This glossary explains technical terms commonly employed in the description of dinosaurbody fossils. Besidesdinosaur-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 coverichnological and bone histological terms, nor does it cover measurements.

acetabulum

In dinosaurs, theacetabulum (plural:acetabula) or hip socket is an opening in thepelvis formed by theilium,pubis, andischium that is visible in lateral and medial views. It accommodates the head of thefemur, forming the hip joint. Mosttetrapods show aclosed acetabulum, in which the socket is completely filled withbone, forming a depression. Dinosaurs are unique in showing aperforate oropen acetabulum, where the full extent of the socket is a hole without infilling bone.^[1]

acromion

The acromion is a bony ridge on the lower part of the scapula that functions in providing an attachment for theclavicle.^[2][3]Nodosaurids develop a pronounced spur known as thepseudoacromion, which probably formed an attachment site for theMusculus scapulohumeralis anterior, and therefore isanalogous with theacromion ofmammals.^[4]

adductor fossa

The adductor fossa orMeckelian orifice in reptiles and dinosaurs is the major opening into thelower jaw, located between thetooth-bearing region and the jaw articulation. It opens dorsally, and is laterally walled by thesurangular and medially by theprearticular; as the latter is usually much lower than the former, thefossa is visible in medial view. The floor of the opening is formed by theangular 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 thelung. 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 limitedgas 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 aspneumatic diverticula (singular:diverticulum); these can be numerous and present in most of the body. Some pneumatic diverticula will resorb and penetratebones as an animal grows, creatingfossae (depressions) on the bone surface as well as internal chambers within the bones, a process known aspostcranial 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 thejaws containing the roots ofteeth. 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 asthecodont, and is present throughoutArchosauria, including dinosaurs. Mostsquamates, on the other hand, show thepleurodont condition, where the teeth are fused to the inner sides of the jaw bones, or theacrodontdentition, in which the teeth are fused to the top of the jaw bones without sockets. The thecodont condition was historically used to define aclade, theThecodontia, which is now consideredparaphyletic and thus obsolete.^[7]

angular

Theangular is a dermalbone of thelower jaw. In lateral view, it covers a larger area of the posteroventral region of the jaw, being located behind thedentary and below thesurangular. In medial view of the lower jaw, it is visible below theprearticular. It forms the floor of theadductor fossa and supports the posterior portion of the Meckeliancartilage.^[5]

antorbital fenestra

Theantorbital fenestra is one of the five major openings of theskull, located between theorbit and theexternal naris (nostril). Present in most earlyArchosauriformes (includingArchosauria), 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, theantorbital fossa. Thisfossa can contain additional, smaller openings, namely themaxillary fenestra (also:accessory antorbital fenestra) and thepromaxillary fenestra.^[1]

articular

The articular is the hindmost bone of thelower jaw. On its dorsal surface, it contains theglenoid, adepression into which fits the lower end of thequadratebone of theskull, forming the jaw joint. It is the onlyendochondral bone of the lower jaw, forming directly out of the posterior part of theMeckelian cartilage. In mammals, the articular migrated into the skull, forming themalleus of themiddle ear.^[5]

articulated

Anarticulation is anyjoint betweenbones. 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, adisarticulated 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 anassociated 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 majorbone in theankle. It is located directly below thetibia and medial to thecalcaneum, which sits below thefibula. 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 thevertebral column. It receives theoccipital condyle of the base of theskull, thus forming the connection between skull andspine. 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, theaxis. The atlas in dinosaurs is primitive in comprising three separate elements, reflecting the basal condition in reptiles: theatlantal neurapophysis (plural: atlantal neurapophyses; also: atlantalneural arch) at the top;^[10] the intercentrum at the front and thecentrum at the back. An additional small bone, theproatlas, sits in front of the neural arch.^[11] The centrum part of the atlas is also termed theodontoid.^[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 singlebone.^[11]

arctometatarsal

In thearctometatarsalian condition (from Latinarctus – "compressed"), the middle (third)metatarsal of the foot is pinched between the surrounding metatarsals (the second and fourth) 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 ofcursoriality. Originally, this character was used to define a clade, the Arctometatarsalia, which is now consideredpolyphyletic.^[13][14]

axis

The axis is the secondvertebra of thespine, following theatlas. It is larger than the latter, with itscentrum fused with itsneural arch, and with theneural 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 ismesaxonic. Anentaxonic hand or foot has its dominant digit medially (either digit I or II); this rare condition can be found in sauropod feet. Conversely, theectaxonic 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 termedparaxonic.^{[15]: 47–48}

basal tubera

The basal tubera (singular:basal tuber) orsphenoccipital tubercles^[16] are a pair of tubercle-like extensions on the underside of thebraincase that function as attachment sites for ventralneck muscles. They are formed by thebasioccipital and thebasisphenoid bones, and are strongly pronounced in saurischians but only moderately so in ornithischians.^[17][18]

basicranium

The basicranium is the floor of thebraincase, and formed by thebasioccipital,basisphenoid, andparasphenoid.^[19]

basisphenoid

The basisphenoid bone forms the front part of the floor of thebraincase. Not visible from the outside in an articulatedskull, it is fused to thebasioccipital at the rear and to theparasphenoid at the front. A pair of distinctive processes, thebasipterygoid processes, extend from its underside and articulate with thepterygoids of thepalate.^[20]

basioccipital

The basioccipital is an unpaired bone of theskull, and one of four bones forming theocciput of thebraincase. It is exposed in rear view of the articulated skull. It forms most of theoccipital condyle, a rounded process connecting the skull to theatlas, the firstvertebra of theneck. Above, it is fused with the pairedexoccipitals. It also forms the posterior part of the floor of the braincase, being fused with thebasisphenoid at its front.^[20]

A: right side view. B: left side view.

A: bottom view. B: top view. C: front view. D: rear view.

3D-reconstruction of thebraincase ofMassospondylus in various views, and with individual bones colored. Abbreviations:bo –basioccipital;bs – fusedbasisphenoid andparasphenoid;CN – cranial nerve passage;eo –exoccipital-opisthotic;ls –laterosphenoid;os –orbitosphenoid;pa –parietal (commonly regarded part of theskull roof);pr –prootic;so –supraoccipital;sta –stapes.

