New evidence adds to a growing body of information that points to a very different conclusion. Indeed, ever since its inception by an astrophysicist, many biologists and paleontologists have been skeptical of the idea that a meteor strike could be so selective in its ability to cause extinction. Why would it only kill off all species of larger dinosaurs, and leave many species of smaller mammals, reptiles, amphibians, fish and bird-like dinosaurs unscathed? Mass extinction events caused by catastrophes are rarely so selective in their culling.
The theory preferred by a number of biologists and paleontologists, and the theory that I have given the greatest credence to, is that continental movements before and during the extinction event had dramatic effects on the earth’s environment, culminating in the Deccan Traps volcanism which occurred approximately 66 million years ago. This occurred when the Indian subcontinent was moved by plate tectonic forces into the southern edge of the Asian continent, forming the Himalaya mountain range. The Indian sub-continental plate passed under the Asian plate (subduction), and moved into a position over a deep mantle plume (the Réunion hotspot) which then resulted in massive volcanic eruptions that lasted for tens of thousands of years. As much as half of modern day India may have been formed by the lava outflows that occurred at the time.
The massive release of particulate matter almost certainly reduced global temperatures and altered global weather patterns for tens of thousands of years. The effects on plant and animal life were devastating. However, the Deccan Traps volcanic event may have only been the nail in the coffin for many species already driven to the brink.
It has long been known that the number of species of large dinosaurs was diminishing over a several million year period leading up to the K/T boundary in the geological record. In fact, species from all branches of the plant and animal kingdoms were dying off for thousands of millenia preceding the final extinction event. Even the number of species of ocean-going ammonites declined for millions of years before the final catastrophe. Due to continental drift the earth was undergoing drastic changes in climate, weather and temperature range. Eventually even ocean acidity may have changed substantially due to increased levels of CO2 released by the prolonged volcanic activity. The earth was becoming a place that was no longer hospitable for large dinosaur species, but perfect for smaller bird and mammalian species, which evolved quickly to adapt to the changing conditions. Smaller animals with shorter reproductive cycles can evolve and adapt to changing conditions much more rapidly than large, slowly reproducing species. This is probably why the bird-like dinosaurs did not die off as did their much larger cousins.