Glossary of dinosaur anatomy

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.

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.^[1]
acromion: The acromion is a bony ridge on the outer distal end of the scapula that functions in providing an attachment for the clavicle.^[2]^[3] 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.^[4]
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.^[5]
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.^[6]
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.^[7]
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.^[5]
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.^[1]
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.^[5]
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.^[8]
astragalus: The astragalus (plural: astragali^[1]) 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).^[9]
atlas: The atlas is the first (foremost) vertebra of the vertebral column. It receives the occipital condyle of the base of the skull, thus forming the connection between skull and spine. Dinosaurs possessed single condyles allowing for rotational movement between atlas and skull. Mammals, on the other hand, show double condyles, limiting movement to one plane; here, rotational movement is mostly taking place between the atlas and the second vertebra, the axis. The atlas in dinosaurs is primitive in comprising three separate elements, reflecting the basal condition in reptiles: the atlantal neurapophysis (plural: atlantal neurapophyses; also: atlantal neural arch) at the top;^[10] the intercentrum at the front and the centrum at the back. An additional small bone, the proatlas, sits in front of the neural arch.^[11] The centrum part of the atlas is also termed the odontoid.^[12] The three elements generally remain separate in dinosaurs, but can be fused together. In ceratopsians, the atlas, as well as the axis and third vertebra, are fused into a single bone.^[11]
arctometatarsal: In the arctometatarsalian condition (from Latin arctus – "compressed"), the middle (third) metatarsal of the foot is pinched between the surrounding metatarsals (the second and forth) at its upper end, thus disappearing in anterior view. This condition is found in some derived theropods, including ornithomimids, tyrannosaurids, troodontids, elmisaurids, and avimimids, and was hypothesized to function as a shock absorber during running, allowing for higher degrees of cursoriality. Originally, this character was used to define a clade, the Arctometatarsalia, which is now considered polyphyletic.^[13]^[14]
axis: The axis is the second vertebra of the spine, following the atlas. It is larger than the latter, with its centrum fused with its neural arch, and with the neural spine well-developed. Its small intercentrum, located in front of the centrum, is separate in theropods but becomes fused with the centrum of the atlas in ornithischians.^[11]
axony: The term axony describes the location of the dominant (largest and most projecting) digit in a hand or foot, and is most commonly employed in the description of footprints. When the central digit (generally digit III) is dominant, as is the case in the feet of most bipedal dinosaurs, the hand or foot is mesaxonic. An entaxonic hand or foot has its dominant digit medially (either digit I or II); this rare condition can be found in sauropod feet. Conversely, the ectaxonic hand/foot has its dominant digit exteriorly (often digit IV). When either two or four digits are present and neither half of the hand or foot is more dominant than the other, the condition is termed paraxonic.^[15]^: 47–48

basal tubera: The basal tubera (singular: basal tuber) or sphenoccipital tubercles^[16] are a pair of tubercle-like extensions on the underside of the braincase that function as attachment sites for ventral neck muscles. They are formed by the basioccipital and the basisphenoid bones, and are strongly pronounced in saurischians but only moderately so in ornithischians.^[17]^[18]
basicranium: The basicranium is the floor of the braincase, and formed by the basioccipital, basisphenoid, and parasphenoid.^[19]
basisphenoid: The basisphenoid bone forms the front part of the floor of the braincase. Not visible from the outside in an articulated skull, it is fused to the basioccipital at the rear and to the parasphenoid at the front. A pair of distinctive processes, the basipterygoid processes, extend from its underside and articulate with the pterygoids of the palate.^[20]
basioccipital: The basioccipital is an unpaired bone of the skull, and one of four bones forming the occiput of the braincase. It is exposed in rear view of the articulated skull. It forms most of the occipital condyle, a rounded process connecting the skull to the atlas, the first vertebra of the neck. Above, it is fused with the paired exoccipitals. It also forms the posterior part of the floor of the braincase, being fused with the basisphenoid at its front.^[20]
bone: Bones are, besides teeth, the most common dinosaur body fossils. Composed of hydroxylapatite, bones are hard mineralized and thus durable. Soft parts, in contrast, may only be preserved as impressions, and are directly preserved only in exceptional cases. Bony parts include the skull, the axial skeleton (spine and ribs), and the appendicular skeleton (girdles and limbs).^[21] Most of these bones are paired, with a mirror-inverted counterpart on the other side of the body, or unpaired, in which case they usually lie at the body midline and are divided by the latter into two symmetrical halves.^[1] Furthermore, bones include a number of elements formed in the skin such as gastralia, bony scutes, and spikes.^[21] Paleontologists most frequently study the morphology of bones, but also their histology (the inner microstructure up to the cellular level) and chemical composition provided important insights into dinosaur biology.^[22] There are two principal types of bones: Dermal bone is directly formed in the dermis (skin), usually growing from initially thin plates. Among others, most bones forming the outer surface of the skull and lower jaws are dermal bones. In contrast, endochondral bone is formed from a cartilaginous precursor, which ossifies (turn into bone).^[1]
braincase: The braincase is the part of the skull housing the brain. In an articulated skull, it is not visible from the outside except from its rear part, the occiput. A complex structure, it is pierced by numerous foramina containing blood vessels and cranial nerves. Individual bones of the braincase tend to be completely fused in adults, with demarcations between the original elements often not visible. In dinosaurs, the anatomy of the braincase is conservative, but for this reason can be used to infer relationships of a group when other skeletal features underwent changes so profound that their origins can no longer be traced. The braincase may also allow for reconstructing the brain and inner ear, with inferences on senses and intelligence.^[20]

