Geologic Column
By Frank Bliss

Extinction of the Dinosaurs?

I am an active participant of  a 600 member strong internet mailing list consisting of experts from many countries in the many fields of Vertebrate Paleontology. The vertebrate part deals primarily only with animals that have backbones and the paleontology part means the study of ancient life. I am certainly one of the least educated members of the list and some of the discussions can get a bit hard to decipher even with a graduate degree in geology behind me.  One of hottest topics in the field these days is what killed the dinosaurs.

All the full time professionals have their own pet opinions and of course when you have 3 geologists in a room you have at least 4 professional opinions.  I of course have my own ideas that I will distill down as much as possible to save room.

For the last 18 years, the really popular story is that about 65 million years ago, a good sized impactor (about a 7 mile across asteroid or comet) smashed into the earth near the village of Chicxulub (pronounced Chik sil ub) in the Yucatan Peninsula of Mexico.  It was very close to the same time of this impact event that 75 percent of all the species on our planet went extinct. When this impact occurred, ejected rock from the crater enveloped the earth and rained back to the surface. Some scientists speculate the radiant heat from the huge amounts of suborbital rock reentering the atmosphere baked any animal that was not under ground, or under water at the time.  There were other effects similar to a nuclear winter where dust blocked out sunlight preventing photosynthesis for up to a year.  It might have been too dark to even see for up to 6 months.  Besides, the solid rocks thrown up, the carbonate/gypsum rich rocks at the impact site released sulfur dioxides in concentrations that produced very strong sulfuric acid rain and with the damage to the ozone layer adding to the insult by adding nitric acid rain to the mix.  This certainly defoliated vast areas of vegetation and changed the chemistry of waters by direct action and leaching poisonous metals from the ground. Massive forest fires destroyed the environment and cooled the climate even more. There is worldwide evidence for this event but this event alone does not explain all the rocks tell us.

An idea based on evidence from Alberta, Wyoming and Montana, is that dinosaur diversity was in decline for several million years before the end.  Around 7 million years before the disappearance of the dinosaurs, there were 30 families of dinosaurs but near the end of their reign, only 12 families remained. This decline may have been in response to gradual environmental changes combined with disease, competition or over specialization. Slow reproductive rates of large animals would also hinder their adaptability to change. The problem with this idea is that the fossil record is incomplete and most of the field work leading to this theory is localized to this region. It does not apply universally to world wide data.

My personal opinion is based on all of these ideas. It may well be that dinosaurs were in decline or were mostly killed by an asteroid impact. It makes more sense to me (and others) that animal populations (dinosaurs included) were slowly changing (dare I use the word evolve?) into more diverse creatures similar to modern birds and mammals.  The impact event in Mexico certainly added a huge stress to dinosaurians and upset their reproductive cycle which no doubt played a major part in the end result.  These factors operating in tandem would certainly result in a dramatic decrease in dinosaur population and the resultant increase in other forms.  These smaller, more diverse, more active and smarter competitors would easily fill in the ecological niches left empty by the declining numbers of larger species.  In effect, all the popular theories were right but the resulting extinction of large dinosaurs was the result of the combination of events not any particular one.

In my humble opinion, enjoy watching those little dinosaurs flying around in your back yard.