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)

3. Life Finds a Way

Richard Dawkins has dubbed evolution “The Greatest Show on Earth”. Truly, the origin of life, much of which is still unclear, and the advancement of it with the most surprising modifications is too vast a concept to fathom. Indeed, even the very ‘definition’ of life is a much debated topic among scientists and philosophers. But it is generally agreed upon that living things consume materials and energy, display growth, and reproduce.

But how did it come to be as it is? ‘Life’ on earth began 3.8 billion years ago and has undergone constant modifications and alterations, sometimes for better, sometimes for worse. Life forms that were equipped to go along with the environment survived, others perished. And over this vast range of time the environment underwent a lot of changes forcing the organisms to change accordingly for survival. In the game of survival, staying ‘fit’ is the mantra.

Before we follow the path of life, let us quickly go through some key concepts which enable the scientists to determine the course of evolution and to understand the relations between the diverse biosphere of the earth. This will come in particularly handy when we talk about dinosaurs.

When an organism evolves from another with new features to suit its environment, it does not mean that it has severed all ties from its ancestors. It has genetic connections with its ancestors. And this is where the principle of homology hits the mark, which basically says that the genetic relationship among organisms characterise genetic relationship among their parts. Such anatomical structures are called homologs and they can be traced back to a parent anatomical feature in a common ancestor.

                                     Fig 3.1- Homologous anatomical structures

 
Figure 3.1 makes the structural similarity clear. From this we conclude that the similarity between a seal’s flipper and a human hand is not an independent occurrence but an indication of the fact that they share a common ancestor somewhere back in time. In other words, they are just two different branches which can be traced back to a single node in the tree of life. They have their forelimbs modified according to their environmental needs. These structurally similar organs may serve different purposes in different organisms, as compared to analogs which serve the same purpose but are structurally different.

                                                 
                                            Fig 3.2- Analogous organs

Now, as we have already learned that fossils are very rare, it is understandably much rarer that a fossil found is an ancestor of some other fossil. But what we may get from the fossil is some features that the ancestors might have possessed. Thus we get an approximation to how the ancestor was like.

The next important concept is that of phylogeny. It would not take an evolutionary biologist to conclude that a dog and a wolf are more closely related to each other than a dog and a horse. The common sense we are applying is that more specific features shared imply closer relation. Phylogenetic systematics looks for unique features to infer about relationships between organisms. This is done using the knowledge of hierarchy.

All features of living organisms are hierarchically distributed. To take an example, all organisms with feathers form a subset of all creatures with a spinal cord, which is a subset of all living beings. So we recognise birds as creatures with feathers. So, it is easy to tell that a sparrow and a crow are more closely related than a crow and a bat. But if we were to determine this same relationship among an eagle, a hawk and a duck, the feature of having feathers would be useless and we would have to go down deeper in the hierarchical ranking looking for other features to conclude that the eagle and the hawk are closer. Scientists look at these features and carefully set up the hierarchy and on the basis of that try to draw a picture of the evolution. This process is often quite difficult with the volume of features to deal with, and a wrong hierarchical set up would give an erroneous picture.

Now that we have a high level idea of how scientists infer about evolution, let’s see what they have inferred about evolution.

I have mentioned earlier that the biosphere is 3.8 billion years old. However, we will omit discussions about molecular life as it requires considerable training in relevant disciplines, making it somewhat beyond the scope of our discussion. We can only mention that we eventually had bacteria and other single-celled organisms, which gradually changed into multi-cellular organisms. This is the Precambrian period, a time also known as Cryptozoic (“hidden life”). About 590 million years ago (the unit ‘million years ago’ is abbreviated as Ma), the Precambrian period came to end and is the start of the Cambrian period. It is from this time proper fossil records are found. And the turn evolution took in this period is called “Cambrian explosion”.

Some organisms at this time developed the ability to absorb mineral calcite from the seawater and form chitin, giving rise to the first shelled creatures. These creatures now had an advantage over its predators. Consequently, some predators developed new structures to even out the odd. And the evolutionary arm race had begun.

The early life forms were all marine. The Cambrian oceans were full of diverse creatures with unusual features most of which did not survive towards the end of the Cambrian period (505 Ma). Only a few did. These creatures were the pioneers of the stable evolutionary lines. There was the Trilobite, an early arthropod.

                                                Fig 3.3- Trilobite fossil

Also, there were early chordates, creatures with spinal cords, like the Pikaia. The time from the Cambrian to present day is termed phanerozoic (“obvious life”).