calcaneum

The calcaneum (plural: calcanea^[1]) is a major bone of the ankle (at the rear of the foot), and together with the astragalus forms the upper row of tarsal bones. It is located lateral to the astragalus and distal to the fibula.^[9]

caputegulum

Caputegulae (Latin "skull tiles") are flat bones covering the skull bones of ankylosaurs. Together with the pyramidal-shaped horns, they form the ornamentation of the skull. Coined by William T. Blowes in 2001,^[23] the term can be used for elements representing both co-ossified osteoderms or cranial sculpturing. The position of a caputegulum on the skull can be specified with the use of modifiers; e.g., the nasal caputegulae sit atop the nasal bones.^[24]

carinae

Carinae (singular: carina) are enamel ridges that form the cutting edges on the front and rear margins of teeth. They are typically found in carnivorous dinosaurs, and often bear serrations.^[25]^: 41

carpals

The carpal bones form the wrist or carpus, which connects the forearm (radius and ulna) to the metacarpals of the hand.^[26] The corresponding part of the foot is the tarsus. Basal reptiles show three rows of carpals. In dinosaurs, the carpus is often not fully ossified, and the number and identity of carpal elements remain unclear in many cases. The carpus is especially well ossified in basal ornithischian Heterodontosaurus, where it consisted of nine elements. The proximal row consisted of the radiale (below the radius), the ulnare (below the ulna), and the pisiform (a small element below the ulna and lateral to the ulnare). The distal row consisted of five elements sitting above the five metatarsals, which are denoted as distal carpals 1–5. The middle row is represented by a single element, the centrale.^[27]^[28]

carpometacarpus

The carpometacarpus is a bony element of the hand consisting of the fused carpals and metacarpals. This structure occurs in modern birds, but was also present in some theropods closely related to birds, and evolved independently in alvarezsaurids.^[26]

caudals

The caudals (from Latin caudum — tail), or caudal vertebrae, are the vertebrae that make up the tail.^[26] Ancestrally, dinosaurs showed approximately 50 caudal vertebrae, although their number, size, and shape varied considerably in the separate groups. The number of caudals decreased along the evolutionary line leading to modern birds; in the latter, the remaining caudals are fused together into a pygostyle. Although flexible in early dinosaurs, several clades stiffened their tail with the help of ossified tendons (as in many ornithischians) or elongated prezygapophyses (as in some theropods such as dromaeosaurids). Diplodocid sauropods featured an elongated, whip-like tail, while the tails of some ornithischians are equipped with clubs, spikes, and/or plates.^[25]^: 45

caudofemoralis

The caudofemoralis muscles, or Musculus caudofemoralis, are the main locomotory muscles in all long-tailed dinosaurs. Located mainly in the tail, they pull the femur of the hind limb backwards when contracted (femoral retraction/hip extension), thus providing propulsion. In long-tailed dinosaurs, the Musculus caudofemoralis is exceptionally large, and can be estimated at 58% of the total mass of the tail in Tyrannosaurus. Two parts are pronounced in dinosaurs: The Musculus caudofemoralis brevis originated on the lower edge of the hind part of the ilium, while the Musculus caudofemoralis longus originated on the anterior portion of the tail, ventral to the transverse processes and beneath the superficial hypaxial tail musculature. Both parts attached to the fourth trochanter on the back of the femur. The caudofemoralis muscles got reduced during theropod evolution, and are mostly lost in modern birds. Birds, in contrast to long-tailed dinosaurs, do not rely on femoral retraction for propulsion, but instead retracted the lower leg around the knee joint.^[4]^[29]

