Anatomical directions (land vertebrates)

 Another of my old articles from my previous website. This one is about the names of the basic anatomical directions. Albeit confusing at first, this jargon is actually pretty useful once you get the hang of it! 

(I introduced the topic with a silly short poem...)

Right is wrong. 

It should be left. 

Left right there. 

Hehe, no, seriously...

There are four main contrasting pairs of terms of directions in anatomy. Note that these apply mainly to tetrapods, or four-limbed vertebrates, to which dinosaurs do belong, but many animals such as molluscs, insects and spiders do not. However, since this site is about dinosaurs, these are the most relevant ones. 

These terms rely on the fact that tetrapods have a distinct head, trunk and tail, four limbs, and clear up and down sides. 

Dorsal means toward and beyond the animal’s back, and ventral means toward and beyond the animal’s belly. Note, however, that most tetrapods have their trunk lying horizontally, unlike in humans (and kangaroos!), meaning that their back faces upward, and their belly downward. Therefore, dorsal and ventral may loosely refer to up and down. So, the ribs would be located ventrally to the spinal column. We could apply this to the skull too, for example by saying that the upper jaw is dorsal to the lower jaw, or that the tooth sockets lie ventrally to the eye sockets. Moreover, stegosaurs are characterised by large plates on their backs, which we could call dorsal plates (in this case, maybe the word dorsal refers more to the fact that the plates sit on the actual back of the animal, but they are nevertheless the most dorsal part of the stegosaur).  

Anterior refers to structures toward the tip of the snout, while posterior is used for things toward the tip of the tail. In simple terms, this means forward and backward, since most tetrapods have their snout facing forward, and so on. Thus, the forelimbs are located anteriorly to the hindlimbs, and the hip region lies posteriorly to the rib cage. Ceratopsians, including Triceratops, have a special beak-like bone (called the rostral bone) attached anteriorly to their upper jaw. Stegosaurs had formidable spikes on the posterior end of their tails. 

Proximal and distal apply chiefly to the limbs (and sometimes to the tail). Proximal means closer to the trunk, while distal means further away from the trunk. Thus, the fingertips are the distal ends of the forelimb, and the femur (thigh bone) is proximal to the knee cap. Many theropods had the distal end of their pubis (a bone in the hip region facing ventrally, or downward) formed like a boot.

Tetrapods, like many other animals, have a clear midline that separates the body from top to bottom into two identical but mirror-imaged halves (the line of symmetry). Referring to this so-called sagittal plane, we use the terms lateral, meaning away from the midline, and medial, meaning toward the middle. The shoulders are generally lateral to the skull, while the tail would be medial to the shoulders. Some nodosaurs (a type of ankylosaur), eg. Edmontonia, had enormous spikes extending forward and laterally from their shoulders. 

These terms can of course be combined. The two subgroups within Dinosauria, Saurischia and Ornithischia, are usually distinguished by the former having the pubis facing anterioventrally (i.e. forward and downward) and the latter having it oriented posterioventrally (i.e. backward and downward). The characteristic neck frill of the ceratopsians can be said to be facing posteriodorsally (backward and upward), while it is also expanded laterally (broadened). In addition, one can refer to structures running along a bone or an axis: theropod teeth had serrations running proximodistally along the edge – that is, they extend from the proximal to the distal part of the tooth. 

How would you describe the direction of Edmontonia's shoulder spikes, using the combined formal terms? 

This system is rather useful, being fundamentally simple (although the words may be confusing if you are not used to them) and concise, and understanding these will surely help you understand more formal texts, especially descriptions. Trust me: once you get the hang of it, you will find it very convenient!