   Fig 3.4- Artist's impression of Pikaia

The periods that followed the Cambrian time were the Ordovician and Silurian times (505-408 Ma), the time when first vertebrates arrived. These were fish, but they had not formed a jaw yet, they had suckers to collect food from the ocean floor. Then gradually they developed well-structured skeletons made of cartilage, very much like today's sharks. Later formed bones and scales, very similar to the fish we know today. This was the period Devonian (408-360 Ma), “the Age of Fish”. 

                                    Fig 3.5- Dunkleosteus, armoured fish            

Something very significant occurred in this time. Plants, the pioneers of life on land, began colonising the landscapes. They could flourish due to the absence of land herbivores. And they made lands inhabitable for other beings by increasing the oxygen content in the air. Arthropods infested those forests and then followed the fish. Some fish developed lungs to be able to breathe out of water, some even developed muscular fins to be able to drag themselves and crawl on land as well. It is suggested that this ocean to land journey occurred in temptation of new food source. A rival theory says that it was a measure to avoid the predator infested waters. Either way, life began on land.

The end of the Devonian times witnessed the first amphibians, creatures that spend their time on land, but had to go back to the waters to breed. Such a creature was the Ichthhyostega (“fish roof”). It had the head and tail of a fish, but had developed four eight-toed 'legs'. 

                                              Fig 3.6- Devonian Life 

 

The next big evolutionary achievement was the ability to breed on land. A group of creatures called the amniotes (the membrane that holds the young in the egg is called amnion) laid hard-shelled eggs that had no need of water. The migration from sea to land was now complete. This was the Carboniferous period. 

                                      Fig 3.7 - Westlothiana, an amniote; 

                                                      fossil and restoration

 

Now let us look at another type of creatures that evolved during the late Carboniferous. These were the reptiles. They can be classified on the basis of arrangement of holes in the skull.

The anapsids had no hole in the skull other than the eye-socket and the nasal holes. Their contemporary relative are turtles and tortoises.

The synapsids had a hole at each side of the skull. They were the mammal-like reptiles. These holes in the skull made the skull lighter and the jaws more dynamic for efficient biting. The fittest creatures were in making.

           Fig 3.8- Skull comparison: A-anapsid, B-synopsid, C-diapsid

   

The diapsids had two such pairs of holes behind the eye-sockets. Their modern day relatives are snakes, lizards, crocodiles and birds. A group of such diapsids, called the archosaurs (“ruling lizard”) made their way into the Mesozoic era to establish themselves as the apex creatures. It was from these archosaurs, during the middle Triassic period, evolved the dinosaurs.

 

                

 

2. Geological Time— An Overview

   

In the previous chapter, we somewhat casually spoke of periods hundreds of millions years ago. But it is not easy to grasp such a overwhelmingly massive concept. Palaeontologists use these very scales to study the ancient life forms. Just like historians categorised the continuum of human history based on socio-political events and patterns (e.g. Italy during renaissance, Victorian England, US during prohibition), geologists and palaeontologists have done the same for geological time based on the rock sequences formed at that time. And by dating these rock sequences we come to know how old they are.

The first to shape the discipline of geology as a science was the Danish Geologist Nicholaus Steno. In 1699, he proposed the principles for relative timing of geological events. They were:

(1) The principle of superposition, in which each rock layer was laid down one upon another,

(2) The principle of original horizontality, in which rock layers were originally laid down horizontally and were inclined only if secondarily tilted, and

(3) The principle of original continuity, in which rock layers were assumed to be continuous over great distances.

These three, taken together with the principle of crosscutting (Charles Lyell, 1830) which states that any feature cutting across another must be the younger of the two, gives the solid scientific foundation for relative geological dating.

However, modern science enables us to determine geological age by absolute dating. This involves studying the radioactive decay of the elements that are in the rocks. Note that the previous method can only tell us which rocks are older than others while the later can give us an idea how old the rocks are. Let us see how it is done.

The method of absolute dating is also known as chronostratigraphy. The layers of rock are usually called strata and chronos (Greek) means “time”. We know that radioactive elements are unstable and constantly decay by means of radiation. Many elements also have such unstable isotopes. Isotopes are variants of an element differing in the number of neutrons.  For example, the element carbon has three isotopes. carbon-12, carbon-13 and carbon-14. A carbon atom has 6 electrons. Therefore the number of neutrons in these isotopes are 6,7 and 8 respectively(the numbers 12, 13 and 14 are called mass numbers. Mass number of an atom is the total number of protons and neutrons in the atom and an atom has equal number of electrons and protons). Of these, carbon-12 and carbon-13 are stable isotopes and carbon-14 is radioactive and therefore undergoes decay and produces a stable “daughter” isotope, which is nitrogen.

carbon-14 –> nitrogen-14 + energy

Now, suppose we found a rock/fossil with such an isotope. If we know

·       The initial amount of parent isotope at the time of rock formation/death of the creature,

·       The residual amount of the parent isotope,

·       The rate of decay of the isotope (which is usually constant)

We can estimate the age of the specimen in years. In this regard, scientists often use the half-life, which is basically the time taken for the isotope to decay 50% of its initial amount, as a marker for these isotopes.