centrocoel

The centrocoel is the cavernous marrow cavity inside the centrum of a vertebra.^[30]^: 31, 47

centroprezygapophyseal fossa

The centroprezygapophyseal fossa is a depression on the anterior side of the neural arch pedicles defined by the centroprezygapophyseal lamina.^[31]

centrum

The centrum (plural: centra), also vertebral body or corpus, is a spool- or cylinder-shaped element that, together with the neural arch, forms a vertebra.^[1] In juveniles, both centra and neural arches are separate elements, and fusion of these elements is an important criterion to determine adulthood. The anterior and posterior surfaces of the centrum form the articulation with the centra of the preceding and following vertebra. Centra can be classified based on the morphology of these articular surfaces:

amphicoelous (noun: amphicoely) centra are strongly concave on both ends.
platycoelous (noun: platycoely) are slightly concave at both ends.
amphiplatian (noun: amphiplaty. Also: acoelous) centra are flat on both ends.
procoelous (noun: procoely) centra are anteriorly concave and posteriorly convex.
opisthocoelous (noun: opisthocoely) centra are posteriorly concave and anteriorly convex.
heterocoelous (noun: heterocoely) centra are saddle-shaped at both ends.

Amphicoely is the primitive condition tetrapods. In fishes, the ends of the centra are deeply excavated and connected via a small opening, the passage for the notochord. In reptiles, this type of centrum is present in embryos, and in adult forms of some species; in most species including dinosaurs, centra are more ossified with the notochordal opening closed, improving resistance against compressional forces. Heterocoelous vertebrae allow flexibility while preventing rotation. Procoelous and opisthocoelous centra form concavo-convex (ball and socket) joints, where the convex end, the condyle, fits into the concave end, the cotyle (also: cotyla). This configuration allows for greater stability without restricting mobility. In long necks and tails, this stabilization works best when the convex part is pointing away from the body. In sauropods, vertebrae in front of the sacrum are therefore typically opisthocoelous, while those of the tail are procoelous. As a vertebral column can contain different types of central morphologies, transitional centra with the two ends shaped differently may occur.^[11]^[32]^[33]^[34]

cerebellum

The cerebellum ("little brain") is a dorsal part of the hind brain between the brain stem and the cerebrum and serves in controlling balance, posture, and movement. This part of the brain usually cannot be observed in dinosaur fossils as it is rarely seen on endocasts; an exception is the possible preservation of cerebellar folia in Conchoraptor.^[35]^[36]^[26]

cervicals

The cervicals, or cervical vertebrae, are the neck vertebrae.^[26] Most dinosaurs possessed 9 to 10 cervicals, although higher numbers were achieved in some groups including sauropods, both by increasing the vertebral count and by integrating dorsal vertebrae into the neck.^[25]^: 44

cervical half-ring

A cervical half-ring is a transversally oriented, collar-like row of osteoderms protecting the upper side of the neck in ankylosaurs (the underside of the neck is left bare). An individual usually possessed two cervical half-rings, with the anterior one being smaller than the posterior one. Cervical half-rings are a synapomorphy of Ankylosauria.^[37]

cervical rib

Cervical ribs are bones that attach lateroventrally to the cervical vertebrae. They are plesiomorphic for amniotes (although lost in mammals) and comprise an anterior and a posterior process. Some sauropodomorphs, especially some long-necked sauropods such as Giraffatitan, possessed hyperelongated cervical ribs with posterior processes overlapping two or three preceding vertebrae. Bone histological analysis has shown these elongated processes to represent ossified tendons, meaning that their ends were connected to muscles. The great length of the processes would have increased the distance between the muscle body and the vertebra the muscle is operating, allowing the muscle to be located close to the body, lightening the neck.^[38]^[39]

