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