Dinosaur skeletal anatomy

The skeletal anatomy glossary was another part of the old Thescelosaurus website. Given its ancestry, it is dinosaur-focused, but most of the information is applicable to other tetrapods, and it's kind of nice to have all of the information in one place. This page also goes with the anatomical terms of location page.

Skull and jaws

Ceratosaurus from Marsh 1884. Abbreviations: PO=postorbital; PRF=prefrontal; QJ=quadratojugal; SQ=squamosal; 1=naris; 2=antorbital fenestra; 3=orbit; 4=infratemporal fenestra; 5=mandibular fenestra

Edmontosaurus from Marsh 1893. Abbreviations: L=lacrimal; PMX=premaxilla; PO=postorbital; PRF=prefrontal; QJ=quadratojugal; SQ=squamosal; 1=naris; 3=orbital; 4=infratemporal fenestra

I borrowed a couple of Marsh's skulls for diagrams. The Ceratosaurus skull comes from Marsh 1884 (Marsh, O. C. 1884. Principal characters of American Jurassic dinosaurs. Part VIII. The Order Theropoda. American Journal of Science [3rd series] 27:329–340.). The Edmontosaurus skull comes from Marsh 1893, back when it was still Claosaurus (Marsh, O. C. 1893. The skull and brain of Claosaurus. American Journal of Science [3rd series] 45:83–86.). Many of the important bones are labeled in these pictures. Dinosaur skulls are typically rather open affairs, with large fenestrae (holes, numbered here, to be detailed later). They are notably different from mammalian skulls, only partially because of different names for the same bones; mammals have fewer fenestrae, several bones in the rear of the skull were turned into inner-ear bones (see below), and the lower jaw of a mammal consists only of dentaries (besides the teeth). Now, here are some details. I'll generally be referring to them in the plural, to emphasize that most of them are paired bones.

Premaxillae: There are two premaxillae (singular, premaxilla) in a classic dinosaur's upper jaw. Typically they bear a handful of teeth, 3 to 5 in most cases. Ornithischians and some theropods (ornithomimids and oviraptorosauroids especially) lose some or all of these teeth, and sometimes turn them into beaks. In ceratopsians, an extra bone called the rostral is present in front of the premaxillae and forms the characteristic sharp beak. In lambeosaurines, the premaxillae make up a large part of the hollow crest.

Maxillae: Like the premaxillae, there are two maxillae in the upper jaw. Maxillae are the main tooth-bearing bones of the upper jaw, and are typically fairly tall and robust, with an ascending process.

Nasals: The two nasals run up along the dorsal margin of the anterior end of the skull. Ceratopsians sometimes grow a horn out of these bones. With the premaxillae, they form part of the hollow crests of lambeosaurines.

Lacrimals (also lachrymals): Known as the "tear-duct" bones in humans due to location, these bones run in front of the orbits in dinosaurs. Often horns or roughened patches form from these bones, such as the prominent triangular horns of Allosaurus. They can be full of holes as well.

Prefrontals: In front of the frontals one will find the aptly-named prefrontals, which usually form part of the dorsal margin of the orbit.

Jugals: These are the "cheek-bones" of dinosaurs. They can have ornate margins, like those of hadrosaurids, or they can be fairly straight and simple bones. They do not bear teeth. Ceratopsians have short bosses on these bones.

Postorbitals: As their name suggests, these bones are found posterior to the eyes. These bones give rise to the brow horns of ceratopsids. In most dinosaurs, they are triradiate, with a process involved in the dorsal margin of the orbit, another involved in separating the orbit from the infratemporal fenestra, and a third messing about the skull roof region, usually in dubious pursuits with the squamosal.

Frontals: The frontals are bones of the skull roof. They contact the nasals in front, the lacrimals and postorbitals to the sides, and the parietals in back. Majungasaurus, an unusual abelisaurid, has a horn growing from the frontals, and the dome of pachycephalosaurids is formed of these bones and the parietals (frontoparietal dome).

Parietals: The parietals form the upper rear of the skull. With the squamosals, they overhang the rear of the skull in marginocephalians (pachycephalosaurians and ceratopsians). Fused into a single unit, they form the midline of the ceratopsian frill, and fused with the frontals, they form the pachycephalosaurid dome.

