If we were to seek a reason as to why so many of the great creatures that once roamed the earth are now extinct, we might guess that the post-catastrophic world is not conducive to their survival.

Salt Levels in the Water

Firstly, there is evidence for a massive increase in the salinity of the oceans. As a comparative physiologist, I have always been fascinated by the fact that marine fish (both the cartilaginous and bony fishes) are anatomically and physiologically adapted to a fresh water environment.

A fish's internal salt concentration is approximately one third of that of seawater and its kidneys are adapted for the elimination of water—although this function is not required in seawater. In fact, their low salt concentration causes them to lose water by osmosis so that they cannot afford to lose water via the kidneys.

The cartilaginous fishes (sharks and rays) solve this problem by retaining urea (a toxin) to raise their osmolarity to a level higher than seawater so that they can gain water by osmosis, whereas bony fishes desalinate the seawater with a salt pump in their gills. Obviously these organisms were adapted to much lower in the past and only survive because of their ability to osmoregulate under these circumstances.

The retention of toxins by a cartilaginous fish is an indication of an emergency solution to which they eventually adjusted. Only organisms that could either conform to the new conditions or regulate their salt content survived. The more sensitive are now extinct.

How could marine and fresh water organisms survive if the waters of the earth were all dumped together during the global Flood? The separation between fresh and salt water would only have been re-established once the continents rose above the water level. Surprisingly, however, large bodies of fresh and saline waters, or even large bodies of fresh water from different sources can coexist side by side without much mingling along the contact zones.

This is seen in the great Amazon River, where two bodies of water run side by side for kilometers on end with limited mingling at the contact zone. During the Flood, organisms adapted only to fresh water would have been able to survive in large bodies of fresh water that remained relatively distinct. Moreover, the marine life is enhanced where great bodies of fresh water come into contact with the ocean, and many species can only spawn in fresh or brackish water, thus indicating that these were the conditions to which they were earlier accustomed.

One would expect large-scale destruction of aquatic life in areas where the water composition was rapidly and radically changed due to hypersalination from underground aqueducts and through catastrophic mingling of the water masses, and this is exactly what we do find. The fact that so many marine organisms use rivers and estuaries as their spawning grounds indicate that the best survival salinities for these creatures must exist under these low saline conditions and that is why they will migrate long distances to spawn in such areas.

The Thermal Environment

In terms of coping with the thermal environment, only two categories of land organisms exist today. Terrestrial animals are either endothermic or ectothermic. Endothermic animals (largely mammals and birds) control their body temperatures by increasing their metabolic rates where environmental temperatures drop. Ectothermic animals control their body temperatures by selectively utilizing external sources such as solar radiation. In the absence of solar radiation, the body temperatures of these animals is the same as that of their environment. We also speak of these categories as warm-blooded and cold-blooded animals.

In a world with climatic extremes, all animals would have to belong to one of these two categories to survive. There is evidence that the great reptiles of the past were probably neither endotherms nor ectotherms, but somewhere in between. The same probably holds true for many of the now extinct giant amphibians and mammal-like reptiles. Studies of bone-to-marrow ratios show that the dinosaurs and other creatures were in this intermediary condition and would thus require stable environmental conditions. Plant fossils suggest that the earth had a relatively warm climate prior to the catastrophe, but the post-catastrophic climate was likely less stable. Moreover, the large scale reduction in vegetation associated with the destruction means that many food sources were no longer available and precludes survival of these animals.

The paleontological record shows that far greater varieties of plants and animals existed in the past than are living today. It is also possible that only non-specialist feeders could have survived destruction of a preferred food source, so that many of the great creatures of the past are no longer with us because the planet does no longer provide their niche food.