lets be realistic about this , we are talking about an
extinction event that did happen according to current knowledge.
smaller animals did survive but the larger ones did die off
the only real reason that there could be would be the lack of plants and as a result the lack of oxygen.
smaller animals would survive because they would require less plants for food.
the earths air may have been much richer in oxygen before the asteroid or more ppm of oxygen in the air.
after the larger plants died off there would still be some smaller plants closer to the ground and close to the entrances of caves and depressions where the smaller animals lived.
the larger animals just couldn't survive in the low oxygen environment.
Im certain that in some areas there were hardy forest that did survive for a while after the asteroid and there were several large animals that survived in them especially those that were used to low oxygen in high elevation areas.
I don't think that a lack of diversity was the culprit it
must have been the lack of oxygen.
also when the sun light faded the temperatures plummeted any animal that lived in areas with higher temperatures just froze to death unless they could migrate in time to warmer areas and the smaller animals could simply lay on the ground or enter caves to stay warm.
really large animals were left out in the cold or they huddled
together in the depressions or pockets of air that were available.
I looked in my minds eye and that is what I saw.
