II. The History of “What is a Dinosaur?”
Despite their prominence with both scientists and the public, it has not always been easy to identify the unique characteristics of dinosaurs. Indeed, the very question “what is a dinosaur?” occupied paleontologists for decades. For many years, scientists did not think that all dinosaurs were closely related to one another. Instead, they thought that two or three groups of dinosaurs might have evolved independently from different ancestral reptiles. These early workers did not recognize any features that could be used to distinguish dinosaurs from all other animals.
Today, however, paleontologists accept that dinosaurs are a single group of animals that share a unique set of features. For example, all dinosaurs have jaw muscles that extend onto the roof of the skull, a large muscle ridge on the humerus, and well-developed processes at the hip, knee, and ankle. Scientists use
such shared, derived charecteristics to diagnose dinosaurs because they are always present in dinosaurs, and they are only present in dinosaurs. Such shared, derived characteristics (also called synapomorphies) are important for classifying organisms because they help to identify unique events in evolutionary history. In this case, these features arose in the ancestor of dinosaurs and were inherited by all its descendants, including birds.