4. Enter the Dinosaur

Knowledge of dinosaur fossils has been with us for millennia. Of course, their true nature was not known. Dinosaur remains and trackways are mentioned in oral and documented accounts throughout the ancient, medieval and later times, giving birth to mythical monsters like the dragon, the griffin, the roc (or rukh) and what not! In this context, an expedition by Roy Chapman Andrews in the Gobi desert in Mongolia is worth mentioning, where he found “bits of dinosaur egg shell, drilled with neat round holes – evidently used in necklaces by primitive peoples” from a site aged back to late Palaeolithic to early Neolithic.

Due to the lack of proper scientific knowledge and prevailing biblical ideas, not much was made of all these evidences. However, the 17^th century, the ‘Age of Reason’ was bold enough to make considerable progress in the field of geology and biology. Freethinking researchers were able to propose the possibility of long-extinct animals.

The beginning of modern dinosaur research can be assigned to 1820, when teeth and bones of the carnivore Megalosaurus and the herbivore Iguanodon and later of Hylaeosaurus were found in England. They were initially thought to be larger versions of modern reptiles. It was Richard Owen who first proposed that these were not usual reptiles. He used the  term ‘Dinosauria’ (Greek deinos meaning “terrible, powerful, wonderful” and sauros meaning “lizard”) to describe these new type of reptiles. However, he described them to be ponderous quadrupedal reptiles.

Fig 4.1- Richard Owen

Based on his descriptions, the dinosaur sculptures in Crystal Palace were made in 1850, which are there till date.

Fig 4.2 - Megalosaurus Statue, Crystal Palace, London

The  first complete fossils discovered were in Europe, some time before the Civil War in America. They were of Scelidosaurus and Compsognathus, followed by Archaeopteryx, with teeth and feathers intact (Germany, 1861).

Fig 4.3- The 'Berlin Specimen', discovered in 1874

This was a turning point in dinosaur research. The presence of both reptilian and avian features caused quite a stir. Also, by that time, Darwin’s theory of evolution was published and scientists were ready to inspect dinosaurs with proper scientific outlook. Gradually dinosaurs discoveries spread out to the United States and then to the rest of the world.

But what are dinosaurs?

To answer this, we must pick up the trail of evolution where we left it— with the archosaurs.

Remember that in the last chapter we talked about diapsids that evolved in the middle to late Carboniferous period. They had two fenestrae (Latin for “windows”) in their skulls. A particular clade of the diapsids (A clade is the collection of all the descendants of a common ancestor, from Greek clados, meaning “branch”), called the Archosauromorpha consisted of a sub-population that developed even more distinguishable features; these creatures are the archosaurs that we had mentioned. The Archosauria contains crocodiles, dinosaurs and birds.

The archosaurs had developed two more openings in the skull; one at the side of snout, right in front of the eye, called the antorbital fenestra and one below the lower jaw, called the mandibular fenestra. The teeth were set in sockets and the femur had larger provision for attachment of muscles. Proceeding a little further from this point, we reach the clade Dinosauria.

The most unique feature of the dinosaurs is their terrestrial locomotion, i.e. how they moved. If you observe a lizard, you will see that it has a sprawling movement. Its limbs sprawl out to either side of its body. That’s why a lizard flexes its body sideways while moving, creating an S shape. Dinosaurs, on the other hand, had an erect posture. The femur had a cylindrical head at a right angle to the shaft, which would go in a socket in the hip bone, making the legs directly under the body, supporting its weight. But the cylindrical head of the femur would allow the leg to move only in forward-backward direction, restricting sidewise movements.

Fig 4.4 - Posture Comparison

Other than this, there are skeletal features that are unique to dinosauria, but we skip them here because they require some anatomical knowledge.

This stance gave the dinosaurs a special edge— running. Air breathing vertebrates with two lungs, which need to flex their bodies during locomotion cannot be good runners. It is because the flexing compresses the lung on one side and expands the other, thus causing irregularity in air supply. This is known as Carrier’s Constraint after David R. Carrier, who made this observation. The English palaeontologist Richard Cowen described this fact in his limerick:

                      
                      The reptilian idea of fun
                      Is to bask all day in the sun.
                      A physiological barrier,
                      Discovered by Carrier,
                      Says they can't breathe, if they run.

At this point, there are two things I would like to point out. Birds are dinosaurs, in the sense that they belong in dinosauria. So, from now on, by dinosaur I will mean the non-avian dinosaurs.

And, dinosaurs were exclusively terrestrial creatures. The marine reptiles that prowled the Mesozoic seas were not dinosaurs. We will come to them in due time.

Now that we know who the dinosaurs were, let us venture further to classify them.

The dinosauria is divided into two orders based on then structure of the pelvic bone— saurischia (lizard-hipped) and ornithischia (bird-hipped). But these terms can be misleading in the sense that, we may be tempted to think that birds evolved from the ornithischians. But that is not the case. Birds are descendants of the lizard-hipped dinosaurs, though they have a reversed pubis. Hips, it appears, do sometimes lie. 

Fig 4.5 - Hip structure comparison

It is not entirely clear which of the two is more primitive. Initially it was thought that the saurischians, with their teeth and claws, resembling more to the archosaurs were the first. But latter evidences have actually nullified this idea.

The ornithischians were a diverse group of dinosaurs, existing throughout the dinosaur era, with remarkable anatomical variability. A distinguishing feature other than the hip bone structure is the predentary, a unique bone capping the front portion of the lower jaw. The ornithischians were herbivores, their backward directed pubic bone allowing for a large stomach and intestines suitable for plant digestion. Also, they had blocks of cheek teeth they used for chewing food, while the cheek held the food during grinding. (At this point, I would like to mention that ‘chewing’ is rare in vertebrates. Their teeth are used mostly to bite and slice food.) Another important feature is a sharp beak in front of the mouth, supported by the predentary, creating an efficient food-gathering mechanism.

Fig 4.6- Typical Ornithischian skull, the predentary is darkened

The ornithischia gives rise to further branches. Some of the remarkable ones are—

Thyreofora: The name means “armour-bearer”. These were armoured dinosaurs like the Stegosaurus and Ankylosaurus.

Fig 4.7- Stegosaurus

Marginocephalia: These were “fringed head” dinosaurs. Boneheads or pachycephalosaurians  and horned heads aka ceratopsians fall under this group.

Fig 4.8- Triceratops

Ornithopoda: The “bird-footed” dinosaurs. They mostly had three-toed feet, just like a bird. An example of an orbithopod is the Iguanodon.

Fig 4.9- Iguanodon

We will talk about these dinosaurs in detail in due time. But presently, let us look at their lizard-hipped counterparts. 

(To be continued)