Fig 2.1- radioactive decay

  But, not all isotopes are equally important. Some have a half-life of over billions of years; some have half-life in thousand years (carbon-14 has half-life of about 5,730 years). Therefore choosing the right isotope to analyse is of single most importance; after all, you don’t want to measure your height in light-years or nanometres.

Now we shall look at how the geological time is structured.

The largest unit of time is supereon, composed of eons. Each eon is divided into eras, an era into periods, periods are further divided into epochs and each epoch is composed of ages. This enables us to give a specific “address” to whatever time we are talking about. For example, the dinosaurs belonged in the Mesozoic era of the Phanaerozoic eon, in the later part of the Triassic period and throughout the subsequent periods Jurassic and Cretaceous. 

Fig 2.2- Geological time  

We will learn about these periods, the condition of the earth and of course, we will learn about the dinosaurs. But before that, it is necessary to know a brief sketch of the evolution of life in order to fully understand the dinosaurs. That will be the topic of our next discussion.

1. Fossils – Evidences from the Past

It is important that, to know and understand about dinosaurs, we first learn about the principal instrument in understanding them— fossils. Fossils are remains or impressions of a prehistoric plant or animal preserved in rocks in a petrified form. Depending on the nature of the fossil, experts can make deductions about the form, habits and habitat of the creature, as well as some other important aspects worth knowing about it. We need to keep in mind that it has been about 65 million years since dinosaurs became extinct. The earth has undergone many changes ever since, along with what are buried underneath it. Therefore, what we often recover as fossils are not open books ready to divulge all the necessary information but cryptic puzzles that require extensive training and skill to solve them. The discipline taphonomy (Greek taphos, meaning "burial", nomos, meaning "law") deals with the process of fossilisation, that is, what happens to an organism after its death until it becomes a fossil. Researchers who are interested in prehistoric life aka palaeontologists (Greek palaois, meaning “old”, ontos, meaning “creature” and logos, meaning “study”) therefore are hugely dependent on the knowledge of taphonomy, along with the biological and geological sciences. Equipped with all the trainings, these detectives take on the quest of unfolding the ancient mysteries that surround the dinosaurs.

But to investigate a fossil, one must find one first. That alone is a difficult task. Let us now look at all the hurdles a palaeontologist may face in finding a fossil, and the ones that come after finding one.

Let us think a little about what all may take place when a dinosaur dies. The body may be dismembered by predators and then scavengers. The bones and body parts may be carried and scattered away. The bones may be trampled by other animals. After the bones are free of flesh by predators, scavengers and decomposers, they are exposed to the sun and weather. Bones are made of calcium-sodium peroxy apatite, a material highly prone to weathering, i.e. if exposed for a long time, the minerals in the bones break down and the bones disintegrate. Only quick burial after death can prevent weathering. And the body becomes qualified to become a fossil.

Sediments turn into sedimentary rocks through a process called diagenesis. The same process is responsible for fossilising the buried body. Diagenesis involves compression by the weight of overlying sediments which helps compact the grains of sediments and cementation by ground water flowing through the deposits adhering them together.

But just because a body has been buried does not mean it will be helpful in future. Even after burial, significant changes may occur to the bones. Bones may come in contact with fluids rich in minerals that can significantly alter the calcium-sodium peroxy apatite content in them. Bones are porous, and they can be filled through the pores with minerals, a process known as permineralisation. Also, original bone contents may be entirely replaced by other minerals. Another scenario is that the compression, along with earth’s internal heat changes the sedimentary rocks to metamorphic rocks, completely destroying the fossil in the process.  

Thus, the chance that a dead dinosaur will turn up as a fossil is rather low. But, the enormity of the number of dinosaurs ever lived is also worth remembering. Dinosaurs existed for about 165 million years in the Mesozoic era (we will talk about this timeline in details) and it is estimated that at any given time, there were about a few billion dinosaurs alive. Therefore, the number of fossils existing in earth is considerably large. But we do not get a fossil too often. Why?

