Thursday, October 20, 2016
A quick Google search confirmed my impression that the dinosaurs became extinct about 65,000,000 years ago at the end of the Cretaceous. An exception was indicated by referring to the extinction as not including non-avian dinosaurs. That distinction was made to satisfy scientists who have determined that bird’s ancestral reptilian roots was most likely in a specialized line of dinosaur-like ancestors. It seems to me that it is unnecessary to continually repeat that connection in discussing non-phylogenetic issues.
Birds are sufficiently different from dinosaurs that they deserve their independency as a group. Feather’s, no teeth, and homeothermy (maintaining a temperature independent of environmental temperature) may be avian characteristics that had a role in the survival of birds, but that is another topic.
THE MAJOR CAUSE, AN ASTEROID STRIKE
Multiple causes may have operated in making dinosaurs unable to survive the asteroid strike(s) that produced world-wide evidence of the disaster in the geological record found in sediments. Several other major geological periods ended with similar disruptions, all also associated with the extinction of a large percentage of previously existing species.
In the several billion years of the earth’s existence earth has grown by the impact of space debris of varying sizes as indicated by the assortment of craters on the moon and other planets Such bombardment was so intense in pre-Cambrian times that animals left a very skimpy fossil record, partially because large fossil forming animals seldom evolved or survived.
The abyss as a refuge during extinction events
The one place that had relatively stable conditions during extinction events was the abyssal region of the oceans. There, any animals that could survive on the nutrients deposited in sediments had a better chance of survival in some location because most of the earth was covered with deep oceans. The deep-water pogonophorans were one of the survivors. The oxygenated region was so extensive and so slow to be replenished by polar surface waters that much of the fauna adapted to the region persisted to the present. Those abyssal conditions made low metabolic rates and extended life cycles contribute to a very slow, almost absent, evolutionary rate for animals living there. (see 2015/05 listed at end of this post)
BEYOND THE ASTEROID IMPACT ZONE
Direct hits by the asteroid, its fragments, debris blasted from the impact zone did not even have to kill a single dinosaur. But the world-wide atmospheric debris may have persisted for a year or more and made it difficult for significant plant growth of the type needed by large herbivores. Carnivorous dinosaurs like T. rex would miss their normal food after large herbivores starved. Continents isolated by oceans and partitioned by deserts or mountain rages would have made it difficult for the large dinosaurs to escape the drastic climatic shifts temporarily making their existing range uninhabitable.
POSSIBLE SECONDARY CAUSES OF DINOSAUR EXTINCTION
Pre-strike population declines or increases may have contributed to the death of many species. Such declines may have been in the dinosaur species and/or other groups important in their food chains and are not limited to food organisms but could include microscopic disease organisms, parasites, and competitors of various types.
The complexity of population changes resulting from decline or increase of one species is difficult to predict with certainty. The decrease in one species may result in survival of more of those they prey upon and fewer of those feeding upon them. The changes can ripple up and down the food, predator, and parasite chains existing in the ecological community. Add to these changes each of the physical changes produced by each species and the variability of possible community changes becomes astronomical. If the vast number of buffalos grazing on our prairies had not been replaced by cattle, sheep, and farmers plowing- would much of it reverted to forest or scrubby vegetation providing homes for a different group of animals and plants? Such changes had far reaching effects including soil erosion, flooding, less retention of water and less rainfall; all changes having additional effects on climate and life.
The cooling accompanying the atmospheric debris which reduced light energy and it warming effect makes a possible sufficient cause for extinction of many dinosaur species by shifting sex ratios of eggs hatching to all of one sex. It is known that alligators and some other reptiles deposit large numbers of eggs in holes that they dig in the ground. The holes are then covered over and the deeper the egg in the ground the cooler the temperature it experiences during embryonic development. Higher temperatures induce one sex, cooler temperatures induce the other sex. If cooler temperatures make all become the sex normally found in the deeper part of the nest, all will be of one sex. It is not known if the phenomenon is the cause of alligators being restricted to warmer latitudes.
Cooler temperatures would make animals other than birds and mammals become inactive and unable to respond to egg predation or even being eaten by smaller animals with feather or fur insulation and higher metabolic rates maintaining body temperatures needed for activity. The immense size of dinosaurs made them less susceptible to short term cold temperatures because it takes days for the largest to cool down, but the asteroid caused global cooling would persist so long it might have been sufficient to cause their demise, even those many other causes may have speeded the process of extinction.
Joseph Engemann Kalamazoo, Michigan October 20, 2016