bone

Bones are, besidesteeth, the most common dinosaur body fossils. Composed ofhydroxylapatite, 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 theskull, theaxial skeleton (spine andribs), and theappendicular skeleton (girdles andlimbs).^[21] Most of these bones arepaired, with a mirror-inverted counterpart on the other side of the body, orunpaired, 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 theskin such asgastralia,bony scutes, andspikes.^[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 thedermis (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 acartilaginous precursor, whichossifies (turn into bone).^[1]

braincase

The braincase is the part of the skull housing thebrain. In an articulatedskull, it is not visible from the outside except from its rear part, theocciput. A complex structure, it is pierced by numerousforamina 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 andinner ear, with inferences on senses and intelligence.^[20]

calcaneum

The calcaneum (plural:calcanea^[1]) is a majorbone of the ankle (at the rear of the foot), and together with theastragalus forms the upper row of tarsal bones. It is located lateral to the astragalus and distal to thefibula.^[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-ossifiedosteoderms 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 bearserrations.^[25]: 41

carpals

The carpal bones form the wrist orcarpus, which connects the forearm (radius andulna) to themetacarpals of the hand.^[26] The corresponding part of the foot is thetarsus. 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 ornithischianHeterodontosaurus, where it consisted of nine elements. The proximal row consisted of theradiale (below the radius), theulnare (below the ulna), and thepisiform (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 asdistal carpals 1–5. The middle row is represented by a single element, thecentrale.^[27][28]

carpometacarpus

The carpometacarpus is a bony element of the hand consisting of the fusedcarpals andmetacarpals. This structure occurs in modern birds, but was also present in some theropods closely related to birds, and evolved independently inalvarezsaurids.^[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 apygostyle. Although flexible in early dinosaurs, several clades stiffened their tail with the help ofossified tendons (as in many ornithischians) or elongatedprezygapophyses (as in some theropods such asdromaeosaurids). 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, orMusculus caudofemoralis, are the main locomotory muscles in all long-tailed dinosaurs. Located mainly in the tail, they pull thefemur 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 inTyrannosaurus. Two parts are pronounced in dinosaurs: TheMusculus caudofemoralis brevis originated on the lower edge of the hind part of the ilium, while theMusculus 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 thefourth 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 thecentrum 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), alsovertebral body orcorpus, is a spool- or cylinder-shaped element that, together with theneural arch, forms avertebra.^[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 thenotochord. 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, thecondyle, fits into the concave end, thecotyle (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 onendocasts; an exception is the possible preservation of cerebellar folia inConchoraptor.^[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 ofosteoderms protecting the upper side of the neck inankylosaurs (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 asynapomorphy of Ankylosauria.^[37]

cervical rib

Cervical ribs are bones that attach lateroventrally to the cervical vertebrae. They areplesiomorphic foramniotes (although lost in mammals) and comprise an anterior and a posterior process. Some sauropodomorphs, especially some long-necked sauropods such asGiraffatitan, possessed hyperelongated cervical ribs with posterior processes overlapping two or three preceding vertebrae. Bone histological analysis has shown these elongated processes to representossified 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, orhaemal arches, are bones attached to the underside ofcaudal 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), orinternal 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 thevomer. 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 atooth crown.^[41]

clavicles

The clavicles, alsoclaviculae (singular:clavicula) or collarbones, are a pair of strut-like and curved bones located above thecoracoid in the shoulder. These dermal bones are attached to theacromion 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 lowpreservation potential.^{[42][25]: 49}

coracoid

The coracoid is a paired bone of the pectoral girdle that is attached to the lower end of thescapula. A flat and in dinosaurs typically semicircle- to square shaped element, it isendochondral 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, thecoracoid 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 thequadrate around thesquamosal 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 thebraincase 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 thebraincase against the palate around the basipterygoid joint (or basal joint).^[44]
• pleurokinesis: The ability of themaxilla 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 thequadrate against thesquamosal (a modified form of streptostyly) as well as basal joint kinesis.^[44]

death pose

Adeath pose, oropisthotonic posture, is a common posture seen inarticulated dinosaur and bird fossils where the spine is strongly curved upwards (hyperextended).^[45] Different possible causes are debated, and includeopisthotonus (death throes)^[45] and the release of theLigamentum elasticum interlaminare during decomposition.^[46]

dental battery

Adental battery is a type ofdentition in some herbivorous dinosaurs where individual teeth are packed tightly together to form a continuous grinding surface. In the hadrosauridEdmontosaurus, 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 sophisticateddentition 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 themandible. 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 thepredentary. At their anterior ends, the dentaries of the left and right jaw are connected together, forming themandibular symphysis.^[25]: 40

dentition

Dentition is a collective term for allteeth present within the jaws of an individual dinosaur. The dentition can behomodont, when only a single type of teeth is present, orheterodont 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 thefunctional teeth. For each tooth position, there are typically one or two uneruptedreplacement 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 calledshed teeth.^[41]

deltopectoral crest

The deltopectoral crest is a forward directed bony flange on the upper part of thehumerus. An especially long and prominent deltopectoral crest is a dinosauriansynapomorphy, 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 theautopodium, following after themetacarpus of the hand and themetatarsus 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 morephalanges (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, thebrachyiliac 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 byEdwin Colbert in 1964.^[50][51]

dorsals

The dorsals, also termed dorsal vertebrae or back vertebrae, are the vertebrae of the back region between thecervicals of the neck and thesacrals of the hip. The Nomina Anatomica Avium (NAA) terminology instead uses the termthoracic vertebrae. In mammals, the termthoracic vertebrae refers only to the rib-bearing vertebrae of the chest, while the vertebrae of the lower back, which lack ribs, are referred to aslumbar 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, theirneural spines longer, and theirtransverse processes more robust and directed outwards.^[25]: 45

ectopterygoid

The ectopterygoid is a smaller bone of thepalate.Paired anddermal in origin, it connects to thejugal laterally and to thepterygoid medially.^{[25]: 39–40}

endocast

Endocasts are infillings (moulds) of neural cavities, including thebraincase and theneural 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. InArrhinoceratops andPentaceratops, the epijugal forms a distinct jugal horn. The bone can be found inYamaceratops 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 thesquamosal orparietal, depending on their position. The epoccipitals of the squamosal are also known asepisquamosals, and those of the parietal asepiparietals. 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 thecervical vertebrae found in dinosaurs and some fossil basal birds. Thesepaired processes sit above thepostzygapophyses on the rear of the vertebralneural 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 asynapomorphy of dinosaurs.^[25]: 14

epiossifications

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

• Therostral, an additional bone in front of the premaxilla of the upper jaw.
• Theepinasal, which forms the horn core of the nose horn.
• Theepijugals, a pair of bones which often form prominent cheek horns.
• Theepiparietals andepisquamosals, small ossifications lining the edges of the neck frill (the parietal and squamosal bones, respectively). These ossifications are also collectively termed asepoccipitals.^[54]

epipterygoid

Theepipterygoid is apairedendochondral bone at the rear part of thepalate. A small, plate-like bone, it connects with thepterygoid and thebraincase.^{[25]: 39–40}

exoccipital

The exoccipital is apaired bone that, together with theunpairedsupraoccipital andbasioccipital, forms theocciput at the back of the skull. In adult dinosaurs, the exoccipitals are fused to theopisthotic, forming a structure also known as theexoccipital-opisthotic. Below, the exoccipitals are fused to thebasioccipital.^{[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 theantorbital fenestra near the tip of the snout. External nares are sometimes referred to as nostrils; this usage is discouraged, however, as the termnostril 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, termedinternal nares, or choanae.^[25]: 39 The often enlarged frontmost part of the nasal cavity, directly below the margins of the external naris, is termed thenasal vestibule.^[55]

femur

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

• Thelesser 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 atrochanteric 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 thelesser trochanter of mammals.^{[25]: 19 [4]: 179}
• Thegreater 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
• Thefourth 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 asynapomorphy 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

