It is a story that dates back to the times of a great naturalist called Charles Darwin and even before him. Before him, organisms were being classified into kingdoms and lower taxa, but the basis for their classification was mainly focused on morphology.
When Darwin came up with his “theory of evolution”, taxonomy was probably the first field in biology that needed reconstruction. The very basis of classification changed from morphological and anatomical features to the lookout for the common ancestor.
When we look at what defines a dinosaur, we have to go back to that idea of a common ancestor. A British paleontologist Richard Owen was the first to coin the term “dinosauria” for sub-order of reptiles, meaning “terrible lizards”. This term is misleading today as lizards are biologically distinct and not considered dinosaurs.
Dinosaurs Aren’t Lizards
What distinguishes lizards from the dinosaurs is the structure of hip bones, shape of skull and sometimes skin. Dinosaurs were the first group of animals that became exclusively terrestrial (some of them of course, not all) and whose legs were put directly under their body, like birds. They had a ball and socket joint in their pelvis that allowed their legs to come straight under their body unlike reptiles whose legs appear protruding from the sides. Dinosaurs enjoyed a free forward and backward movement of their legs that allowed for greater mobility, not like reptiles who have a side to side movement which is actually reminiscent of movement in water. Dinosaurs said to themselves “we are moving towards land away from water and so we don’t need side limbs”. It was a game changer.
What Makes an Animal a Dinosaur
Now we look at what exactly constitutes a dinosaur? An animal that has scales or feathers on its skin, and legs directly under its body is for sure a dinosaur.
Note that many animals portrayed in movies like “Jurassic Park” are not true representations of dinosaurs. Dinosaurs in film are often badly portrayed with regard to biological point of reference. Pop culture and many children books have a habit of portraying some unrelated animals as dinosaurs, which is misleading.
What Happened to the Dinosaurs?
At this point, we see what remains of those magnificent creatures besides fossils?
Luckily, at least one group of them managed to survive somehow.
Birds are considered modern day dinosaurs. It is not that every dinosaur was a bird, but that every bird is a dinosaur. Avian species are the one and only survivors of a diverse group of animals called dinosaurs.
There were herbivore dinosaurs, bipedal and quadrupedal dinosaurs, beast feet dinosaurs and ornithopods or bird feet dinosaurs. Some were 10 meters tall while some were as small as 20 centimeters and didn’t survive.
There is no difference however in an innocent looking modern day pigeon and an extinct predatory remote age gigantic dinosaur. They share a common ancestor and many common features that place them in a single order.