chevron

Chevrons, or haemal arches, are bones attached to the underside of caudal vertebrae, forming the ventral surface of the tail. A chevron comprises a left and right part, which are typically fused to each other in a V-shaped fashion, enclosing a large opening that can be seen in front or back view. The openings of multiple consecutive chevrons form the haemal canal of the tail, which protects nerves and blood vessels.^[26]^[25]^: 46

choanae

The choanae (singular: choana), or internal nares (singular: internal naris), are a pair of openings in the roof of the mouth that are continuous with the external nares, or nostrils, forming the nasal passage. The left and right openings of the pair are separated along midline of the skull by the vomer. In dinosaurs, choanae are usually very large and elongate.^[25]^: 39^[40]

cingulum

A cingulum (plural: cingula) is a shelf-like bulge surrounding the base of a tooth crown.^[41]

clavicles

The clavicles, also claviculae (singular: clavicula) or collarbones, are a pair of strut-like and curved bones located above the coracoid in the shoulder. These dermal bones are attached to the acromion of the scapula, and are best seen in front view of a skeleton. Clavicles are infrequently found with dinosaur skeletons, which probably be due to their low preservation potential.^[42]^[25]^: 49

coracoid

The coracoid is a paired bone of the pectoral girdle that is attached to the lower end of the scapula. A flat and in dinosaurs typically semicircle- to square shaped element, it is endochondral in origin. Its bottom margin forms part of the glenoid of the shoulder joint, together with the scapula.^[25]^: 47–49 The coracoid shows an opening on its lateral surface, the coracoid foramen, through which the supracoracoid nerve passes.^[3]

coronoid process

The coronoid process is a bony projection that extends upwards from the upper surface of the lower jaw behind the tooth row. This process serves as an attachment site for muscles that close the jaw; a higher coronoid process increases the lever arm of these muscles, and thus the bite force.^[43]^[26] (see Figure here)

cranial kinesis

Cranial kinesis is the ability of parts of the skull to move against each other at joints within the skull. Though cranial kinesis had been proposed for a number of non-avian dinosaur taxa, a 2008 review found most of these inferences problematic.^[44] Forms of cranial kinesis suggested to occur in dinosaurs include:

streptostyly: The movement of the quadrate around the squamosal at the otic joint. The quadrate would mostly swing parallel to the skull midline, although transverse movements may be involved.^[44]
prokinesis: The movement between the braincase and the facial skeleton around the nasofrontal joint (a joint between the nasal and frontal), or movement between the left and right nasal. This type occurs in modern birds.^[44]
neurokinesis (also: basal joint kinesis): The movement of the braincase against the palate around the basipterygoid joint (or basal joint).^[44]
pleurokinesis: The ability of the maxilla to swing outwards. Pleurokinesis had been proposed as a novel type of cranial kinesis for ornithopods, allowing for the use of transverse jaw movements in chewing. This complex form of cranial kinesis requires, besides the mobility of the maxilla and facial skeleton, transverse movements of the quadrate against the squamosal (a modified form of streptostyly) as well as basal joint kinesis.^[44]

