Most natural history museums do not display mammals with dinosaurs. But it turns out mammals are not all that rare in dinosaur rock layers. Nearly 300 genus groups ("kinds") of mammals have now been found in dinosaur rock layers. Dinosaur-age mammals have been found in Europe, Canada, China, Australia and the USA.
There has been discoveries of a 12 pound opossum-like mammal and a three-foot long, 30 pound collie-sized Tasmanian Devil-like mammal.
With new information about mammals being found in Jurassic rock layers, the term "The Age of the Reptiles" is considered a misnomer. But unfortunately, some museums perpetuate this myth even today.
"In a sense, 'The Age of Dinosaurs' or 'The Age of Reptiles' for the Mesozoic is a misnomer ... Mammals are just one such important group that lived with the dinosaurs, coexisted with the dinosaurs, and survived the dinosaurs" (Dr. Zhe-Xi Luo , curator of vertebrate paleontology and associate director of research and collections at the Carnegie Museum of Natural History in Pittsburgh, Pa.).
According to Dr. Zhe-Xi Luo, there is nearly 100 complete fossil skeletons of mammals that have been found in dinosaur rock layers.
In the beginning of fossil discoveries, paleontologists would only collect the dinosaur bones and leave the mammal bones behind. The Morrison Formation had many large dinosaur bones which the scientists easily spotted. The so-called "bone wars" began. When they came upon mammal bones they discarded them. Because of this disparity in fauna, they referred to the dinosaur era as the "Age of the Reptiles". These scientists missed the great wealth of mammal bones alongside dinosaurs. But I believe they did not want to give these mammal fossil bones that were found alongside dinosaur bones any credence that would destroy their evolution thinking that dinosaurs died out long before mammals appeared.
The reason there is dinosaur and mammal bones together, is that they were both living together at the time of the great cataclysmic global flood of Noah's day (Genesis 6-9). They both died in the flood of waters of muddy sediments that rapidly buried these creatures together, which later became the rock layers we have today.