Directional Terms and Anatomical Glossary

    In order to understand dinosaurs beyond the level of remembering names, places and times, one should be able to read the special and sometimes arcane terminology that scientists use to describe things.  This includes directional terms, which orient you in space, and anatomical terms, which tell you what you're looking at.  The two go together; if you ever peruse a technical work on dinosaurs, you'll be greeted with phrases like "preacetabular process of the dorsoventrally compressed ilium."  I can't promise to give you everything you need to know to figure out technical works (especially since there are things that give me pause), but I can offer you a good foundation of terminology.

Directional Terms

    Directional terms can be very confusing, which is unfortunate because they're so common.  I have a habit of writing in pencil above the word the approximate translation, in case I come back later with my terms confused.  I am going to capitalize the terms so they stand out, but this is not how they are written (unless of course they start a sentence).  There are three major whole-body groups of terms and a few other smaller groups.  (note-I reworked most of these definitions from the introduction of The Dinosauria [Weishampel, Dodson, and Osmolska, eds., 1992 edition])

    Several of the basic terms you may encounter vary depending on the identity of the animal in question.  This is because one set of terms originated with human anatomy, and another set originated with the anatomy of animals that do not walk with an erect spine.  As is typical, there is some mixing and repurposing of terminology.
    Anterior (on a human)=Ventral=toward the belly
    Posterior=Dorsal=towards the back
    Superior=Cranial=toward the head
    Inferior=Caudal=toward the tail (or ground, in our case)
    Note, though, that paleontologists may use Anterior instead of Cranial, and Posterior instead of Caudal.  This muddles with the original definitions, but has the advantage of not including the potentially-confusing Cranial and Caudal.
    These terms are used relatively as well, i.e. x-feature is more dorsally-positioned on a vertebra than y-feature.
    When we get to the skull, Cranial no longer applies (you're already there).  Thus, Rostral, referring to the tip (rostrum) of the head, is its replacement.
   
    Another set of directions is used when we consider the midline of an animal.  Suppose we observe an animal along the center of the back, along the animal's length.  Features close to the midline of the body are designated Medial (toward the midline), in opposition to features farther from the body's midline, which are designated Lateral (away from the midline).
   
    The third and final main set of terms is used with the limbs and tail.  The term Proximal refers to features or bones toward the mass of the body, while the term Distal refers to features away from the body.  Thus, the hand of our friend is considered Distal compared to the upper arm, and in the hand itself the palm bones (metacarpals) are considered proximal compared to the fingertip bones (unguals).  This works the same way with features: A limb bone with an expansion on the end farthest from the body can be said to have distal expansion.

    A number of other groups of terms exists as well.  For example, the surface of a hand or foot in contact with the ground is known as Palmar or Plantar, respectively, and the opposite surface is known as Dorsal for both extremities.  In teeth, the edge of the tooth facing the front of the jaw is known as Mesial, while the opposite side, facing the rear of the skull, is known as Distal.  The surface of a tooth facing the inside of the mouth is known as Lingual, while the surface of a tooth facing away from the oral cavity is known as Labial (in reference to lips) or Buccal (in reference to cheeks).

Let's go back over what we know:

Group One:

Group Two:

Group Three:

Other Terms: Hands\Feet

Other Terms: Teeth

Next we will cover anatomy, including most of the bones of the skeleton (the numerous itty-bitty bones of the skull's braincase will be overlooked for the moment).

