DINOSAUR EXTINCTION
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
http://evolutioninsights.blogspot.com/2014/03/science-extinctions-and-evolution.html
http://evolutioninsights.blogspot.com/2015/05/abyssal-ocean-environment-and-extreme.html
