MASS EXTINCTIONS : A CASE STUDY

  

     Life is always being a precious gift to planet Earth. Not only about the present, but from very beginning the varying species diversities who have inhabitated and those who are still inhabitating on the blue planet, play a vital role as an individual species. Apart from insects, who remained the most successful species till date, only a fewer tends to be the primal survivor.

   

     Ancestors of humans have ruled over the planet since 24 Million years ago as The Hominids [ Dryopithecus: ancestors of modern apes and man]. Who can ever imagine that someone more dominant than us can eventually become extinct. It seems impossibly right. If you do think of the same, then let me take back to the Triassic and Jurassic Periods [ Mesozoic Era(248 MYA - 63 MYA)] where someone more powerful and dominant had ruled the planet. Yes you guessed it right, The Dinosaurs. Their size and power surpassed man in every aspect. Though they were huge and dominant, but nature decided to write the story in it's own way. And it so happened, that dinosaurs disappeared in a mass extinction. 

   

      8.7 million is the number of eukaryotic species present on Earth today, and also many species are to be discovered. But, research says that the species count in the present time is just 0.1 per cent of all species that ever inhabitated the planet. So, is this true that all 99.9 per cent of species became extinct?

      With this point of view, if a creature as powerful as dinosaurs can go extinct, then we fragile humans are nothing. Well talks apart, let's dive into the topic of how mass extinction changed the planet? Will it happen again? What would be the case if it didn't? Before focusing on such aspects, we must know first what is mass extinction and it's occurrence in history.

   

       First of all what comes to your mind when you hear the word 'mass extinctions' ? Let's answer to this question first. It's an event that occurred with several impacts which are so severe that, it swept away large number of biodiversity in a small period of geological time. Their are various possibility that includes asteroid impacts, volcanic eruptions, climate changes and also severe natural calamities that resulted in a huge loss in species diversities and took millions of years for recovery. There are 5 known mass extinctions happened in history which includes :-

1. Ordovician - Silurian Extinction [ End Ordovician (450 MYA - 440 MYA)]

2. Devonian - Carboniferous Extinction [ Late Devonian (375 MYA - 360 MYA)]

3. Permian - Triassic Extinction [ End Permian (252 MYA)]

4. Triassic - Jurassic Extinction [ End Triassic (201 MYA)]

5. Cretaceous - Paleogene [ End Cretaceous (66 MYA)]

         It's clear that nearly all types of species were swept away by the deadly events but the Marine inhabitants suffered the most. Let's discuss about the mass extinctions in brief.

   1. LATE ORDOVICIAN MASS EXTINCTION :

         Ordovician period started about 485 million years ago after the end of Cambrian period and before the beginning of Silurian period of Palaeozoic era.

         Life flourished in the oceans during this period. Many different types of fishes evolved during this period along with other animals such as Trilobites, which came to existence in Cambrian period. But the Ordovician period ended as the second largest mass extinction in the history.

   
                                

     

        There is not any proper evidence about what must had happened, or the causes related to the extinction. But, as per the suggestions of paleontologists, there must be some drastic climate changes occurred, which have leaded to the extinction of various species. Or, the extinction might had occurred in several phases, like the major falling in sea level causing a severe downfall of carbon dioxide content in atmosphere, affecting survival of numerous of species.

           

         Further, glaciation took place by global warming which resulted in depletion of green house gases converting them into toxic pollutants in atmosphere. There were no such asteroid impacts recorded during late ordovician mass extinction.

         Near about half of Marine fauna got extinct including most families related to trilobite and graptolites. Lamp shells, also known as Brachiopods that served as one of the key invertebrates, also vanished during the extinction. 

    2. DEVONIAN MASS EXTINCTION :

        Devonian period started about 419 million years ago and ended before the beginning of Carboniferous, 358 million years ago. 

        The devonian period is also known as the age of fishes as so many new types evolved. Some of these fishes gave rise to four-legged amphibians (e.g. Eryops) that were the ancestors of all land vertebrates, such as reptiles and mammals. Reef ecosystem began to develop which have horn corals and Brachiopods in major sections.

         The extinction was similar to late ordovician mass extinction in case of impact on life, that in maximum range affected the Marine inhabitants. The remaining trilobite families completely swept away by the extinction.

 

         Unlike ordovician extinction, this extinction reports asteroid impact with reference to the Siljan structure in Sweden. Dissolved oxygen inbalance (hypoxic and anoxic sedimentation) in underwater affected a large number of marine species. Global warming and cooling also included in the devonian mass extinction.

