Evolution and Extinction of Dinosaurs

Introduction

In the field of geology, especially dealing with palaeontology, the varied group of gigantic extinct animals commonly termed as dinosaurs, have probably stimulated extensive interest to all, including the scientists, principally because of their enormous dimension. Dinosaurs consist of diverse group of vertebrate animals having more than 900 distinct genera and are mostly bipedal and egg-laying. The group also includes birds, which have been confirmed from fossils. A most intriguing subject to all scientists is the cause of sudden disappearance of these species which once ruled the earth.

Discussion

For understanding the situation responsible for extinction of the giant animals [1] it is necessary to go to the bottom of the problem, namely, the reason why a large group of animals grew up to gigantic dimension and after ruling the planet for considerable period suddenly disappeared. In a previous work the author [2] has discussed the views on extinction of dinosaurs put forward by several scientists, amongst which the work put forward by Nobel Laurate physicist Alverez and co-workers attracted much attention, even though many scientists disagreed with the thesis. In 1980 Luis Alvarez [1] along with his geologist son Walter Alvarez suggested that mass extinction of dinosaurs and certain other fauna was caused due to impact of an enormous meteorite over the surface of the earth during the Cretaceous–Paleogene period. The theory has been supported by many including a team of scientists who consider that a giant meteorite of about 15 km thickness fell at Chicxulub in Mexico causing this unusual event.  Alvarez  and co-workers consider that such impact would inject about 60 times the object’s mass into the atmosphere as pulverized rock, a fraction of which would stay in the stratosphere for several years and distributed worldwide. The resulting darkness would hinder the process of photosynthesis, and the expected biological consequences which, according to them, match with the extinction observed in the paleontological record. The present author considers that this view is evidently not based on actual geological evidence but appears to be a fantasy-driven opinion and should have been forthwith rejected. The view of an outstanding chemist or physicist in the field of geology may not be correct and, on the other hand, the opinion of a geologist or a chemist may not solve crucial problem of physics. In the event of fall of a large meteorite, if large and robust animals like dinosaurs would face extinction, before that almost all the flora and fauna would be wiped out. Some of the objections which could be raised against the meteorite pelting thesis are: -

1. Some large and robust animals would have escaped extinction while small and relatively weaker animals would have perished. Perhaps some dinosaurs would have been severely injured but would have survived.
2. The view cannot explain the reason of selective extinction of all dinosaurs during the K-T period.
3. It is not clear how the pulverized rocks are distributed worldwide and remained in the stratosphere defying the force of gravity.
4. The theoretical concept that pulverized rocks would have stayed in atmosphere for several years cannot be considered as sacrosanct and beyond any doubt. In all probability owing to gravitational attraction such debris would soon fall over the surface of the earth and due to that many [3] animals, especially, the smaller ones would have died while larger ones too would have either died or severely injured.
5. Extra-iridium content in rocks on earth’s surface could have also been caused owing to igneous intrusion, especially like the event of Deccan volcanism.
6. Charles Officer  and Jake Page [3] contemplate that Chicxulub structure of Cretaceous-Tertiary age may represent remnant of a volcano of late Cretaceous age and that iridium might have been ejected from the volcano.
7. Officer and Page [3] also opined that even if a meteoric impact occurred at K-T time causing interruption of sunlight, many species remained unaffected.
8. According to Gerta Keller [4] extinction of dinosaurs has been caused by Deccan volcanism.

Conclusion

The author considers that the cause of extinction of large sized animals of various types, grouped as dinosaurs which include birds, bipedal and quadrupedal animals of both herbivorous and carnivorous types, was not caused by impact of meteorites, but owing to depletion of oxygen of the global atmosphere. Impact of meteorite, even if had occurred in the past in a particular geological time affecting several parts of the globe, it is most unlikely to be a general world-wide phenomenon that would kill selectively only the large size animals in every nook and corner of the planet. In contrast, oxygen content of the global atmosphere would virtually be the same everywhere near the surface of the earth where all land animals dwell upon. Even if oxygen content in several corners is diminished due to prolong burning caused by igneous intrusion, its effect unlike stone pelting, would be rapidly spread throughout the globe, thereby causing it to be a global effect. Geological records reveal that during the Triassic period oxygen content of the atmosphere was greatly enhanced owing widespread photosynthesis of the glossopteris forests. In such an affable oxygen-enriched environment, where abundant food was also available, some animals grew up to enormous size. In contrast, in the next Cretaceous age due to incidences of igneous activities oxygen content of the global atmosphere was significantly depleted and in consequence of that dinosaurs that required more oxygen selectively faced extinction while the smaller animals remained unaffected.