Squamosals: These bones form the lateral upper rear corner of the skull. In ceratopsids, they are greatly expanded and form the lateral margins of the frill. In ankylosaurids, they bear triangular horns. In humans, the squamosals have become part of the temporal bones.

Quadrates: To make a long story short, the quadrates articulate with the lower jaw in tetrapods (the aptly named articular bone) except for mammals. The external ears are located posterior to them. In mammals, this bone has become the incus, one of the three bones of the ear (other amniotes, like dinosaurs, have or had only one, the stapes).

Quadratojugals: As their name makes clear, these bones contact both the quadrate and jugal.

Other Skull Bones: There are a number of other skull bones in non-avian dinosaurs, most of which are found in the braincase or the palate. They tend to be mostly of interest to specialists, with a few exceptions. In the rear of the skull are the occipital-group bones, most identified in space by their prefixes. The basioccipital (~base) and the two exoccipitals, which are dorsal to it, form the occipital condyle and some of the rear of the skull. The occipital condyle is a bony knob that articulates with the vertebrae and allows the head to move in a variety of directions. Above the occipital condyle is a large hole known as the "foramen magnum," through which the spinal cord leaves the brain. Finally, above this hole are the supraoccipitals, which form the upper rear of the caudal face of the skull. Anterior to posterior, the palate has paired palatines and pterygoids, with the narrow vomer bones running down the midline. Another group of bones, the palpebrals, are found only in ornithischians. In ornithopods they form stout bars that project over the orbit and probably protected the eye.

Mandible

The mandible has a few large bones and several small bones.

Dentaries: The dentaries typically form the majority of the length of the mandible, and are the only tooth-bearing mandibular bones. The joint they make is called a symphysis. On the medial surfaces are found grooves, one per dentary, called Meckelian grooves. In ornithischians, a bone called the predentary appears at the anterior tip of the dentaries. This bone is always toothless, and gives the ornithischians their other name, the predentatans. The paired dentaries are the only bones left in the lower jaw in mammals, and because they fuse, the whole thing is just called the mandible.

Angulars: These bones form the lower rear corners of the mandible. In ornithischians, they are expanded dorsally, and form a coronoid process. In mammals, they get to be part of the auditory apparatus.

Surangulars: These bones form the upper rear corners of the mandible.

Other Mandibular Bones: The articulars are found at the rear of the mandible, and, as their name suggests, articulate with the upper jaw. In mammals, these bones have become malleus bones, the third ear bone, and the jaw, consisting of only dentaries, contacts the squamosals. The splenials are long bones found on the medial surfaces of the mandibles, and overlap the dentaries, surangulars and angulars. The coronoids, found in ornithischians, contribute to the coronoid processes that provide anchors for their jaw muscles. A few other small bones are also sometimes found.

Fenestrae

There are several prominent paired holes in the dinosaurian skull, although many groups fiddled with the number, either reducing or increasing the number. "Reptiles" are often classified by these holes. Anapsids, such as turtles, have no temporal fenestrae (the holes posterior to the eyes). Synapsids, including mammals, start off with one pair, the infratemporal fenestrae ventral to the postorbitals and squamosals. Diapsids, including dinosaurs, start off with two pairs, the infratemporal fenestrae and the supratemporal fenestrae dorsal to the postorbitals and squamosals. Within diapsids, archosaurs (dinosaurs, crocs, etc.) start off with another pair anterior to the eyes, the antorbital fenestrae. Five fenestrae are numbered in the diagrams.

1. Nares (singular naris): The respiratory passages went through the nares, although the nostrils themselves would have been at the tip of the snout.
2. Antorbital fenestrae: Found anterior to the orbits, these large holes are characteristic of archosaurs. A number of different functions have been suggested for these holes. They are frequently lost in ornithischians.
3. Orbits: The eyes were in these holes. In a number of dinosaurs, a ring of bones called the sclerotic ring supported the eyeball.
4. Infratemporal fenestrae: These holes are found below the contact of the postorbitals and squamosals. They are one of the two pairs of fenestrae unique to diapsids.
5. Mandibular fenestrae: Another archosaur trait, these holes are found in the lower jaw. Again, they are often lost in ornithischians.