To answer this, we must return to the process of fossil formation we were discussing. After a fossil is formed, it is of no use unless it reaches us. Because million years has passed and more sedimentation has occurred in layers above it, sometimes changing the landscape. The fossil can make its way to the surface only when the rock containing it is lifted through movements of the earth, usually mountain building. Then it is the role of erosion to wear away the rocks and expose the fossil. But too much erosion will wear away the fossil itself. And then someone has to be there to recognise it as a potential fossil. All this taken into account, we now understand why fossils are a rare discovery.

 Fig 1.1- The process of fossilisation

         1.      The dinosaur’s body on a river bank.

         2.      Then the body is carried underwater (by flood, say). The body is decomposed and the skeleton is revealed.

         3.      Over time, more mud and sand are deposited in layers and the sediments containing the body now transforms into sedimentary rocks and the body is fossilised.

         4.      Erosion occurs in the region, eventually bringing it to the surface.

         5.      A group of palaeontologists working in the site.

   

     

      Bones and teeth are so talked about because they are robust. But there are other fossils which are no less important. Fossilised eggs, skin impressions and even tissues have been found in some cases. Occasionally fossilised faeces known as coprolites can be found, giving us an idea of dinosaur diet. Another very important type of fossil is trace fossils. These are footprints or complete trackways of a dinosaur. Trace fossils tell us the dinosaurs’ walking postures, their speed and sometimes significant tales like hunting habits.

Now we will take a quick look at how fossils are found and collected.

By now, we have some idea that luck plays an important role in discovering a fossil. However, there are ways to locate a zone where the likelihood of finding a fossil is higher. The guiding factor is geology; one needs to look in the right rocks. And what rock is that?

Sedimentary, my dear Watson!

We have already learned how fossils are formed and preserved in sedimentary rocks and destroyed in metamorphic rocks. It is needless to mention that igneous rocks cannot hold any fossil.

But not all sedimentary rocks are worth investigating. Remember that the dinosaurs lived in a certain era, ranging from 231 million years ago to 66 million years ago. Only sedimentary rocks formed in that period of time are likely candidates.

Also, dinosaurs were terrestrial creatures, i.e. they lived on the land. Thus, searches are to be made in rocks that are from ancient river systems, deserts, lakes etc. The richest fossil localities are badlands and deserts, where the rock exposure is extensive and air is dry as compared to forests where the terrain is covered in vegetation and the weathering rates are too high.

Once the site has been identified, comes the task of collecting the fossil and transporting it back to the lab. This entire procedure is extremely painstaking. Digging up a fossil is a one shot job and once something goes wrong, it cannot be undone. There are certain risks for the workers. Large dinosaur bones are usually heavy and their removal itself is hazardous. The fossil may be in a steep terrain or unstable land where working is extremely dangerous.

The first thing that is done in case a fossil is located is a palaeoenvironmental study, which involves assessing the local geology in order to have ideas about where the creature lived, when it lived and how it died. There are several political issues involved, especially if the site is a foreign one. And there are questions to be asked before digging up the terrain:

      ·       Are there important floral and faunal association?

      ·       Are there paleoenvironmental associations that make the dinosaur important?

      ·       Does the dinosaur have any unusual features?

      ·       Is it possible that it is a new genus or species?

      ·       Is there any indication to its cause of death?

      ·       Is the dinosaur fit for display in a museum?

There are other concerns as well, like is there a possibility of damage to the landscape? Is there any endangered species under potential threat by the work? What will be the effect of the excavation on local life? An excavation is executed only if these questions are satisfactorily answered. 

                 Fig 1.2- Palaeontologists working on a stegosaurus fossil in Garden Park, Colorado.

The job of separating the fossil from the rock (called the matrix) requires both finesse and force. After carefully isolating the bone from the matrix, it is cushioned in wet toilet paper. Then it is jacketed with strips of burlap cloth soaked in plaster. After the plaster has hardened, the specimen is ready to be moved to the museum.

This procedure can be experienced in this wonderful interactive:

Virtual Dino Dig

But all these are easier said than done. When bones buried for millions of years are suddenly exposed, they often undergo reactions like shrinking, changing colour etc. The job of collecting a fossil therefore requires extensive level of expertise, not to mention the profound patience. Such excavations usually last for months, depending on the geography and nature of the fossil.

The entire procedure is documented by means of logs, field notes, sketches, maps and photographs for achieving.

Then the fossil brought undergoes curation for long term use for researchers or viewing for the interested public.

           Fig 1.3- A Tyrannosaurus rex specimen at the Carnegie Museum of Natural History, Pittsburgh