Posted: Dec 23, 2010 9:58 pm
by theropod
Popular misconception:

"All Prehistoric Beasts were Dinosaurs, and They Were All Huge"

In popular culture there exists a notion that any large animal that existed in the distant past was a dinosaur, and that all dinosaurs were huge lumbering giants. This group of misconceptions has been extended to cover anything from the Permian pelycosaurs Dimetrodon to the Pleistocene Wooly Mammoths. Pterosaurs, as well as marine reptiles such as Ichthyosaurs and Mosasaurs, have also been lumped into the false dinosaur group. This erroneous idea is the direct result of lack of proper education in combination with those holding an agenda to discredit the hard science of vertebrate paleontology. Books aimed at children often depict these other creatures as dinosaurs and the notion sticks with people the rest of their lives.

In order to understand why this is a problem we should first examine the morphological features that either include or exclude a creature from the group of organisms classified as dinosaurs. While it would be interesting and educational to examine the evolutionary pathways that lead to the emergence of dinosaurs that aspect of the matter is not the focus of this effort. Dinosaurs exhibit a specific set of features that distinguish them from all other creatures that have ever lived. While dinosaurs share a great many characteristics with other animals they also have exclusive skeletal elements only found in dinosaurs. Most notable of these features is the pelvic structure. At some point in the evolution of dinosaurs a split occurred wherein two families of dinosaurs emerged. The "Bird Hipped" Ornithischians and the "Lizard Hipped" Saurischians.

The common names of dinosaurs don't help the lay person gain any better understanding of what constitutes a dinosaur. Usually when such people think about dinosaurs they see one term or the other and come to the false conclusion that the dinosaurs with "Lizard Hips", for example, are somehow just big lizards. Even the term dinosaur translates to thunder lizard. Partly this confusion arises from the fact that the earliest paleontologists didn't really understand the fossils that they were finding and describing. There is no need to place blame here as without a base upon which they could build they were forced to work from what was already known. One of the first described dinosaurs, Iguanodon bernissartensis was named the way it was because the material looked, to those early workers, like large versions of known extant lizards. In hindsight these early efforts were clearly in error, but they didn't have the luxury of our current knowledge base.

The structural morphology, which supported the weight of dinosaurs during locomotion, is the key feature which sets them apart from all other creatures. In all other reptiles, including lizards, the legs are splayed apart in a bowing fashion, whereas in dinosaurs the legs reside directly under the body like pillars. Both the Ornithischians and the Saurischians display this characteristic, but differ in the supporting pubic skeletal elements. In the Saurischians, or "Lizard Hipped", dinosaurs the pubis is oriented more toward the the front of the dinosaur and in the Ornithischians, or "Bird Hipped", dinosaurs the arrangement of this feature is modified to favor an orientation toward the rear of the dinosaur.

Among the Saurischians one of the more commonly know groups of dinosaurs, the theropods, reside. This includes the giant Tyrannosaurus rex, which is not the largest theropod to ever exist, as well as the diminutive Microraptor zhaoianus, which had feathers on all four limbs and was a possible insectivore, although the type specimen contained ingested small mammalian skeletal elements that were discovered during preparation. The very largest known land animals to ever exist were the Sauropods and also fall within this classification. Evidence of the evolutionary relationship between Saurischians and extant birds is quite robust, and quite interesting, but again is not the focus of this subject.

Among the Ornithischians the horned Ceratopsia, armored Ankylosauria and well known duck billed Hadrosauridae all reside. Most, if not all, of these dinosaurs were herbivores.

When one considers the classification of a creature to determine if it lies within dinosauria one must also take into account the temporal setting in which the fossil is deposited. While many of the organisms incorrectly lumped into dinosauria were extant over the span of time when true dinosaurs lived this alone is an insufficient qualification. The pterosaurs and marine reptiles shared none of the key skeletal elements, and in fact differ greatly from the accepted standards cited earlier, they did share the environment with dinosaurs. It is also interesting that these animals suffered the same fate as the dinosaurs they lived alongside when the end of the Cretaceous epoch came. This temporal restriction automatically eliminates those creatures that preceded dinosaurs and those that arose after dinosaurs became extinct. Excluding birds, which evidence strongly suggests are a highly derived line of theropods, no good evidence exists that any non-avian dinosaur survived the Cretaceous-Tertiary boundary. Obviously the mega-mammals that arose after the K-T event fail to meet the temporal requirements as well as the skeletal construct true dinosaurs display.

In conclusion it becomes obvious that the dinosaurs were a highly specific group of creatures that were very successful over a very long span of time, and that lumping any prehistoric animal into this grouping is a mistake. While a great deal of progress has been made in educating the general public a continuing effort must be undertaken to assure that children, and interested adults, are not allowed to think of dinosaurs as anything other than the very specific creatures that lived and died so long ago.


Weishampel, D.B., P. Dodson, and H. Osmólska (eds.). 1992. The Dinosauria. University of California Press, Berkeley.

Carpenter, K. and P. J. Currie, eds. 1990. Dinosaur Systematics: Perspectives and Approaches. Cambridge University Press, Cambridge.