Stress on Thin Eggshells Theory
This theory can be largely attributed to Heinrich K. Erben of Bonn University in Germany. His research led him to the conclusion that a species of dinosaur procreated a shell that, through time, grew progressively thinner. He continued to reason that luxurious swamps and warm climate activity led to “Biological Prosperity”. These factors contributed to problems such as under population, overcrowding and other such related stresses. Assuming the relation of dinosaurs and birds, stress could have increased estrogen in female dinosaurs. This in turn, would result in thin eggshells, thus the extinction of the species.
A flaw that this theory suffers is which species were affected and which weren’t.
Egg Predation by Mammals Theory
Mammals eating dinosaurs’ eggs has been suggested as a cause of the K-T extinction.
This theory has many problems with it. First of all , evidence was found suggesting parental care and feeding of the hatchlings. It is difficult to imagine that dinosaurs would have survived if they couldn’t defend their nests from predators.
Secondly, dinosaurs appear to have nested in large rookeries, with many nests together, each separated by about one adult dinosaur-length. This undoubtedly made predation more difficult, as a predator would have to run the gamut of many dinosaur feet to get to any but the outermost nests.
Epidemic Diseases Theory
Dr. Robert T. Baker explains in his book “The Dinosaur Heresies” that the mass extinction that wiped out the dinosaurs was caused by the lowering of sea level at the end of the Cretaceous. Prior to its close, the oceans of the Cretaceous covered about 90% of the land surface, forming vast shallow waters. As these drained off, land bridges such as the Bering Straits were created between formerly isolated continents, allowing large animals to migrate form one continent to another. Each population of large animals had evolved with its own unique parasites and diseases, and had developed a degree of immunity to them. When populations mixed at the close of the Cretaceous, each acquired and succumbed to each other’s diseases. Smaller animals were spared simply because they were incapable of such long migrations. Marine species died out because their favorite habitat, the shallow seas, dried up and went away.
The problems with this theory is that sea levels today are not tighter than they were at the K-T boundary, and there are still abundant enough shallow sea environments to support a rich variety of organisms. Also, epidemic diseases may wipe out large segments of a population, but rarely if ever has disease caused the complete extinction of a species, let alone an entire lineage of dozens of genera.
Drop in Sea Level Theory
Changes in sea-level are commonly suggested as a cause of mass extinctions. Oceans contain many different habitats, which in turn contain a wealth of ecological niches in which animals and plant can flourish. When sea-levels change, these habitats are disrupted, or lost completely, along with any species that lived in them, which were unable to move away or adapt to the new conditions.
Sea-level is closely liked to the formation of ice-sheets in periods of glaciation, as seawater is taken up into the ice-sheets the oceans shrink until the ice melts once again, and sea-level rises. Shallow, warm water seas (such as those that contain coral reefs) are some of the most biologically diverse habitats, but are also among those most susceptible to even small changes in sea-level. Changes in sea-level may have other, secondary, effects. Changes in the distribution of oxygen of the oceans, or in their salinity, have both been associated with fluctuations in sea-level, and could easily have altered marine environments enough to cause extinctions
A change in sea-level has been implicated in most of the mass extinctions, and almost certainly played a major part in those of the late Cambrian, the late Ordovician, and the end of the Permian.
Global Cooling Theory
Changes in the Earth’s orbit that could have caused climactic cooling might have caused the extinction. In this scenario, the dinosaurs couldn’t adapt to the cold, but the furry mammals could. This is consistent with the climate in the late Cretaceous; toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures.
Seventy million years ago, the Earth’s surface was changing very quickly. Many mountain ranges were formed. These changes in the Earth’s surface caused weather patterns and climates to change. These changes caused global cooling and bigger changes in seasons. All of these changes in the Earth’s surface caused volcanoes to erupt, thus spreading lots of dust into the air. The dust blocked the sunlight, causing even cooler temperatures. The cooler weather killed many of the dinosaurs. The cold weather also caused thinner egg shells. Thinner egg shells have been seen in birds that have been exposed to cold, chemical poisoning, or overcrowding.
Mammals that were smaller were better able to survive the temperature changes, and so they competed successfully for food.
Arctic Ocean Spillover Theory
This theory suggests that for a brief time the ocean surface waters became cooler, not warmer. During the Mesozoic continental drift the arctic ocean could have become isolated. Brackish or even freshwater would result form such an event. Colder. lighter arctic water would have mixed with the warmer Atlantic, forming a frigid layer on top of heavier salty sea-water. Drop in world temperatures as much as ten degrees Celsius would have resulted. If this happened, according to Stefan Gartnew and James P. McGuirk of Texas A&M University, a devastating chill would have spread over much of the world. However, no evidence supports the idea of the Arctic ocean ever being fresh or Brackish.
