I find a logic which indicates grass caused the transition. Modern plants and animals had been evolving slowly, and in very low numbers, during the dinosaur years; but only after a change in the ecology could modern species take off.

It is clear and beyond question, based on the total ecology and biology, that the factor which shaped the transition to modern plants and animals is grass. This would be true even if an asteroid were the triggering factor; but two factors cannot interact without interference; so it is unlikely that an asteroid was involved, but it cannot be totally ruled out.

At the time of the transition, mammals replace dinosaurs, and woody and flowering plants replaced most of the nonwoody plants. The characteristics of modern plants and animals show that grass was a major factor influencing their evolution.

Consider what grass is. Unlike most plants, grass absorbs water above the ground. The absorption point is at the base of the leaves, which is at ground level. Of course, water can also be absorbed by the roots.

The advantage is that the lightest rains provide usable moisture. In fact, grass can grow on morning dew, which is why it has long, narrow leaves. Dew drops roll down the leaves to a point where they come together and water is absorbed.

With nongrassy plants, moisture has to soak into the ground before roots can absorb it. Then, only heavy rains are beneficial.

These characteristics allow grasses to produce a faster expansion of green growth in dryer areas than most other plants, which in turn allows grass to grow thick and crowd out other small, nonwoody plants. But grass does not crowd out the woody plants, because they can grow above it.

The nonwoody plants of the dinosaur years had large leaves, and foliage covered the ground creating difficulty for animals to move through it. The environment would have been like a huge compost pile. The dinosaurs had to be large to walk over it and through it, while mammals had to be small to move around within it.

We can conclude that there would not have been a large amount of grass at the time of dinosaurs, because it would have crowded out the nonwoody plants which dinosaurs were eating. Grass also would have eliminated the bare ground which dinosaurs needed for laying their eggs. If grass were prevalent, dinosaurs would have had to carry their young like mammals do. Therefore, it would have been grass that caused mammals to evolve.

Dinosaurs were reptiles. After grass covered bare ground, the only way reptiles could survive was to lay their eggs in holes under the ground.

Recently, paleontologists found that a few modern flowering and woody plants (beyond conifers, which were 300 million years old) existed more than 200 million years ago, while the modern biological era began 65 million years ago. This means the modern type plants were not competing well with nonwoody plants until the modern era began. When grass shoved out many of the nonwoody plants, modern trees could flourish.

At this point, the basic nature of evolution needs to be considered. There are two types of evolution: the gradual changes that occur continuously with all species, and the large leaps that occur sporadically. The major leaps must be caused by changes in environmental conditions, because all advantageous characteristics must be adaptations to external influences. Grass would have created a major change in environmental conditions for nearly all other land species. Ferns increased in numbers at that time, apparently because the change in ecology was favorable to them. Grass also gave the mammals mobility and allowed them to thrive.

The selective advantage of dinosaurs over mammals was that they could get around in the brush better by not having to carry young. They could leave eggs in a nest on the ground while tromping through the brush. But grass reversed the dynamic. When the brush disappeared, the dinosaurs had no advantage, and their large size was a disadvantage, because they could not eat the woody plants which replaced the nonwoody plants.

New Evidence from a Dinosaur Fossil

A new dinosaur fossil (Anzu wyliei) is thin and long legged and about ten feet (three meters) tall. It was one of the last dinosaurs. This means grass existed before or during the die-off of dinosaurs.

Long legs and thin bones are needed to go through grass, as demonstrated by deer. The normal dinosaurs were extremely heavy, because they needed to go through thick brush created by non-woody plants.

Since there was enough grass to evolve a new dinosaur before they all became extinct, the evolution of grass could explain dinosaur extinction with no asteroid being required.

Link to UCMP National Geographic