Other fenestrae: The supratemporal fenestrae are found above the contact of the postorbitals and squamosals. They are the other pair of unique diapsid skull openings.

Vertebrae

Dinosaur vertebrae, as well as most tetrapod vertebrae once you get past the early experimental phase, are composed of a cylindrical body known as the centrum (or vertebral body), with various bony struts and processes attached to it, known as the neural arch (one way to place it is to know it arches over the passage for the spinal cord).

There are a variety of terms for the various sheets and struts of a vertebra. Some of the more commonly encountered parts include the neural spine, which is the "stick-up" part of the neural arch that makes the exaggerated fin of spinosaurs; the transverse processes, which are the "stick-out-to-the-side" parts of the neural arch; and the zygapophyses, which are the parts that stick out fore or aft to articulate with the vertebrae in front and behind. Logically enough, you've got prezygapophyses on the anterior and postzygapophyses on the posterior. You can think of them as hands gripping each other for support: the prezygapophysis reaches forward and under and the postzygapophysis reaches backward and over. (This also means that if you've got a vertebra with at least one of the four, you can figure out how it was oriented.) Some dinosaurs, particularly those of a titanosaurian persuasion, have an additional vertebral articulation formed by a hyposphene (a processes on the posterior surface of a vertebra) and a hypantrum (a depression on the anterior surface of the following vertebra). Dinosaurian ribs have two articulation processes that meet two articular processes on a vertebra, one on the transverse process called the diapophysis, and one on the centrum called the parapophysis.

Vertebrae in dinosaurs are typically divided into four subgroups:

• Cervicals: the neck verts. The first two are known as the atlas and the axis, helpfully in alphabetical order.
• Dorsals: the back verts (you can do thoracic and lumbar, but it's not that helpful with dinosaurs, who have ribs down the whole trunk, unlike mammals). A fused group of dorsals in the shoulder region is known as a notarium and is found in birds and pterosaurs.
• Sacrals: the hip verts. Together they form the sacrum, a solid unit to support the pelvis. If dorsal or caudal vertebrae are also included in this fusion, you have a synsacrum.
• Caudals: the tail verts. In birds and some nonavian dinosaurs the final few caudals fuse to form a pygostyle, or "parson's nose".

Vertebrae can articulate in a number of ways, and of course there are a variety of terms for various styles. The most commonly encountered have to do with the way the anterior and posterior surfaces of the centra are shaped. (These terms are not to be confused with pleurocoels, which are hollows in vertebrae particularly beloved of sauropods.) Multiple kinds of articulations can often be found in one animal, usually depending on the part of the vertebral column you're looking at.

• Procoelous vertebrae have a hollow or socket (a cotyle for the specific anatomic term) on the anterior face (pro-), and a rounded surface (a condyle) on the posterior to fit the hollow on the next vertebra back; this is a ball-and-socket articulation.
• Opisthocoelous vertebrae are the opposite (round front, socket back).
• Amphicoelous vertebrae have a socket on both sides.
• Amphyplatyan verts are flat on both sides; they are sometimes called acoelous, to go with the -coelous theme.
• Finally, heterocoelous vertebrae have sort of saddle-like surfaces, which prevent twisting.

Dinosaurs restricted the movement of vertebrae in a variety of ways, including ossified tendons (just what they sound like, rods of bone formed from tendons, found especially in ornithischians), really long cervical ribs (especially in some sauropod groups), and really long articular processes on the vertebrae (seen in dromaeosaur tails).

Ribs and Friends

Well, what can you say about ribs? Cervical ribs are typically very short, although sometimes with prongs projecting posteriorly for a significant distance, as in some sauropods. These sauropods often have overlapping cervical ribs, which would have helped stability. Cervical ribs fuse to their vertebrae in some groups. Dorsal ribs are the big curved things that most people identify with ribs. Birds have strut-like bones crossing ribs which are called uncinate processes. The sacral vertebrae have ribs that are firmly fused. There are also caudal ribs, but you don't need to think about them.

Chevrons (or hemal/haemal arches) are the bones seen on the underside of the tail, where two caudals meet. There are articulation facets on the caudals for these bones. The skid-like chevrons of some sauropods are sometimes interpreted as features that aided using the tail as a tripod for rearing.

