Dinosaurs Categorized By Their Biology 

"Focusing on the biological aspect of dinosaurs || Probing what shapes and sizes dinosaurs had, both herbivores and carnivores."

The dinosaurs were these majestic creatures who ruled the world before humans did. With all the archeological information we received, we can now mentally paint a picture of how they appeared, what they ate, their characteristics, and how they lived on this earth. 

In this article, we look into the biology of dinosaurs, touching more specifically on the different shapes and sizes of dinosaurs.

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General classification

There are presently over 700 species of dinosaurs that have been discovered, studied, and named. Paleontologists say that there are more species to be discovered. From these 700 species there, there is a system of categorization created by the English biologist H.G. Seeley in 1887.

His categorization system focuses on the two different pelvis shapes that dinosaurs have, and from that observation, we have the two general groups. The first is called the Saurischia, which means “lizard hips,” and the second is Ornithiscia which means “bird hips.” 

Like most four-legged animals, the dinosaur’s pelvis had a paired structure made of three separate bones on each side connected to the sacrum of their backbone. The ilium, the biggest and superior bone of the hip and pelvis, was attached to the spine. Then you would have the ischium, a paired bone that gives the shape of the lower back section of the hip bone, and the pubis located at the bottom and forms a bone plate. 

The acetabulum, the name for the hip socket, was placed like a hollow cup at the center of each plate. The hip socket was outward-facing and opened in the middle to unite with the thighbone.

Saurischian pelvis

The pelvic bone on Saurischians was triangular when we look at it from the side. The pubis was facing down and forward from the hip socket, and the ischium was facing down and backward. The pelvis, hind leg, and tail muscles were attached like a vertical plate to the ilium. 

There weren’t too many variations with the saurischian pelvis among species. 

The Saurichians are divided into two subcategories: the Sauropodomorph and the Theropoda.

The Sauropodomorph includes:

  • Prosauropoda
  • Sauropoda

The Theropoda includes:

  • Ceratosauria
  • Tetanurae  

Ornithischian pelvis

The ornithischian pelvis still had the sacral vertebrae joined to three separate bones on each side like all four-legged animals. When looking at its overall shape from the side, there is a striking difference from the Saurischian pelvis. This pelvis had a low and long iliac blade (extension of the ilium over the acetabulum level) that rested above the hip socket and was attached to a joint structure of the pubis and ischium, situated further below.

Here, the ischium is thinner and longer and continues downward and backward from the hip socket. From the back, the pubis extended into a longer and thinner pubic area that lay beneath and was parallel to the ischium. It is this arrangement that gives the pelvis its bird-like shape.

The Ornithischians are divided into two subcategories: the Cerapoda and the Thyreophora.

The Cerapoda includes:

  • Ornithopoda
  • Pachycephalosaurus
  • Ceratopsia

The Thyreophora includes:

  • Stegosauria
  • Ankylosauria  

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Secondary factors

The secondary factors that differentiated the Ornithischians from the Saurichians were their limbs, skulls, jaws, and teeth. The shape of their teeth would reveal if a dinosaur was a herbivore or a carnivore. Most dinosaurs from both the Saurischian and Orthiscian groups laid eggs; only a few were warm-blooded and carried a fetus until birth.

Ornithischia dinosaurs were mostly herbivores, and apart from the common pelvic structure, other features which set them apart were their teeth, which were leaf-shaped and crenulated on the top edges. Ornithischians all had a bone that joined their two-layered jaws, a small opening in their skull near the eyes and lower jaw. The pubis had a shaft that extended backward and downward and was closely parallel to the ischium.

Saurischian dinosaurs, however, all shared a long posterior neck vertebra, smaller joints on their trunk vertebrae, and their hands were half the length of their arms or longer in some cases. The second finger on the hand was the longest, the thumb sprouts from a metacarpal bone that is off-balance at the end, making the dumb stick out a bit from the other fingers. Most of the bird species we see today come from the Saurichians group.

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Sizes of dinosaurs

We will now look into the sizes of some dinosaurs to get a mental image of how big and small these creatures were. We’ll also mention some biological features that made them belong to one of the categories or subcategories mentioned.

The Tyrannosaurus (T-Rex)

The Tyrannosaurus is a popular name among dinosaurs, and if we remember, this was the type of dinosaur cast in scary movies for the most part. This dinosaur was a carnivore and measured 46 feet in length and 12 feet high; if he could stand upright, it would measure 21 feet. The T-Rex belongs to the Theropods subcategory from the Saurichians, who were all bipeds. 

They had a similar body shape which consisted of strong hind legs, which served their purpose for body support and movement and were also weight-bearing. They had three toes split in a V-shape, and one toe faced the back and an elevated heel.

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This ornithischian is one of the smallest of the group and only measures three feet and is 1 metre in length. It is so small that most of its bones are hollow. This little was a biped omnivore that travelled on two slender legs, small arms with hands with five fingers that made it easy to hold things. The tail was also very light, which means it was a fast runner and easily avoided being someone’s prey.

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Image courtesy of Conty


The Triceratops is also an ornithischian dinosaur that belongs to the Ceratopsia subcategory. The Triceratops was about 26 feet long and was three meters in height. The Ceratopsia means horn-faced; the neck frill that this dinosaur was almost three meters tall overall. 

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The Diplodocus belongs to the longest dinosaurs that are the sauropods from the Saurischian group. This subcategory is known to have small heads, eat plants, and have very long necks that help them reach tall trees for food. The Diplodocus could hevea a maximum length of 90 feet with a 26-foot-long neck, a tail that was approximately 45 feet long, but its head was less than two meters long. Even though it walked on all fours, its front legs were shorter than the back legs. He had five toes on each foot like us humans.

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With such a large category of species of dinosaurs, it would take quite a while to name and describe all the shapes and sizes of dinosaurs; then, I would be writing a book about it. These few examples show how many very small and how very big and tall some of these dinosaurs were millions of years ago.

Apart from the pelvic structure, which was the most significant trait that differentiated the dinosaurs, secondary features were taken into account, too, such as the teeth, limbs, and skull shape. We hope this information helped with understanding the general biology of dinosaurs, both herbivores and carnivores. 

Picture of Elisha Hosea

Elisha Hosea


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