IMPLICATIONS
If periodicity of extinctions in the geologic past can be demonstrated, the implications are broad and fundamental. A first question is whether we are seeing the effects of a purely biological phenomenon or whether periodic extinction results from recurrent events or cycles in the physical environment.If the forcing agent is in the physical environment, does this reflect an earthbound process or something in space?
If the later, are the extraterrestrial influences solar, solar system, or galactic? Although none of these alternatives can be ruled out now, we favor extraterrestrial causes for the reason that purely biological or earthbound physical cycles seem incredible, where the cycles are of fixed length and measured on a time scale of tens of millions of years. By contrast, astronomical and astrophysical cycles of this order are plausible even though candidates for the particular cycle observed in the extinction data are few. One possibility is the passage of our solar system through the spiral arms of the Milky Way Galaxy, which has been estimated to occur on the order of 108 years (21). Shoemaker has argued (21) that passage through galactic arms should increase the comet flux and this could, following the Alvarez hypothesis (2), provide an explanation for the biological extinctions. Two of the extinction events being considered here (Late Cretaceous and Late Eocene) are associated with evidence for meteorite impact (23,24). However, much more information is needed before definitive statements about causes can be made. It may turnout that the biological extinction record is sensitive to periodic phenomena that other indicators have failed to recognize.
The implications of periodicity for evolutionary biology are profound. The most obvious is that the evolutionary system is not "alone"in the sense that it is partially dependent upon external influences more profound than the local and regional environmental changes normally considered. Much has been written about the "bottlenecking" effect of mass extinction. With kill rates for species estimated to have been as high as 77% and 96% for the largest extinctions (11,25), the biosphere is forced through narrow bottlenecks and the recovery from these events is usually accompanied by fundamental changes in biotic composition (26). Without these perturbations, the general course of macroevolution could have been very different.
Structures on Earth under the current gravity can only be so large using known materials. When a structure's mass exceeds the strength of the building material, the structure collapses. Which is why we cannot build structures miles high. On the Moon with its lighter gravity, building such structures would be possible.
The largest animal structures on Earth are found in the oceans as water's buoyancy provides the needed cushion against the pull of gravity. A large dinosaur simply has too much mass for the bone destiny to support on Earth in this geological period. Their structures are simply too flimsy.
Then there are the hydralic problems: how does the blood get pumped that high (30 feer or more) under reasonable preasure so as not to explode the heart?
Where do you put these structures? Large Dinos vs. Human
For the smaller land dwelling Brachiosaurus: 85 feet, 55,100 to 96,800 lbs and up from there for the really big ones.
Blue Whale vs. Human
For the water dwelling Bule Whale: 98 feet. 400,000 lbs plus
More here on the problems with dinosaurs and better explainions:
The Paradox of Large Dinosaurs
Sorry about the large graphics, but I could not dind smaller ones.