Diverse fossil insect assemblages near the Jurassic/Cretaceous transition from the Shar-Teg in Mongolia comprise frequent deformed species. These (first known) mass fossil animal deformities, expressed as fusions of veins changing the wing geometry, probably represent heritable mutations. They accumulated as a result of a changed structure of selective pressure, and are unique in showing how individual variations may be fixed to form higher taxa, significantly contributing to the process of evolution. Similar deformities were also recorded in recent ecosystems undergoing elevated environmental stress. The occurrence of deformities indicate a long-lasting (100 kyr–1 Myr) ecological stress in the continental environment before the J/K boundary and a biotic character of the changes: high evolutionary tempo and consequent radiation of newly evolved taxa forming new control mechanisms including social decompositors and new predators, resulted in temporary more or less destabilized ecosystems and uncontrolled, rapid evolution of its elements. Accordingly, ecosystems with higher diversity stabilized and some of their elements remained virtually unchanged for over 30-million-years at least in Laurasia. Notably, occurrences of true flowering plants and some advanced insects during the lowermost Cretaceous are limited to the region.