Thefibula (plural:fibulae) or calf bone is along bone located lateral to thetibia; 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

Thefrontal is apaireddermal bone of theskull 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 thebraincase.^[25]: 38

fontanelles

Fontanelles (also:fontanels) are gaps between bones—typically skull bones—that are primarily present in embryos but often close as the animal grows.^[56]: 25 Ceratopsians feature openings in between the frontal and parietal bones called the frontal fontanelles, which represent the openings of the supracranial cavity, a feature unique to the group.^[52]

fossa

Fossa (plural:fossae) is a general term for a depression or groove in bone surfaces.^[26]

gastralia

Main article:Gastralium

Gastralia (singular:gastralium) are dermal bones present within thedermis of the abdomen between thesternum andpubis. Gastralia are primitively present in dinosaurs and can be found in basalsauropodomorphs andtheropods, but have been independently lost insauropods,ornithischians, andneornithines (today's birds). When present, they are arranged in 8 to 21 rows, each of which consisting of four elements: A right and leftmedial gastralium articulate at the body midline in a zig-zag like fashion, while alateral gastralium articulates with the lateral end of each medial gastralium. The first and last row may be fused into a single, bow-like structure termed achevron-shaped gastralium. The use of the term"abdominal rib", which has been sometimes used for gastralia and other abdominal structures, is ambiguous thus discouraged.^[57]

glenoid

The glenoid is the articulation surface formed by thecoracoid andscapula that receives the head of thehumerus, forming theshoulder joint. A concave and circular structure, it is edged by a bony rim, and located on the posterior surface of the coracoid and the posterodistal surface of the scapula.^{[3][1]: 144}

heel

The heel is the rear end of thefoot. The term also refers to the rear end of a footprint, in which case it does not necessarily correlate with the anatomical heel, which is elevated above the ground in many dinosaurs.^[15]: 47

horn

Horns are outgrowths of skull bones covered by akeratinous sheath. The bony core of the horn is termed thehorn core. As the keratinous sheath is generally not preserved in fossils, its presence in a number of dinosaurs was inferred from comparisons to today'sbovids. Other cranial outgrowths or rugosities may be covered by skin structures, as has been proposed for, e.g., the nasal boss ofPachyrhinosaurus and the domes ofpachycephalosaurians.^[58]

humerus

Thehumerus (plural:humeri) is the single upper arm bone, and commonly the largest bone in the forelimb.^{[1]: 145 [25]: 49}

hyposphene-hypantrum articulation

Main article:Hyposphene-hypantrum articulation

Hyposphene-hypantrum articulations are accessory joints connecting neighboring vertebrae. They consist of a process on the posterior side of a vertebra, thehyposphene, that fits in a depression on the anterior side of the next vertebrae, thehypantrum. Hyposphene-hypantrum articulations occur in the dorsal vertebrae and sometimes also in the posteriormostcervical and anteriormostcaudal vertebrae.^[59]

hyoid

Hyoids are bones of the throat region below and behind the skull that supported the tongue. These elements are seldom found in dinosaurs, as they are not articulated with the remaining skeleton and thus are easily lost.^[4]: 155

hypex

A hypex (plural:hypexes orhypices) is theapex of the angle formed between twodigits, or, in other words, the most proximal point of the space between two digits. The hypex can only be identified when soft tissue is present, and is thus mostly employed in the study of fossil footprints.^{[15]: 46–47}

ilium

The ilium (plural:ilia) is the upper and largest bone of the hip. It consists of a main body, theiliac blade, whose medial side is attached to thesacral vertebrae. Ventrally, it forms a thickened, curved ridge overhanging theacetabulum, thesupraacetabular ridge orcrest, which distributes stresses transmitted through thefemur of the hind limb. The anterior expansion of the iliac blade is termed thepreacetabular process (also:preacetabular ala), and the posterior expansion thepostacetabular process (also:postacetabular ala);^[60] both expansions allow for the optimal placement of the origins of limb muscles. In front of the acetabulum, a robust process, thepubic peduncle, is extending ventrally to attach to the pubis, while anischiadic peduncle is extending behind the acetabulum to attach to the ischium.^[61]: 33 Immediately behind the acetabulum on the ischiadic peduncle lies theantitrochanter, an articular surface that extents ventrally onto the ischium and articulates with the greater trochanter of the femur.^{[25]: 19 [62]: 726} The ventral surface of the postacetabular process showed a depression termed thebrevis fossa, where the caudofemoralis brevis muscle originated.^[61]: 21 Bordering the brevis fossa laterally is thebrevis shelf, a ridge running along the ventrolateral margin of the ilium between the ischiadic peduncle and the rear end of the bone.^{[62]: 733–734}

ischium

The ischium (plural:ischia) is one of three bones comprising the pelvis, and located below theilium and behind thepubis. It is a rod-like element with an expanded shelf at its proximal end, theobturator plate, that makes contact with the ilium and pubis (a corresponding obturator plate is also found in the pubis). As the pubis, the left and right ischia are fused along their entire length. The expanded symphysis of the caudoventrally directed shaft is termed theischiadic apron.^[63]: 139 The distal end of the ischium is often expanded into anischial boot. The ventral surface of the shaft featured a longitudinal ridge, theobturator process. While proximally located in basal taxa, the obturator process is located on the distal end of the ischium in coelurosaurs.^[63]: 152

infratemporal fenestra

The infratemporal fenestra (also:lateral temporal fenestra) is a major opening on the side of the skull behind theorbit.^[1]: 140 It is bordered by thepostorbital in its upper front, thesquamosal in its upper back, thequadratojugal in its lower back, and thejugal in its lower front corner.^[25]: 37

intercentrum

Intercentra are part of thevertebral column found betweencentra in thecervical region of some dinosaurs. An intercentrum can often be found associated with the first cervical, though it can fuse to the centrum indistinguishably.^[64]

interclavicle

The interclavicle is a single, rod-like dermal bone located between theclacivles in the shoulder girdle.^[42] A primitive feature in tetrapods, interclavicles are generally assumed to be absent in dinosaurs, having been lost in Dinosauriformes.^[65] The presence of interclavicles in several dinosaur taxa has, however, been repeatedly suggested.^[42]

interdental plates

Interdental plates (also:transverse septa,rugosae^[30]) are bony plates on the lingual (inner) margin of the tooth row, that rise from the alveolar margin of the jaw to fill the region between the teeth.^[66][67]

jugal

The jugal, or cheek-bone, is a skull bone that defines the lower border of the orbit and connects to themaxilla anteriorly and to thequadratojugal posteriorly. Its posterior end is bifurcated at its articulation with the quadratojugal, which is considered asynapomorphy of dinosaurs.^{[1]: 140 [25]: 12}

lacrimal

The lacrimal is a strut-like bone in the skull separating theantorbital fenestra from theorbit. It is pierced by a canal for thelacrimal duct (tear duct), which connected the orbit with the antorbital fenestra. In some species the upper end of the lacrimal features a triangular crest or horn that rises above theskull roof.^{[25]: 37 [17]: 62}

laminae

Vertebral laminae (singular:lamina) orosseous septa (singular:osseus septum) are ridges on vertebrae that divide pneumatic chambers. They originate and terminate on different processes and regions of the vertebra depending on their terminology, and named from their points of start and finish. Laminae are divided into four arbitrary categories based on their terminus –diapophyseal laminae,parapophyseal laminae,zygapophyseal laminae andspinal laminae. They may also be divided intosublaminae if they split along their length, orcomposite laminae if more than one lamina forms a ridge. Example laminae:^[68]

