Dinosaur Extinction From Egg Predation by Mammals
Some people make the mistake of thinking that the mammals only evolved after the dinosaurs had already become extinct. In fact there were mammals at the time, but they tended to be small insectivores. The largest mammal was Repenomamus giganticus which was 1m (30 in) long. Compare this to the smallest dinosaur discovered at 40cm (15.7 in), as yet it is unnamed.
When looking at our fossil records we notice a gradual decline in the number of dinosaur species. This, combined with the fact that mammals existed during the time of the dinosaurs and flourished at the demise of the terrible lizards has made some people hypothesize that maybe the dinosaurs had their eggs eaten by some of these mammals.
Unfortunately, there are many problems with this hypothesis. The first of which is that even though there had been a gradual decline in the number of species of dinosaurs there is also evidence of a mass extinction of animal and plant life, known as the Cretaceous-Paleogene (K-Pg) extinction event (previously known as the Cretaceous-Tertiary extinction event). Egg predation by mammals would not cause this event.
The second reason is that large amounts of iridium are found at the geological K-Pg boundary. This element is very rare on Earth and adds leverage to impact hypothesize.
Another explanation for why this is unlikely is that mammals and dinosaurs evolved at the same time and if mammals evolved to eat dinosaur eggs, the dinosaurs would have evolved to better protect their eggs. Large nesting colonies have also been discovered with nests spaced by the size of an adult of that species apart. This would have made it very difficult for predators to have eaten enough of the eggs to cause extinction. It would also not explain the extinction of aquatic animals at the same time.
When looking at the evidence it begins to look like this is a highly unlikely explanation for the dinosaur extinction.