Other Unusual Theories
Some scientists have suggested that subatomic particles called neutrinos, released as stars collapse, could have bombarded the earth and resulted in widespread cancer among animals. However, such events are rare, and do not necessarily explain all the extinctions, but further research may yet prove that they have played some part in the mass extinctions.
An extension of the asteroid impact theory of mass extinction suggests that nickel, which is common in asteroids, could be spread across the globe in the cloud of debris released by such an impact. Nickel prevents plants from photosynthesizing, stopping them from growing. There would have been a huge shortage of food for both plant-eating animals, and the predators with ate them.
A quantity of other absurd theories have been proposed over the years. For a list of these theories see the appendix.
Species Affected
During the End-Cretaceous (K-T) extinction (65 million years ago) 85% of all species disappeared, making it the second largest mass extinction event in geological history. This mass mass extinction event has generated considerable public interest primarily because of its role in the demise of the dinosaurs. One of the foremost problems that any successful theory of dinosaur extinction must explain is the fact that not just dinosaurs were affected. In fact, dinosaurs represent but a small portion of the species that became extinct at or near the end of the Cretaceous Period. The extinction even that brought the Cretaceous Period to a close was truly a “mass extinction,” in that a wide variety of taxonomic groups from many different habitats were wiped out essentially at the same time.
Many of the groups of organisms that were hit hardest by the K-T extinction lived in the ocean. Ammonites and belemnites, shelled cephalopod mollusks related to the octopus and squid, were abundant in the seas of the Cretaceous Period, but had disappeared entirely by its end. Another squid relative, the nautiloids, were also severely affected; only two species, the Chambered Nautilus and the King Nautilus, have survived to the present. Plesiosaurs, long necked, fish-eating marine reptiles, and mosasaurs, ferocious giant seagoing lizards, also vanished form the seas at this time, although their smaller land-dwelling reptilian cousins survived mostly unscathed.
Bivalve mollusks and other shelled invertebrates of the sea floor also suffered greatly at the end of the Cretaceous. Rudists, bivalve mollusks and the dominant reef builders of the Cretaceous seas, declined sharply towards the end of the Period, and disappeared entirely at its close. Brachiopods, which are bivalved but not mollusks, also suffered greatly, but managed to survive in severely restricted numbers to the present.
Perhaps the most dramatic extinctions in the sea were among the nannoplankton, minute calcium-secreting algae, and the foraminiferans, calcium-secreting protozoans. Their abandoned shells piled up in immense thickness to form the great chalk cliffs that give the Cretaceous Period its name. Marine sediments during the Cretaceous Period were comprised almost entirely of this chalk, with only a small percentage of clay particles. Sediments deposited immediately after the K-T boundary is dominated by clay particles, with only 20 to 40% being chalk. This clay later, known as the “Fish Clay” in Europe, is widely accepted worldwide as the boundary between Cretaceous and Tertiary sediments. It ranges in thickness from less than one-half inch to over three feet in thickness.
Thus the K-T boundary exhibits a drastic reduction in the abundance of calcium secreting organisms. Assuming that the clay particles, derived from the erosion of nearby continents, continued at the same rate across the K-T boundary, this represents approximately a 97% reduction in the abundance of marine calcareous algae.
Dinosaurs were the undisputed rulers of life on land, right up to the catastrophic K-T event, but they were not the only creatures to suffer. Although fossil birds are rare during the Cretaceous (due more to the scarcity of preservation than to a lack of abundance), there were apparently several distinct lineages of Cretaceous birds, only one of which survived the extinction event, to give rise to the birds of today. However, many species within that one lineage survived, as many of the modern bird orders were represented prior to the close of the Cretaceous.
Many species of mammals also survived the extinction, as many mammalian orders also have Cretaceous representatives. Cretaceous mammals, however, tended to be quite small, and probably were predominantly nocturnal. Freshwater animals and the smaller terrestrial cold-blooded vertebrates, reptiles and amphibians, were largely unaffected by the K-T extinction.
Land plants were for the most part unaffected by the extinction event. One prominent plant community, however, was nearly obliterated at the end of the Cretaceous. This assemblage of predominantly by angiosperms (flowering plants) and conifers, is technically known as the Aquilapollenites botanical province, which flourished right up to the end of the Cretaceous Period. Western North America was separated from the rest of the continent throughout the Cretaceous by a body of water known as the Great Interior Seaway. The Aquilapollenites plant community occurred along the wes