• Spinoprezygapophyseal lamina (SPRL): lamina extending from theneural spine (spino-) to theprezygapophysis (-prezygapophyseal).
• Prespinal lamina (PRSL): lamina extending along the front surface (pre-) of theneural spine (-spinal).
• Posterior centrodiapophyseal lamina (PCDL): lamina extending from the posterior end (Posterior) of thecentrum (centro-) to thediapophysis (-diapophyseal).
• Intrapostzygapophyseal lamina (TPOL): lamina extending between (intra-) thepostzygapophyses (-postzygapophyseal).
• Prezygoparapophyseal lamina (PRPL): lamina extending from theprezygapophysis (Prezygo-) to theparapophysis (-parapophyseal).

laterosphenoid

The laterosphenoid (also:pleurosphenoid) is a wing-like bone of the anterior part of the lateral wall of thebraincase. Not visible from the outside, it is located anterior to theprootic and contacts theparietal,frontal, andpostorbital above. The laterosphenoid as a distinct ossification is unique to archosauriforms; in other reptiles this part of the neurocranial cartilage does not ossify.^{[25]: 39 [17]: 83 [20][26]}

ligamentum elasticum interlaminare

The ligamentum elasticum interlaminare (sometimes shortened toligamentum elasticum) is a ligament composed of unpaired bundles that connect theneural spines of adjacent vertebrae in dinosaurs and birds.^[46] This ligament spans the neck and tail, and in modern birds maintains the concavity of the neck.^[46][69] Release of this ligament during decay was proposed to result in the characteristicdeath poses where the spine is strongly curved.^[46]

limb

The limbs form, together with the girdles, part of the appendicular skeleton. Fore- and hind limbs are very similar in structure, and are segmented into three principal parts.^[1] The upper limb, thestylopodium, consists of the humerus (forelimb) or the femur (hind limbs), while the lower limb, thezeugopodium, consists of ulna and radius (forelimb) or tibia and fibula (hind limb). The distal part is theautopodium, comprising the hands or feet. The autopodium itself is subdivided into three segments: the carpalia (forelimb) or tarsalia (hind limb) form thebasipodium; the metacarpalia (forelimb) or metatarsalia (hind limb) themetapodium; and the phalanges of the digits theacropodium.^[70]: 18 An aberrant terminology was proposed by Günter Wagner and Chi-Hua Chiu in 2001. Here, the autopodium is subdivided into only two sections, themesopodium comprising the nodular elements (the carpus or tarsus), and the acropodium, which under this definition comprises all autopodial long bones, including both the metacarpalia (or metatarsalia) and the phalanges.^[71]

maxilla

The maxilla (plural:maxillae) is the posterior, and usually largest, tooth-bearing bone of the upper jaw. Anteriorly, it connects to thepremaxilla and often contributes to the border of the external naris. Over most of its length, the maxilla forms the ventral limits of theantorbital fenestra as well as the depression surrounding this opening, theantorbital fossa, which is deeply recessed within the maxilla. In derived theropods, the maxilla may feature two smaller, additional openings, the promaxillary fenestra and the maxillary fenestra. At its posterior end, the maxilla articulates with the jugal and the lacrimal.^{[25]: 37 [1]: 139–140}

mandible

Themandible, orlower jaw, is the part of theskull below thecranium. Each half of the mandible, termed amandibular ramus (plural:mandibular rami; from Latin "branch") orhemimandible,^[72]: 157 is connected to thequadrate at the back of the skull to form the jaw joint. At the front, both mandibular rami are attached to each other; the region of attachment is termed themandibular symphysis. The mandible consists of bothendochondral bones, which ossified from theMeckelian cartilage, anddermal bones.^[5] In dinosaurs, only thedentary bearsteeth.^[25]: 40

mandibular fenestra

Theexternal mandibular fenestra is an opening in the lower jaw between thedentary,surangular, andangular bones. It is characteristic for archosauriforms, and present in most groups of dinosaurs.^[25]: 40

manus

The manus (plural: manus), or hand, comprises thecarpals,metacarpals, andmanual phalanges of thedigits.^[1]: 145 The first digit is thepollex (or thumb).^[73]

Meckelian groove

The Meckelian groove is a longitudinal channel within the lower jaw. It housed the unossified remnants of theMeckelian cartilage (also:Meckel's cartilage), from which theendochondral elements of the lower jaw ossified. The anterior part of the Meckelian groove is exposed on the medial side of the dentary close to the ventral margin of the bone, while the posterior part is enclosed by bone on both sides, only to open dorsally through theadductor fossa.^{[5][16]: 44}

mesethmoid

The mesethmoid is a bone in the anterior part of thebraincase. It is located beneath thefrontal bones and in front of thesphenethmoid, and articulates with theinterorbital septum anteriorly. The mesethmoid is composed of a horizontal dorsal plate located directly beneath the frontals from which a median septum (a thin wall of bone at the skull midline) extends ventrally to make contact with thebasisphenoid. As is the case with the sphenethmoid and theorbitosphenoid, the mesethmoid is not ossified in many species and thus rarely preserved; it is however commonly found in birds.^{[74][20][25]: 39}

metacarpal

The metacarpals are the long bones of the hand, forming the palm. Together they are called themetacarpus, and are situated between the carpals and the phalanges of the digits. They are identified with Roman numerals from I–V, with metacarpal I the innermost (connecting to the thumb) and metacarpal V the outermost.^[1]: 145

metatarsal

The metatarsals are the long bones of the foot. Together called themetatarsus (plural:metatarsi), they connect the tarsals with the digits. As is the case with the metacarpals of the hand, metatarsals are identified with Roman numerals from I–V, where metatarsal I is the innermost (connecting to the thumb toe) and metatarsal V the outermost.^{[1]: 147–148}

