Out of the many theories on the extinction of the dinosaurs the poisonous fungal theory has some grounds on the fact that the dinosaurs were wiped out completely. The theory states that since most of the flora and fauna on the Earth was destroyed due to the asteroid hitting it fungi served as the major flora. The evidence from 8 cm thick coal layer suggests that it was composed of ferns, conifers and other flowering plants. However, the upper 4 mm layer of the same consisted of fungal spores and remains of various species of fungi.

This discovery suggested that there was a point when all the flora and fauna was destroyed and the Earth was inhabited for a short span of time by fungus. This showed that fungi were a dominant flora at this period if not the only one.  The evidence suggested that some calamities like hitting by an asteroid caused all the photosynthetic plants to die and allowed non photosynthetic flora like fungi to grow as the high humidity and atmospheric dust and aerosol would have diminished the amount of sunlight coming to the Earth. This made the condition favorable for the growth of fungi.

However, most of the fungal species that dominated the plant kingdom on Earth that time were poisonous and detrimental to the animal life. Though it was easy for smaller animals to run around easily and find some alternative food it was difficult for large size dinosaurs to do so and hence lead to death if eaten in large quantity.
Thus, since the fungi was the major flora found as food for the dinosaurs they ate it without realizing the consequences which lead to the mass destruction of dinosaurs. However, there are a few pitfalls for this theory too.