        Talking about the intensity, it was not so severe as compared to the other extinctions but severely affected marine biodiversity in which Jawed vertebrates suffered the most and corals cannot regained their existence till several millions of years.

         

    3. END PERMIAN EXTINCTION :

        End Permian or Permian-Triassic extinction that occurred about 251 million years ago is the greatest known mass extinction till date. 

        The intensity was so huge that it wiped out nearly 90 per cent of Earth's total species, that included 95 per cent of marine and 70 per cent of all terrestrial species. This was the age where life came to an end, where species count is severely low and surviving in harsh conditions. And worse thing is that, the extinction lasted comparatively longer and took millions of years for it's recovery.

        Several causes involved in the process includes heavy meteor impacts, that severely affected the biodiversity of a huge landmass in a very short period of time. Though meteor shower were not globally spread, but fragments reported in parts of Antarctica and Australia destroyed larger area. Apart from meteor shower, other causes including volcanic eruptions also reported in parts of China and Russia. Lastly, temperature differences, disturbances in Carbon cycle and drastic climate changes occurred, that simultaneously led this extinction event to be the world's worst known mass extinction in history.

        Remaining genera of trilobite families got extinct. For the first time, terrestrial world had affected. Land Vertebrates including Sauropsids and Therapsid families got extinct. And the fewer that survived, went through extreme unfavorable conditions for their existence that resulted in millions of years for complete recovery.

    4. END TRIASSIC MASS EXTINCTION :

        Triassic period started about 248 million years ago, and lasted for a comparatively small time as the Triassic Extinction held about 45 years after the beginning of the period, that is about 205 million years ago.

        Triassic marks the period where first marine reptiles appeared i.e. Mixosaurus, many other marine fauna developed. One of the greatest event occurred in mid-triassic period was the vast supercontinent Pangea splitted into 2 subcontinents, Laurasia and Gondwana. Two specialized groups evolved from archosaurs at the end of Triassic Period, the first one was dinosaurs, who later on became dominant in Jurassic Period and the second one Pterosaurs, who remained the first flying vertebrates.

         Possible causes mainly incude volcanic eruptions, that led to increase in atmosphere carbon dioxide content along with exposure of effective greenhouse gas, methane that further led to global warming. Sudden climate change and rise in sea level also responsible for carbon dioxide inbalance in atmosphere, which remained a major cause for wiping out around 75 per cent of marine and terrestrial species. No such extrastellar impacts recorded in end triassic extinction.

          Marine inhabitants were partially affected, where planktons, in some regions lost their diversities. Reefs were affected due to rise in carbon dioxide level. Terrestrial Vertebrates including therapsids completely wiped out. However, only non-mammalian therapsids, Cynodonts survived.

      5. END CRETACEOUS EXTINCTION :

                    

          The creataceous-tertiary extinction or cretaceous- Paleogene extinction marks the end of Mesozoic Era and beginning of Paleogene epoch of cenozoic era, occurred about 66 million years ago. 

           Dinosaurs, the creature that dominated during Jurassic Period wiped out during this extinction. Also, the terrestrial species including both flaura and fauna had affected in a huge scale. As a reason, the Cretaceous extinction became the third hugest extinction after the Permian-Triassic and the late ordovician mass extinction . The Marine inhabitants had also suffered a lot where families related to Jawed fishes and sharks species became extinct.The Marine corals however in fewer amounts survived this extinction event.

       

            The major cause for this extinction remained was massive asteroid impacts in different region of world that includes Mexico, Ukraine and parts of Western India. There was no volcanic eruptions reported in this event.

             The species we came across present day are the evolved form of the species that survived the mass extinctions. Like, the modern birds evolved from the surviving group of dinosaurs of aves category. Furthermore, the mammalian group that survived adapted their features from where horse, whales and other animals formed after millions of years to which we see now.

       A PROSPECTIVE:

            Here comes the vital question one may ask himself/herself after reading this post, that will any mass extinction is possible in future? Will there will be some extinction that will wipe out humanity?

            Well, answer to such questions might not be easier, as this directly points to a prediction. But, in my point of view, it's most probably possible with respect to the present time. If one will sit down to write some possible causes for the future extinction, it will begin with the major cause i.e global warming and will go so on. Anyway, the main cause always remains the rise in human population and I will continue with this in my next article about the sixth extinction and it's possibilities.