myorhabdoi

Myorhabdoi (also: myorhabdoid ossifications) are bones found in the tail ofpachycephalosaurs. Multiple myorhabdoi form a meshlike structure surrounding (but not connecting to) the vertebral column which has sometimes been called acaudal basket. Myorhabdoi form within themyosepta (connective tissues separating blocks of muscle tissue, themyomeres), and represent ossified myoseptaltendons. Therefore, myorhabdoi form in the periphery of the muscle tissue below thedermis (and not within the dermis, as is the case inosteoderms). They are also distinct fromossified tendons, which stiffened the tail in other ornithischian dinosaurs, although they may have had similar functions. Myorhabdoi are primarily known fromteleost fish; pachycephalosaurs are the only knowntetrapods where these structures ossify. An articulated caudal basket is known fromHomalocephale.^[75]

nasal

Thepaired nasal is the frontmost bone of theskull roof, and the largest bone of the top surface of the snout. It is situated between thepremaxilla in front and thefrontal behind.^{[25]: 38 [1]: 141}

neural arch

The neural arch is the arch-shaped upper component of avertebra, lying above thespinal canal andcentrum. In adult dinosaurs, the centrum and neural arch typically fuse together, closing up theneurocentral suture which lies between the components in juveniles. The neural arch may host an assortment of bony processes, such as theneural spines,epipophyses,transverse processes (which connect to theribs, andzygapophyses (which articulate with adjacent vertebrae). Dinosaur have complex neural arches, often ornamented with a system oflaminae,fossae, and/orpleurocoels which define air sacs that lie along the vertebrae.

neural spine

Neural spines (also:spinous processes) are processes rising upward from the top surface of the vertebralneural arch. Singular processes, they form a single row along the midline of the spine, with their tips generally being the highest points of the individual vertebrae. Neural spines provide attachment surfaces for muscles and ligaments running atop the spine.^[76]: 8 In some species, they can be greatly elongated to form a sail, or deeply bifurcated so that their top parts form a double row.^[77][78]

neurocentral suture

The neurocentral suture is the separation between thecentrum andneural arch of eachvertebra. Juvenile dinosaurs typically have "open" (unfused) neurocentral sutures, which become "closed" (fused) as they reach adulthood.

nuchal crest

Thetransverse nuchal crest is a ridge at the back of the skull found in many theropods. Oriented transversally (perpendicular to the skull midline), it forms the border between theskull roof and the occipital region. It is mainly formed by the rear margin of theparietals with contributions by thesquamosals. The crest anchors neck musculature, and its rear face may contain fossae for attachment of thenuchal ligament. The transverse nuchal crest is not to be confused with thesagittal nuchal crest, a midline ridge on thesupraoccipital.^{[79]: 82 [80]: 117 [81]: 167}

nuchal ligament

Thenuchal ligament is an elasticligament that stretches from the back of the skull to the rear part of the neck above the vertebral column.^[82]: 354

occiput

The occiput is an area on the back side of the skull.^[26] It consists of four bones surrounding theforamen magnum, the passage for the spinal cord that opens into the brain cavity: thesupraoccipital on top, thepairedexoccipitals on the sides, and thebasioccipital below. Directly below the foramen magnum is a spherical, knob-like process, theoccipital condyle, which articulates with the atlas, the first vertebrae of the neck. The occipital condyle is mainly formed by the basioccipital, with smaller contributions by the exoccipitals on the top edges.^{[20][25]: 38}

opisthotic

The opisthotic is apaired bone of the rear of thebraincase. In adult dinosaurs, it is always fused with the pairedexoccipitals, forming a structure known as the exoccipital-opisthotics. Together with the exoccipitals, the opisthotics form a pair of large wing-like processes extending backwards and sidewards from the braincase, theparoccipital processes. These processes make contact with theparietal,squamosal, andquadrate.^{[20][25]: 38}

orbitosphenoid

The orbitosphenoid (deprecated:presphenoid) is a bone of the anterior part of thebraincase that underlies the forebrain. Apaired bone, the left and right halves enclose the canal for the second cranial nerve (optic nerve). In dinosaurs it only ossifies occasionally.^{[74][20][25]: 39}

osteoderm

Osteoderms are bones forming in thedermis of the skin. They can form plate or spike-like structures in some species, most extremely in the armored ankylosaurs. Small and irregular osteoderms less than 1 cm (or 5–15 mm^[83]) in diameter are known asossicles.^[84][85]

ossified tendons

Ossified tendons are bony rods bracing and stiffening the vertebral column.^[1]: 148 Their presence on the upper sides of the vertebrae is considered asynapomorphy ofornithischians.^[86]

otic capsule

The otic capsule is the thickened side region of thebraincase that contains the inner ear. It is composed of two ossifications, theprootic in front and theopisthotic behind. Between both bones lies thefenestra ovalis, a large opening receiving thestapes.^[17]

pad

A pad is a fleshy swelling on the underside of a hand or foot that is commonly observed in footprints. Depending on the part of the hand or foot they support, they are described aspalm pads,sole pads, ordigital pads.^[15]: 47 A digit may feature discretephalangeal pads (also:digital nodes^{[87]: 112–116}) that can correspond to individualphalanges (finger/toe bones).^[88] In dinosaurs, phalangeal pads typically show anarthral arrangement, with a pad enclosing the joint between two phalanges. Humans, on the other hand, show themesarthral arrangement, were the transition from one pad to the next occurs beneath the joint. A pad enclosing the joint between a metacarpal (or metatarsal) and the first phalanx is termed ametatarsophalangeal pad.^{[87]: 112–116}

palate

The palate is the part of the skull between the left and right upper jaws that forms the roof of the mouth. It consists of one unpaired bone, thevomer, and five paired bones (palatine,pterygoid,ectopterygoid,epipterygoid, andquadrate). The palate features three pairs of openings, namely thechoanae or internal nostrils, thesuborbital fenestrae between the palatines and ectopterygoids, and thesubtemporal fenestrae behind the ectopterygoids, which are continuous with theinfratemporal fenestrae.^{[25]: 39–40}

palatine

The palatine is apaired,dermal bone of thepalate. It contacts thevomer andpterygoid medially and themaxilla andjugal laterally, and forms the posterior margin of thechoanae as well as the anterior margin of thesuborbital fenestrae.^{[25]: 39–40}

palpebral

The palpebral (also:supraorbital,adlacrimal^[89]) is a small bone attached to, or forming parts of, the upper margin of theorbit (eye socket). Forming in the upper eyelid, it is originally derived fromosteoderms. There may be one or several palpebrals at either side of the skull. The presence of a palpebral is a synapomorphy of ornithischians, although palpebrals also evolved in other reptile clades, includingcrocodyliformes.^[90]

parapophysis

Parapophyses are processes on thecentra of the cervical and dorsal vertebrae that articulate with thecapitulum of theribs. They are typically located close to the front margin of the side surface of the centrum, and well separated from thediapophyses on thetransverse processes of the neural arches, which receive the second rib head, thetuberculum. However, the position of the parapophysis gradually shifts onto the transverse process along the spine from the cervical to the posterior dorsal vertebrae, where it may form a single structure with the diapophysis that receives a single-headed rib.^{[76]: 8 [11]: 225}