Gastralia, or "belly ribs," are groups of bony segments found supporting the belly of theropods and prosauropods. In theropods, the midline elements overlap each other at the tips. Besides support, they may have functioned in breathing.

A few dinosaur specimens have bones of the hyoid apparatus, which otherwise is most commonly featured in crime shows for getting broken during strangulation. These bones tend to be lost before fossilization.

Shoulder Girdle

The dinosaurian shoulder girdle is composed of a large scapula (shoulder blade; plural, scapulae) attached to a smaller bone called the coracoid (when fused, called the scapulocoracoid). These two bones have a recessed articulation on their posterior surface, formed by both, called the glenoid, where the arm articulates. Also often present is a clavicle or furcula, overlaying the anterior edge of this assembly. The coracoid of most tetrapods is not the same as the coracoid process of placental and marsupial mammals. Just to make things complicated, the coracoid bone seen in non-avian and avian dinosaurs, crocodiles, etc., is a bone known as the "procoracoid" in the mammalian lineage, and the mammalian coracoid, a distinct bone, fused to the scapula and became the coracoid process. A parascapula, a cartilaginous continuation of the scapula, ossified in at least one dinosaur skeleton, that of Parksosaurus, and several of its relatives have rough surfaces on their scapulae indicating they had them too.

Although not directly part of the shoulder girdle, dinosaurs also had sternal bones (breast bones) (which often are not found), paired in sauropodomorphs and ornithischians, but single in theropods. Largely cartilaginous elements attached to the sternal elements and rib cage were also present, but rarely are preserved.

Hip Girdle/Pelvis

The dinosaurian pelvis is composed of three bones surrounding the acetabulum, or socket for articulation of the hindlimb. In most dinosaurs the acetabulum is open all the way through, but in ankylosaurians the hole is closed, leaving just a deep socket. The ilium (ilia) is the uppermost hip bone, and is typically very prominent, as it anchored powerful leg muscles in life. The anterior-ventral hip bone, the pubis (plural pubes or pubic bones), points anteriorly in most saurischians outside of birds, but posteriorly in ornithischians and birds. Many theropods developed extensive elongations of the distal end of the pubis, called pubic boots. The last hip bone, the ischium (ischia), typically points down and back. In animals where the three pelvic bones fuse into one unit during growth, such as humans, the whole thing becomes known as a hip bone or innominate bone.

Forelimb

The forelimb is composed of several segments. The upper arm bone is the humerus (humeri), usually with an articular head and a prominent deltopectoral crest for the attachment of muscles.
The forearm is formed by the ulna and radius (ulnae and radii). The ulna is typically the stoutest and articulates with the humerus, having a flange of variable size after the articulation called the olecranon process (elbow), a lever for muscles.

The wrist is made up of a variable number of bones (carpals); maniraptorans possess a wrist with one main bone, a fused semilunate (half-moon) carpal. The palm is made up of metacarpals, of varying number but never more than five. These bones attach to phalanges, or finger bones; the final phalanx is usually called an ungual. Digits are numbered beginning with the thumb (just as on the feet beginning with the big toe), sometimes called the pollex. The dinosaurian pollex is often well-developed, with a large claw.

Hindlimb

Again, the hindlimb is made up of several segments. The thigh bone, or femur (femora) is not always the longest bone in dinosaurs, as it is in humans. It typically has a well-offset head on a neck for articulation with the hip, and a variety of flange-like processes for muscle attachments.

The lower leg is composed of two bones, the robust shin bone, the tibia (tibiae), and the less-developed fibula (fibulae). The tibia usually has a prominent process at the knee known as the cnemial process. Unlike most mammals and more derived birds, non-avian dinosaurs do not have a bony kneecap (patella).

Dinosaurian ankles have as their most prominent contributor the astragalus, which usually has a large flange running up the anterior surface of the tibia. The calcaneum, very important in mammals (the calcaneus or "heel bone"), is reduced and articulates with the fibula in dinosaurs. There may be additional tarsals, as these bones are called, but as with the wrist dinosaurian ankles are fairly simple hinges.

The foot is composed of metatarsals, and like the hand, phalanges for the toes, often terminating in claw-like unguals. The innermost toe is sometimes called the hallux, and in many dinosaurs is much reduced. The fifth metatarsal is commonly reduced to a small splint.