Let’s start doing the ‘action line’ that includes the head and spine.
Here I divided the spine in three segments, for the neck, back and tail. The ‘tail’ is a bit more than the half of the total lenght of the spine, and the ‘back’ is a little longer than the ‘neck’.
The neck presents a soft ‘S’ shape, while the back is an arc. The tail wasn’t very flexible, but it continues the line of the back.
For the head, these simple shapes will be enough, but I always recommend to look carefully at a skull before drawing.
This is a very simplified diagram, the real spine is composed by several bones, the vertebrae, and it is divided in four segments: cervical (neck), dorsal (back), sacral (hip), caudal (tail).
Once we have the spine, the next step is to do the legs. We will use bars and nodes for this.
The first node (big ellipse) is the illium, the hip bone. The bar under the hip is the femur, the thigh bone. The following node represents the knee joint, and the bar under the knee is the lower leg (tibia and fibula). The next node is the ankle joint (including the heel), and finally the lower bar is the foot, divided in two segments: the upper is the foot/metatarsals, and the lower segment are the toes/phalanges (mind you, the dinosaurs walked in their toe bones, just as the birds).
The height from the hip to the ground is more or less 1/3 of the total pine lenght. The knees are placed aproximately at 1/2 of the height from the ground.
Now we have the basic skeleton lines. Next thing to do is to add the volumes that form the body mass.
Start with an ellipse that covers a bit of the neck, all the back, the hips, and a part of the tail (and down to the knees). Make it somewhat pointy on the chest region.
Neck and tail: two muscular parts of a dinosaur, they follow the curve of the spine.
Limbs: pay attention, this is the part where most people commit mistakes.
We have already defined the parts of the leg, now let’s add the muscles.
It is hard to explain the correct placement without using anatomical terminology, but I think that with this diagram will be enough. See how the thigh muscles are attached to the illium and around the knee. The lower leg is more bulky near the knee joint (specially behind it) due to the presence of muscles.
Outline the feet, too.
Arms: ridiculously tiny, at the sides of the body, oriented backwards.
Now you have all the major volumes chained, it is time to draw the outline. Just follow the shape of the muscles, softening the transitions.
For the legs, remember that they are attached to the body and covered by the skin, so they will look integrated to the body at half way between the knee ant the hip.
The feet have four digits: the I is the the tiny dewclaw pointed backwards. The other three (II, III, IV) will support the weight of the animal, but being III the larger, II or IV will be hidden (depending the side) in this view.
The (weird) arms of a Carnotaurus were really small, and their hands possesed four digits.
Finally work in the head, add the eye under the horn, the tiny teeth and the muscles of the jaw and neck.
Once you have the outline, you may want to color the Carnotaurus.
Depending the media you use and your experience, you can paint a solid base or directly go to step 8...
(remember to erase the sketch lines, or transfer the ouline to another sheet of paper).
Volumes. The illusion of volume in 2D drawings is made with lights and shadows.
Let’s suppose that the light comes from above (example: sun).
Then the surfaces in direct contact with that light will be lighter (Use a lighter color paint, or erase softly the pencil base).
The surfaces hidden from the light will look darker (use a darker paint or more pressure on the pencil to make the shadows).
If you are using pencils, you can use a tissue paper or a cotton to blend the strokes.
Optionally, you can equip it with some pattern as camouflage or display. In this case, some big blurry circles will do it.
If you have enough patience (it took me an hour or so), add the texture to the skin. In this case, the skin is scaly (remember than several dinosaurs were covered on feathers, but not this case).
Small circles will do it, but again, take your time and try to not change greatly the size of the circles and the pressure of the pencil
A more advanced technique to improve the realism is to add volume to every single scale: with a withe pencil, highlight a point on the upper half of the circles.
You can keep adding details as the tongue, or adding a background. Something that gives good results when you want to add a background/environment, is to cut carefully the silhouette of the dinosaur and paste it in a previously painted scene like this...
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