More than a century has thus elapsed since the first Mesozoic mammal was made known. In that time, which includes almost the entire history of the science of vertebrate paleontology, most students of fossil mammals have been concerned in some way with the group, from Cuvier to our contemporaries. Yet the known mammalian faunas stand out like lights in the vast darkness of the Age of Reptiles – and very dim lights most of them are. This mammalian prehistory is two to four times as long as the “historical period” which followed it, and yet the materials for the latter are literally many thousand-fold those for the former. This however, only makes close scrutiny of the Mesozoic mammals which are known the more necessary, and the results which are to be obtained from them the more precious.

The bony skeletons in a museum give such good feeling for what the animals must have been like that we tend to forget what a skeleton is. We are looking only at the compression members of a structure whose tension members have rotted away.…. Bones have no functional meaning without their muscles, tendons, and ligaments, which move them and hold them together. Contrarily, of course, muscles and tendons must act against something comparatively unyielding, such as bone, cuticle, or water.