At www.macroevolution.net/mesozoic.html … there is quite a nice discussion of the cataloguing of animal remains to specific periods of time with an emphasis on dinosaurs. The Mesozoic began 250 million years ago and lasted up till the K/T boundary event around 63 million years ago. It is defined as the Age of Reptiles which came out of the idea that reptiles were the dominant species throughout the Triassic, Jurassic and Cretaceous periods of time. The idea goes all the way back to the 19th century where these things were first being discovered by scientists of the modern era. One popular misconception is that dinosaurs were all big animals – giant lumbering beasts. Some dinosaurs weighed up to 80 tons but there were lots of others, some as small as chickens.
Only two orders of Mesozoic reptiles are classified as dinosaurs. These are the well known ones you find in books for children (for example) but some so called dinosaurs are not actually reptiles, such as the pterosaurus (pterodactyls in old speak) as they were basically warm blooded giant bats complete with fur rather than scales. Secondly, the mesosaurs, whale like marine creatures such as the plesisaur (once a candidate for Nessie) are related to creatures of the modern world that paddle or use flippers (including seals). In addition. modern crocodiles, turtles and lizards had their Mesozoic counterparts – as well as sharks as pointed out in a recent post. Many other types of animal lived in the Mesozoic – including mammals. However, although a full inventory of fossils of the Mesozoic is known, few, if any, people have looked at them as a whole. The fossils, instead, are assumed to substantiate the basic chronology of evolutionary biology. The latter is familiar to readers of articles on the fossils but not to actually looking at them as a whole. It is much easier to read and assimulate a story about fossils than to actually go and evaluate the fossil evidence first hand.
One point of the story is that mammal fossils of the Mesozoic are rare – but are they. The appear to have come into existence at least as early as the end of Triassic event as their fossils are found in those levels. As such, there is no reason why mammals did not exist through most of the Triassic as well. A review of the literature prompts the suspicion, he says, that the most significant reason for the reported preponderance of reptile remains in the Mesozoic lies not so much in the actual rarity of mammals but in a rather strong tendency of paleontologists to classify fossils as reptiles if they are found in Mesozoic contexts and mammal if they are found in later layers. For instance, on the discovery of teeth in a Mesozoic layer the paleontologist naturally classifies it as reptilian – in the brain as much as in the boxes provided at the museum archive.
McCarthy goes on to say the reason for this 'bias' is subtle, complex, and part of tradition. Another explanation, he suggests, is stereotyping – thinking which box it 'should' be lodged in rather than which box it is realistically related to – as an artifact. Many mammal traits, such as warm blood. a four chambered heart, mammary glands, diaphragm, external ears, hair and fur etc are not the kind of things to be preserved in a fossil state (but he also notes some animal differ from the norm).