death pose: A death pose, or opisthotonic posture, is a common posture seen in articulated dinosaur and bird fossils where the spine is strongly curved upwards (hyperextended).^[45] Different possible causes are debated, and include opisthotonus (death throes)^[45] and the release of the Ligamentum elasticum interlaminare during decomposition.^[46]
dental battery: A dental battery is a type of dentition in some herbivorous dinosaurs where individual teeth are packed tightly together to form a continuous grinding surface. In the hadrosaurid Edmontosaurus, the dental battery of each half of the upper and lower jaws contains more than 60 rows of teeth, with each row comprising up to three functional teeth stacked upon each other and up to five replacement teeth beneath this stack that would erupt from the jaws once the functional teeth were worn down and shed. The most sophisticated dentition type in dinosaurs, it evolved independently in hadrosaurids, ceratopsians, and some sauropods, and differs in form and function in these separate clades.^[25]^{: 185–186}^[47]
dentary: The dentary is the main bone of the mandible. It is the only mandible bone that bears teeth, and is located anterior to all other jaw bones except in ornithischians, where the tip of the lower jaw is formed by the predentary. At their anterior ends, the dentaries of the left and right jaw are connected together, forming the mandibular symphysis.^[25]^: 40
dentition: Dentition is a collective term for all teeth present within the jaws of an individual dinosaur. The dentition can be homodont, when only a single type of teeth is present, or heterodont in the case of different types.^[48]^{: 232–233} Teeth are continuously replaced during life. Teeth that are erupted and currently in use are termed the functional teeth. For each tooth position, there are typically one or two unerupted replacement teeth at any time, which successively migrate into the tooth socket and replace the functional tooth once the latter fell out. Some herbivorous species may show up to six replacement teeth per tooth position. Tooth turnover started with the resorption of the root of the functional tooth.^[49] Teeth ejected after replacement lack their root, and are called shed teeth.^[41]
deltopectoral crest: The deltopectoral crest is a forward directed bony flange on the upper part of the humerus. An especially long and prominent deltopectoral crest is a dinosaurian synapomorphy, i.e., a feature differentiating the group from other groups. In dinosaurs, the crest measures 30–40% of the length of the humerus. It provided insertion surfaces for muscles of the shoulder and chest (the deltoid and pectoralis muscles, respectively), which, when contracted, drew the arm towards the body.^[25]^: 14
digits: The digits, or fingers and toes, form the distal part of the autopodium, following after the metacarpus of the hand and the metatarsus of the foot. They are identified with Roman numerals from I–V, with I denoting the innermost and V the outermost digit. The individual digits have of one or more phalanges (finger and toe bones).^[1]^: 145
dolichoiliac: Dolichoiliac refers to one of two principal hip bone configurations in early saurischian dinosaurs. The dolichoiliac type is characterised by large front and back extensions of the ilium as well as elongated and narrow pubes and ischia. In contrast, the brachyiliac type is characterised by short ilium extensions and more robust pubes and ischia. The brachyiliac type is typical for basal sauropodomorph dinosaurs of the Triassic, while the dolichoiliac type is characteristic of theropods. Both terms were introduced by Edwin Colbert in 1964.^[50]^[51]
dorsals: The dorsals, also termed dorsal vertebrae or back vertebrae, are the vertebrae of the back region between the cervicals of the neck and the sacrals of the hip. The Nomina Anatomica Avium (NAA) terminology instead uses the term thoracic vertebrae. In mammals, the term thoracic vertebrae refers only to the rib-bearing vertebrae of the chest, while the vertebrae of the lower back, which lack ribs, are referred to as lumbar vertebrae. This subdivision of the dorsal vertebral column is not applicable in dinosaurs, and the NAA term "thoracic vertebrae" refers to the dorsal vertebral column as a whole.^[1]^: 143 Most dinosaurs had around 15 dorsals. In comparison with the cervicals, the dorsals are shorter, their neural spines longer, and their transverse processes more robust and directed outwards.^[25]^: 45

ectopterygoid

The ectopterygoid is a smaller bone of the palate. Paired and dermal in origin, it connects to the jugal laterally and to the pterygoid medially.^[25]^: 39–40

endocast

Endocasts are infillings (moulds) of neural cavities, including the braincase and the neural canal of the vertebrae. They thus can record external features of the neural structures that have been present within these cavities, most importantly the brain. Endocasts are not exact copies of neural structures though, as neural cavities typically contain additional tissue that may obscure the morphology of the neural structure. A natural endocast forms when a neural cavity is filled in by sediment, while artificial endocasts can be produced using casting material.^[35]^: 192

epijugal

The epijugal is a dermal ossification unique to ceratopsians. It caps the bottom end of the downward-facing, triangular flange of the jugal that is typical for the group. In Arrhinoceratops and Pentaceratops, the epijugal forms a distinct jugal horn. The bone can be found in Yamaceratops and all ceratopsians more derived than the latter.^[52]^: 500^[53]^: 538

epoccipital

Epoccipitals are dermal ossifications lining the edges of the frills of ceratopsians. Epoccipitals are distinct bones in juveniles but in adults fuse to either the squamosal or parietal, depending on their position. The epoccipitals of the squamosal are also known as episquamosals, and those of the parietal as epiparietals. Some authors argued against the use of the term epoccipital, as these bones have no connection with the occipital region of the skull, instead preferring the terms episquamosals and epiparietals. Epoccipitals were ornamental instead of functional, and varied widely in shape, forming greatly enlarged spikes in centrosaurines.^[54]^[52]^: 502^[53]^: 540

epipophysis

Main article: epipophyses

Epipophyses are bony projections of the cervical vertebrae found in dinosaurs and some fossil basal birds. These paired processes sit above the postzygapophyses on the rear of the vertebral neural arch. Their morphology is variable and ranges from small, simple, hill-like elevations to large, complex, winglike projections. Epipophyses provided large attachment areas for several neck muscles; large epipophyses are therefore indicative of a strong neck musculature. The presence of epipophyses is considered a synapomorphy of dinosaurs.^[25]^: 14

epiossifications

Epiossifications (also spelled epi-ossifications) are accessory ossifications attached to the skulls of ceratopsians.^[54] They include:

