between the orbit and the external naris (nostril). Present in most early Archosauriformes (including Archosauria), it is primitively present in dinosaurs; it tends to be large in saurischians, but is reduced or entirely closed in ornithischians. The antorbital fenestra lies within a larger depression, the antorbital fossa. This fossa can contain additional, smaller openings, namely the maxillary fenestra (also: accessory antorbital fenestra) and the promaxillary fenestra.
articular
The articular is the hindmost bone of the lower jaw. On its dorsal surface, it contains the glenoid, a depression into which fits the lower end of the quadrate bone of the skull, forming the jaw joint. It is the only endochondral bone of the lower jaw, forming directly out of the posterior part of the Meckelian cartilage. In mammals, the articular migrated into the skull, forming the malleus of the middle ear.
articulated
An articulation is any joint between bones. The term is also used to describe the preservation of specimens: In an articulated specimen, individual bones stick together in their original anatomical position. Conversely, a disarticulated skeleton has its bones moved out of their original anatomical compound. A specimen found with its bones disarticulated but in close proximity to each other is termed an associated skeleton. The transition from a completely articulated skeleton to a cluster of isolated and unassociated bones is fluent.
astragalus
The astragalus (plural: astragali) is a major bone in the ankle. It is located directly below the tibia and medial to the calcaneum, which sits below the fibula. The astragalus does not rotate against the calcaneum or the tibia; instead, it is frequently fused to both elements in bipeds (see also: tibiotarsus).
atlas
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