parasphenoid

The parasphenoid is the only dermal bone of thebraincase. It forms part of the underside of the braincase, being located in front of thebasisphenoid and extending forwards beneath the eyes in an elongated process. This process supported a membrane that extended dorsally to theskull roof, separating the right and left eyeballs. This membrane may be ossified, when it is called aninterorbital septum.^[25]: 39

parietal

Thepairedparietal is the hindmost bone of theskull roof. A dermal bone, it is located behind thefrontals, and roofs thebraincase.^{[25]: 38 [1]: 141 [20]}

parietal fenestrae

The parietal fenestrae are a pair of window-like openings commonly found in the neck frills ofceratopsians. InChasmosaurus andPentaceratops, these openings were especially large.^[91]

pectoral girdle

The pectoral girdle, or shoulder girdle, is the set of bones of theappendicular skeleton that connects to the forelimbs. In the strict sense, the pectoral girdle includes thescapula,coracoid, andclavicle, while thesternum is variously referred to either the pectoral girdle or, together with the ribs, the axial skeleton. Unlike the pelvic girdle, which is rigidly anchored on the spine, the pectoral girdle has no such bony connection but instead is suspended by muscles.^[3]

pelvis

The pelvis, also known as the pelvic girdle, is the collective term for bones of the hip, which connect the vertebral column to the hindlimb. The pelvis includes theilium,pubis, andischium. Its inner surface has a strong connection tosacral ribs of thesacral vertebrae, which may be fused into a sacrum.^[3]

pes

The pes (plural: pedes), or foot, comprises thetarsals,metatarsals, andpedal phalanges.^[1] The first digit of the foot is thehallux, which is short and not contacting the ground in most bipedal dinosaurs, but enlarged and reversed (pointing backwards instead of forwards) in birds.^[2][92]

phalanges

The phalanges (singular:phalanx) are the bones of the digits of the hands and feet. The ultimate phalanx of a digit called anungual and formed the bony core of a horny hoof or claw.^[1]: 145

pineal foramen

The pineal foramen (also:frontoparietal foramen;parietal foramen;postfrontal foramen;interfrontal foramen) is anunpaired opening at the midline of theskull roof present in some dinosaurs, but often closed in adults. Its position is variable; it can be located between thefrontals, theparietals, or at the junction between frontals and parietals. This opening exposed theparietal organ, and thus served forphotoreception.^[93][94][95]

pleurocoel

Pleurocoels are openings on the side surfaces of the vertebra that lead into internal chambers within thecentrum and/orneural arch of the vertebra. A pleurocoel may be a single cavity or a complex of smaller, interconnected cavities.^{[1]: 143–144}

postcranium

The postcranium ("behind the cranium"; plural:postcrania) is the part of the skeleton that follows behind the skull. It is subdivided into theaxial skeleton, which includes the entirety of the vertebrae and ribs, and theappendicular skeleton, which includes girdles and limbs.^[1] Furthermore, individual specimens found without a skull are often referred to as postcrania.

postorbital

The postorbital is apaired bone on the side of the skull that forms much of the back margin of theorbit. In ceratopsians, it is modified to form prominent horns above the eyes.^[25]: 38

postparietal foramen

The postparietal foramen (also:postparietal fontanelle,postparietal fenestra) is anunpaired opening located at the skull midline between theparietal and thesupraoccipital that is found in somesauropodomorphs. As is true for thepineal foramen, it possibly exposed theparietal organ, and thus served forphotoreception.^[96][97][93]

posttemporal fenestra

The posttemporal fenestra (alternative spelling: post-temporal fenestra) is apaired opening located below the rear edge of theparietal of theskull roof and above theparoccipital process of thebraincase. It possibly represents the exit of the occipital ramus of theophthalmic artery.^[98]: 30

prearticular

The prearticular is a bone of the rear portion of thelower jaw. Exposed on the inner side of the latter, it sits in front of thearticular and above theangular. It forms the inner margin of theadductor fossa.^[5]

prefrontal

The prefrontal is a smaller bone on the side margin of theskull roof between thefrontal,lacrimal, andnasal.^[25]: 38

predentary

The predentary is an unpaired bone found in front of thedentaries of the lower jaw that formed the lower beak inornithischians.^[1]: 142 Its presence is considered an ornithischiansynapomorphy.^[99]

premaxilla

Thepaired premaxilla (plural:premaxillae) is the frontmost bone of the upper jaw and forms the tip of the snout. It contains the front part of the upper tooth row, which is continuous with that of themaxilla, which follows behind.^[25]: 36

proatlas

The proatlas is a small paired bone sitting in between theneural arch of theatlas (the first vertebra) and the base of the skull. The elements of each pair are triangular in shape, with the broad end articulating with the neural arch of the atlas, and the acute end with theexoccipital of the skull. Given its small size, the proatlas is easily lost during fossilization, collection or preservation and thus rarely been found.^[11]

prootic

The prootic is apaired bone forming parts of the side surface of thebraincase. Together with theopisthotic, which is located behind it, the prootic forms theotic capsule.^[17] The prootic contains openings for thetrigeminal andfacial nerves, and is internally excavated by sinuses. Its external surface provided rough attachment surfaces for muscles opening the jaw.^[25]: 39

pterygoid

The pterygoid is adermal,paired bone of thepalate. Large and complex, it articulates with both thebraincase and other bones of the palate. At their rear portion, the left and right pterygoid enclose an opening, theinterpterygoid vacuity.^{[25]: 39–40}

pubis

The pubis (plural:pubes) is one of the three bones that comprise the pelvis, and located ventral to theilium and anterior to theischium. Insaurischians, the pubic shaft is primitively directed forward (thepropubic condition). Inornithischians, the original pubic shaft is pointing backwards (theopisthopubic condition); instead, a well-developed anterior ramus orprepubis is present, which can be larger than the posteriorly directed shaft in derived forms.^[100] The left and right pubes are usually fused along their entire length, with the distal portion being expanded, forming thepubic apron.^[63]: 152 In some neotheropods, the distal section of the suture is not continuous but forms a fenestra, thepubic foramen, that is visible in ventral view.^[63]: 141 The proximal part of the pubis features an opening, theobturator foramen, which is located close to the acetabulum and visible in lateral view. The obturator foramen allows for the passage of the obturator nerve. In theropods, the obturator foramen is not completely surrounded by bone as the ventral border is lacking; this condition is termed anobturator notch.^[63]: 143

pygostyle

Main article:pygostyle

A pygostyle is a bony structure composed of the fused hindmost caudal vertebra. It is found inpygostylian birds, where it functions in anchoring long tail feathers.^[26]

quadrate

The quadrate is a large,paired, andendochondral bone at the back of the skull. A columnar structure, its lower end articulates with the lower jaw, forming the jaw joint, while its upper end reaches beneath thesquamosal. The quadrate is part of thepalate.^{[25]: 39–40}

quadratojugal

Main article:quadratojugal bone

The quadratojugal is a bone in the cheek region of the skull that is visible in side view, forming the lower rear corner of thecranium.^{[25]: 37–38}