This restoration is of the skull of the abelisaurid Indosuchus (drawn after the restoration in Chatterjee and Rudra, 1996).  Many of the important bones can be seen labeled in this picture.  As can be plainly seen from this and museum mounts, dinosaur skulls are typically rather open affairs, with large fenestrae (holes, grey and numbered here, to be detailed later). They are a good deal different from mammalian skulls, only partially because of different names for the same bones; mammals have less 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 (note: I'll be referring to them in the plural, as 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.  Some dinosaurs, like the sinraptorids, add holes in the maxilla.
    Nasals:  The two nasals run up along the dorsal margin of the rostral 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 (not seen here, as abelisaurids must not allow their prefrontals to contribute to the visible orbital margin):  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 straight bones.  They do not bear teeth.  Ceratopsians have short bosses on these bones
    Postorbitals:  As their name suggests, these bones are found to the rear of 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 (on top of this skull, out of view):  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.  
    Quadrates:  The quadrates help to brace the upper jaw.  Behind them are located the external ears.  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 their quadrate and jugal.
    Other Skull Bones:  There are a number of other skull bones, many of which call the braincase their home and are very obscure.  Less-obscure, important skull bones are found in two general areas; the palatial region, and the rear of the skull.  From the front to the back, the palatial region has paired palatines and pterygoids, with the narrow vomer bones running down the midline.  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 above 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.  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:  These two bones 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 rostralmost 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 of the lower jaw in mammals.
    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 holes in the dinosaurian skull, although many groups fiddled with the number, either reducing or increasing the hole complement.  "Reptiles" are often classified by these holes.  Five are numbered in the diagram.
    1. Nares:  Through these holes the dinosaurs breathed.
    2. Antorbital fenestrae:  Found in front of the orbits, these large holes are characteristic of archosaurs, a group dinosaurs (and crocodiles, who have rewalled this hole) belonged to.  A number of different functions have been suggested for these holes.
    3. Orbits:  The eyes were in these holes.  In a number of dinosaurs, a ring on 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 diapsid "reptiles."
    5. Mandibular fenestrae:  Another archosaur trait, these holes are found in the lower jaw.
    Other holes:  The supratemporal fenestrae are found above the contact of the postorbitals and squamosals.  They are the other pair of unique diapsid skull openings.

A Brief List of Other Bones:
    
Vertebrae: 
    The bones of the vertebral column-they are composed of a cylindrical body known as the centrum, with a variable series of bony struts 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 the vertebra, and for the various modes of articulation, which I will further detail at a later date.  Various sauropod groups got the most creative with verts.
    Vertebrae in dinosaurs are typically divided into four subgroups:
    1. Cervicals-the neck verts
    2. Dorsals-the back verts (you can do thoracic and lumbar, but it's not as helpful with dinosaurs, who have ribs down the whole trunk, as it is with mammals)
    3. Sacrals-the hip verts
    4. Caudals-the tail verts
Each general type has its own characteristics that help to identify them.  What are those, you may ask?  Go and do your own research!  I'm not going to tell you every single thing!

Ribs and Friends:
    Well, what can you say about ribs?  Cervical ribs are typically very short, sometimes with prongs projecting caudally, as in some sauropods.  These sauropods often have overlapping cervical ribs, which would have help stability.  Dorsal ribs are the big curved things that most people identify with ribs.  
    Chevrons 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 designed to aid 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.

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 an articulation on their caudal surface, formed by both, called the glenoid, where the arm articulates.  Also often present is a clavicle or furcula, overlaying the leading edge of this assembly.
    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:
    The hip girdle is composed of three elements surrounding the acetabulum, or hole for articulation of the hindlimb.  The ilium (ilia) is the uppermost, and is typically very prominent, as it anchored powerful leg muscles in life.  The cranialmost of the lower hip bones, the pubis (pubes), points cranially in most saurischians, but caudally in ornithischians.  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.

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 always less than five.  These bones attach to phalanges, or finger bones; the final phalange 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.

The 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 flanges for muscles.
    The lower leg is composed of the robust tibia (tibiae) and less developed fibula (fibulae).  The tibia usually has a prominent process at the knee known as the cnemial process. 
    Dinosaurian ankles have as their most prominent contributor the astragalus, which usually has a large flange running up the cranial surface of the tibia.  The calcaneum, very important in mammals, is reduced and articulates with the fibula.  There may be additional tarsals, as these bones are called.
    The foot is composed of metatarsals, and like the hand, phalanges terminating in often claw-like unguals.  The innermost toe is sometimes called the hallux, and in many dinosaurs is much reduced.