The rostral, an additional bone in front of the premaxilla of the upper jaw.
The epinasal, which forms the horn core of the nose horn.
The epijugals, a pair of bones which often form prominent cheek horns.
The epiparietals and episquamosals, small ossifications lining the edges of the neck frill (the parietal and squamosal bones, respectively). These ossifications are also collectively termed as epoccipitals.^[54]

epipterygoid

The epipterygoid is a paired endochondral bone at the rear part of the palate. A small, plate-like bone, it connects with the pterygoid and the braincase.^[25]^: 39–40

exoccipital

The exoccipital is a paired bone that, together with the unpaired supraoccipital and basioccipital, forms the occiput at the back of the skull. In adult dinosaurs, the exoccipitals are fused to the opisthotic, forming a structure also known as the exoccipital-opisthotic. Below, the exoccipitals are fused to the basioccipital.^[20]^[25]^: 38

external nares

The external nares (singular: external naris, also: bony nostrils^[55]) are a pair of external skull openings for the nostrils. Important landmarks in the skull, they are primitively located in front of the antorbital fenestra near the tip of the snout. External nares are sometimes referred to as nostrils; this usage is discouraged, however, as the term nostril is reserved for the fleshy opening of the nasal passage (also termed the fleshy nostril^[55]), which in some cases may lay far outside of the external naris.^[1]^: 139 The openings of the nasal passage into the oral cavity are, conversely, termed internal nares, or choanae.^[25]^: 39 The often enlarged frontmost part of the nasal cavity, directly below the margins of the external naris, is termed the nasal vestibule.^[55]

femur

The femur (plural: femora) or thigh bone is the proximal element of the hind limb. Its proximal head features a large femoral head that is directed medially, forming a 90° angle with the shaft; the head is entering the acetabulum of the hip, forming the hip joint.^[25]^: 15 The upper section of the femur features a number of trochanters (processes for muscle attachment):

The lesser trochanter (also: anterior trochanter) is located on the anterior surface immediately below the head. A feature unique to dinosaurs and their immediate outgroups, it anchored the puboischiofemoralis muscles, which draw the hind leg forwards and inwards, and are therefore crucial for locomotion. Many early dinosaurs additionally have a bony ridge or muscle scar known as a trochanteric shelf, which extends from the base of the anterior trochanter towards the posterolateral edge of the femur. The lesser trochanter in dinosaurs is not homologous to the lesser trochanter of mammals.^[25]^: 19^[4]^: 179
The greater trochanter (also: dorsolateral trochanter) is a ridge or bulge on the lateral surface, opposite and continuous with the femoral head.^[30]^: 47 It provided attachment for both the puboischiofemoralis and the ischiotrochantericus muscles.^[4]^: 181
The fourth trochanter is an extensive flange on the posterior surface of the femur, directed rearwards. In dinosaurs, the fourth trochanter is distinctly asymmetrical, with the proximal part more expanded than the distal part. This is a synapomorphy of Dinosauria. The fourth trochanter provided attachment for the caudofemoralis longus muscle, the most important locomotor muscle that pulls the femur backwards when contracted, providing propulsion.^[25]^: 16^[4]^{: 181–182}

fibula

The fibula (plural: fibulae) or calf bone is a long bone located lateral to the tibia; together, the two bones form the lower leg. Generally more slender than the tibia, it is especially thin in derived theropods and birds.^[1]^: 147^[25]^: 53

foramen

A foramen (plural: foramina) is an opening in a bone for the passage of blood vessels, nerves, muscles, and similar entities.^[26] Nutrient foramina are openings allowing blood vessels to enter the inside of bones to enable nutrient supply to the bone.^[25]^: 224

frontal

The frontal is a paired dermal bone of the skull roof that lies between the nasal in front and the parietal at the back. Frontals are typically extensive and form the upper rim of the orbit as well as large parts of the roof of the braincase.

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