radius

The radius (plural:radii) is the smaller and anterior of the two bones of the forearm.^[1]: 145

rhamphotheca

The rhamphotheca (plural:rhamphothecae) or beak is a horny (keratinous) covering on the tips of the jaws commonly found in ornithischians, ornithomimosaurs, and birds.^[26][21] As keratin rarely fossilizes, beaks are only preserved as impressions. The presence of beaks is often inferred from the roughened surface texture of the jaw tips, which are strongly vascularized (containing numerous blood vessels).^[2]

ribs

Dorsal ribs orcostae (singular:costa; this Latin term is less commonly used than the English term "ribs"^[1]) attach to both sides of thedorsal vertebrae and provide protection for organs located in the trunk. Ribs arebicapitate (two-headed):^[101] A dorsal head, thetuberculum (plural:tubercula),articulates with thetransverse processes of theneural arch, while a ventral head, thecapitulum (plural:capitula) articulates with thecentrum. Ribs are also present in the neck (cervical ribs) and sacrum (sacral ribs).^[1]

rostral

Therostral bone is a novel bone found inceratopsians. An unpaired element, it caps the front of thepremaxillae at the tip of the snout.^[25]: 36 When used as an adjective, the termrostral refers to therostrum.^[82]: 357 The rostral and other accessory ossifications attached to the skulls of ceratopsians are collectively termedepiossifications.^[54]

rostrum

Therostrum (plural:rostra) is thesnout region of the skull.^[82]: 357

sacrals

The sacrals, or sacral vertebrae, of the vertebral column are located between the dorsal and caudal vertebrae. They are frequently fused together into a structure known as thesacrum (plural:sacra).^[1]: 143 Sometimes one or more dorsal vertebrae are fused to the sacrum into a structure termed asynsacrum.^[48]: 368

sail

Sails (also:neural spine sails) are structures resulting from elongation of the vertebralneural spines, typically in the dorsal,sacral, andcaudal vertebrae. They vary in size and shape, from shallow ridges to tall, conspicuous structures.^[77][102] In some cases, such as inConcavenator, the neural spines form a "hump"^[103] or, such as inIchthyovenator, asinusoidal (wave-like) sail.^[104] The condition is most prominent inspinosauridtheropods,^[102] but also occurs incarcharodontosaurids,metriacanthosaurids,^[77][105] and a fewornithopod andsauropod dinosaurs.^{[106][107][108]}

scapula

The scapula (plural:scapulae), or shoulder blade, is anendochondral bone and the largest element of the pectoral girdle. In adults, the scapula is usually fused to thecoracoid, forming thescapulocoracoid.^[3]

sclerotic ring

Main article:sclerotic ring

A sclerotic ring (also:scleral ring) is a ring of small plate-like bones located oneye within thesclera and around thepupil. The individual plate-like bones are termedscleral ossicles.^{[109][110][25]: 84}

secondary palate

A secondary palate is a roof-like structure separating the nasal airways from the mouth. Primitively in reptiles, thechoanae open into the oral cavity at the front of the mouth, admitting that food items may block the air passages while feeding. A secondary palate, as it is commonly found in mammals, shifts the choanae backwards, allowing for feeding and breathing simultaneously.^[82] In several clades, includingankylosaurians,^[111]spinosaurids,^[112] andmaniraptoriform coelurosaurs,^[79] the secondary palate was bony and termed anosseous secondary palate. In contrast, asoft secondary palate was inferred for some other taxa such asPlateosaurus andStegosauria based on projections and ridges of the palatal bones.^[113][114] An osseous secondary palate is typically formed by shelf-like extensions of thepremaxillae andmaxillae that meet with thevomer at the skull midline. Ankylosaurids developed a novel additional part of the secondary palate that extends forward from thepterygoid andpalatine at the rear part of the palate, and has been termed thecaudoventral palatal shelf (correspondingly, the anterior part formed by the premaxillae and maxillae is also termed therostrodorsal palatal shelf).^[111]

serrations

Serrations are small protuberances arranged in lines along the cutting edges (carinae) of teeth. A closely related term isdenticle (also:dentelures). Both terms have been defined in different ways, with many authors using them as synonyms. Other authors applied the term "serration" to the smaller structures of carnivorous teeth and the term "denticles" to the coarser protuberances seen in many herbivorous teeth. A recent terminological review defines denticles as a complex type of serration that is formed by both theenamel and the underlyingdentine, while the term serration also encompasses structures solely formed from enamel. According to this definition, the protuberances seen in carnivorous dinosaurs should be termed denticles.^[115] A detailed terminology is employed to describe the complex denticle morphology of carnivorous teeth:

• cellae (singular:cella; also:interdenticular space): The spaces between individual denticles.^[115]
• diaphysis (plural:diaphyses): The point where the enamel caps of two neighboring denticles diverge or are closest to each other.^{[116][117][115]}
• ampulla (plural:ampullae): The flask-shaped chamber beneath the diaphysis.^[115]
• operculum: The enamel layer of the denticle.^[115]
• radix: the core of the denticle beneath the operculum.^[115]
• interdenticular sulcus (plural: sulci, also:blood groove): A groove that runs from the spaces between neighboring denticles a short distance onto the tooth surface at both sides perpendicular to thecarina.^[115]
• cauda (plural:caudae): a bulge arising from the base of a denticle and running perpendicular to thecarina onto the tooth surface between two interdenticular sulci.^[115]

skull

The skull encompasses the bones and teeth of the head. It is composed of an upper part, thecranium (plural:crania), as well as a lower part, themandible. The cranium consists ofbraincase, theskull roof, the upper jaw and cheek region; and thepalate. The braincase is almost completely ofendochondral origin, while the remainder of the cranium is predominantly composed of dermal bones. The mandibles are composed of both dermal and endochondral bones. The skull features a number of openings, which are important landmarks in anatomical descriptions. These include theorbit (plural:orbita), or eye socket, which housed the eye, as well as theexternal naris. In addition, the skull featured additional openings, orfenestrae (singular:fenestra), that were typical fordiapsid reptiles: theantorbital fenestra between the external naris and the orbit; theinfratemporal fenestra behind the orbit, and thesupratemporal fenestra at the back of the skull roof.^{[1]: 138–140 [25]: 32–36 [17]}

skull roof

The skull roof is the upper surface of the skull. In dinosaurs, it consists of fourpaired bones: Thenasals,frontals, andparietals are large elements, while theprefrontals are small and situated between the nasal, frontal, andlacrimal. In the rear part of the skull are thesupratemporal fenestrae.^[25]: 38

sphenethmoid

The sphenethmoid is a tube-shaped bone at the front part of thebraincase. Located beneath thefrontal bones, behind themesethmoid, and in front of theorbitosphenoid, it contained theolfactory bulbs, which housed the sense of smell. As is the case with the mesethmoid and the orbitosphenoid, the sphenethmoid is not ossified in many species and thus rarely preserved.^{[74][20][25]: 39}

splenial

The splenial is adermal bone of thelower jaw that is located behind and medial (towards the inside) of thedentary. The splenial covers most of the inner side of the dentary, and is often the most extensive bone of the lower jaw in medial view.^{[25]: 40 [5]}

squamosal

The squamosal is a paired bone at the rear corners of the skull.^[25]: 38

sternum

The sternum, referred to as sternal plates when unfused, is a bone in the pectoral girdle. The sternum is usually cartilaginous in reptiles, so its absence and relationship with theclavicle may not always be known.^[3]

subnarial foramen

The subnarial foramen is a small opening between thepremaxilla andmaxilla in saurischians and some basal ornithischians.^[25]: 23

subnarial gap

The subnarial gap (also termed a "kink") is a gap between thepremaxilla andmaxilla in the snouts ofcoelophysoid,dilophosaurid, andspinosaurid theropods. The subnarial gap results in adiastema, a gap in the tooth row. The space between the subnarial gap of the snout and the upwards curving tip of the mandible is called thesubrostral notch.^{[118][119][120][121]} InDilophosaurus, the gap contained a deep excavation behind the toothrow of the premaxilla, called thesubnarial pit.^[118]

supraoccipital

The supraoccipital is an unpaired bone of theocciput at the back of the skull. It forms the top margin of theforamen magnum, and connects dorsally to theparietal bones. It frequently shows a midline ridge for insertion of neck muscles.^{[20][25]: 38–39}

supratemporal fenestra

The supratemporal fenestra (also:upper temporal fenestra^[122]) is an opening on the rear top of the skull.^[1]: 140 It is typically bordered by thepostorbital,squamosal, andparietal.^[25]: 38

surangular

The surangular is a dermal bone in the lower jaw that is visible in side view, being located behind thedentary and above theangular.^[25]: 40

syncervical

The syncervical is a structure composed of the fused first threecervical vertebrae in members ofNeoceratopsia. While some interpretations have the first four cervicals fused into the syncervical, evolutionary development inAuroraceratops,Leptoceratops andProtoceratops among other ceratopsians show that the syncervical fuses the first three cervicals and the first twointercentra.^[64]

tarsals

The tarsals are a set of bones in the hind limb between the metatarsals below and thetibia andfibula above.^[1]: 148 They are collectively called thetarsus. The tarsus consists of an upper series formed by theastragalus andcalcaneum, and a lower series of small bones sitting atop the metatarsals. The upper and lower series can move against each other, forming theankle joint.^[25]: 53

teeth

In dinosaurs, teeth are present on themaxilla andpremaxilla of the upper jaw and thedentary of the mandible.^[25]: 40 Only the very basal dinosaursEoraptor andEodromaeus bear rudimentary teeth on thepterygoid of thepalate (palatal teeth).^[123] A jaw bone without teeth is denoted asedentulous.^[1]: 140 Teeth can be divided into two parts: thecrown, which is the upper, exposed part, and theroot, which is nested within the gums.^[48] Teeth are primarily composed of two tissue types, enamel and dentin.Enamel forms the hard and shiny outer layer, while the softer and bone-likedentin fills the inside. Occasionally, a third type,cementum, can be found atop the enamel layer.^[41]

tibia

The tibia (plural:tibiae) is the larger of the two bones of the lower leg. It is located medial to thefibula. The tibia features a prominent ridge extending from its front surface at its upper end, thecnemial crest (also:tibial crest). The cnemial crest anchored the tendons of the triceps femoris muscles, which were responsible for knee extension (stretching of the leg), and increased the leverage of these muscles.^{[4]: 178 [1]: 147}

thagomizer

Athagomizer is the four to ten tail spikes seen in stegosaurians. The term was coined in 1982 by cartoonistGary Larson.^[26]

transverse process

Transverse processes are wing-like projections extending sidewards from the neural arches of the cervical, dorsal, sacral, and the frontmost caudal vertebrae. Their typically horizontal orientation results in the characteristic T-shape of the vertebrae in front or back view. At its tip, a transverse processes articulates with thetuberculum of itsrib; the broadened articulation area for the rib is termed adiapophysis (plural:diapophyses).^[25]: 42 Some authors, however, denominate the entire transverse process as the diapophysis, using both terms as synonyms.^{[11]: 224–225 [76]: 8}

ulna

The ulna (plural:ulnae) is the larger and more posterior of the two bones of the forearm.^[1]: 145 The upper end of the ulna often shows a large process extending past the elbow joint, theolecranon process, to which the triceps muscles (forearm extensor muscles) were attached. The sometimes great elongation of the olecranon process provided these muscles with increased leverage.^{[25]: 50 [4]: 173–174}

vertebrae

Vertebrae (singular:vertebra) are the bony components of the vertebral column (i.e. the backbone or spine). In dinosaurs, the vertebrae are regionalized intocervicals (neck vertebrae),dorsals (trunk vertebrae),sacrals (hip vertebrae) andcaudals (tail vertebrae). Each vertebra has a roughly spool-shaped lower component, thecentrum, and an arch-shaped upper component, theneural arch. Between these components is a tube-shaped tunnel, thespinal canal. The spinal cord of the nervous system runs through the spinal canal of each vertebra in the backbone. The cervical, dorsal, and sacral vertebrae connect toribs viatransverse processes on the sides of their neural arch. The caudal vertebrae connect tochevrons on the underside of their centra.

vomer

The vomer is anunpaired, narrow bone of the front part of thepalate that separates the left and rightmaxillae andchoanae.^{[25]: 39–40}

Zahnreihe

AZahnreihe (German for 'tooth row', pluralZahnreihen) is a series of tooth positions that form a replacement unit. A tooth row typically consists of severalZahnreihen containing two or more teeth each. Tooth replacement always starts at the rearmost tooth position of aZahnreihe and propagates frontward. This way, it is avoided that two neighboring tooth positions are replaced at the same time, which would leave gaps in the tooth row. The number of teeth from oneZahnreihe to the next is theZ-spacing. When the Z-spacing is exactly 2, teeth are replaced in an alternating pattern; if Z-spacing is greater than 2, teeth will be replaced in sequence.^[124]

ziphodont

Ziphodont refers to teeth that are blade-like, with recurved and flattened crowns that typically bear serrations. Adapted to a carnivorous diet, ziphodont teeth are primitively present in dinosaurs, and are found in most non-avian theropods.^[41]

zygapophyses

The zygapophyses (singular:zygapophysis), orarticular processes, are projections of the vertebral arch that connect adjacent vertebrae. Each vertebra has two pairs of zygapophyses, which are articulating with the zygapophyses of the following and preceding vertebra: At the front are theprezygapophyses (also:cranial zygapophyses), which are extending forwards and have their articular surface facing upwards. At the back of a vertebra are thepostzygapophyses (also:caudal zygapophyses), which are extending backwards with their articular surface facing downwards.^[1]: 143

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