Hypsilophodon
Hypsilophodon
(HIP-sih-LOFF-o-dawn)
Name means: Hypsilophus (lizard) Tooth
Systematics: What’s in a Dinosaur Name?
The sign text for Hypsilophodon mentions the term “wastebasket taxon.” Of course, that begs the question of what is a taxon to begin with? The short answer: it’s an abstract scientific model of an organism. For a fuller, clearer explanation, read on! And if you have questions, feel free to ask a member of our staff for clarification.
First, bear in mind that even though we usually think of giving names to things, in paleontology we work backwards and give things to names. When we discover a new fossil, we assign it to a taxon—a name with a specific set of definitions to help us recognize fossils and fit them within a theoretical context. Think of a taxon like a closet full of bones, and the closet door has a sign on the front saying what kinds of bones fit in the closet. When we find a fossil that doesn’t fit the description on any of the doors, we build a new closet—that is, we “erect a new taxon” for those fossils. Each taxon represents a theory on the behavior of the animal, its genetic, evolutionary, and ecological relationships, and even its appearance. IT DOES NOT REPRESENT THE ANIMAL ITSELF, only our ideas about it.
Does that seem backward or unnecessarily convoluted? It might if we had living animals to work with, although biologists follow this same protocol. If we had nothing more to learn about organisms, we could ditch the whole taxon idea. But even when studying the biology of modern animals, we still have an incomplete set of data. There’s always something more to learn. And because of the nature of the fossil record, paleontology doesn’t just work with an incomplete set of data: it works with a set of data that we will never complete. We therefore use taxa in part to acknowledge that fact, to keep our attitudes about these creatures precise, at least formally.
We do keep adding to that data set, though. Taxa help us manage those changes. Depending on the degree of change, we may just revise the description on that closet door, which we call a “diagnosis.” Drastic changes sometimes require us to erect new taxa or to move bones from one taxon to another. Those changes can come in the form of new fossils that completely change our way of thinking about the animal in question, but sometimes they come through changes in methodology, either to correct a specific mistake or to improve scientific techniques in general. In these cases, the “new” dinosaur might represent the reclassification of fossils that scientists have studied for decades, but they get a new name upon review because we have a clearer point of view. Or, to be honest, occasionally we give a dinosaur a new name by mistake even though it doesn’t need one.
Some of the first dinosaurs to get scientific descriptions make good examples of this process. The first theropod described, Megalosaurus, was known only from the tip of a lower jaw. It obviously represented something new to science at the time, but its fragmentary nature led to a rather generalized description. As a result, a bunch of other fragmentary finds ended up added to this taxon; any time somebody found a big carnivorous dinosaur, they named it a new species of Megalosaurus. The sign on the “Megalosaurus” closet door just wasn’t specific enough, so it became a “wastebasket taxon” that held a bunch of junked-up bones that couldn’t give us much information. Eventually, it became obvious that the way scientists described this taxon made it look like Megalosaurus had 20 different species and spanned 30 million years—an implausible scenario at best. So the “Megalosaurus” taxon came under review, starting with the bone used to make that first diagnosis, known as a “holotype.” Scientists found enough unique features that Megalosaurus remains a valid taxon, but all the other bones attributed to it have been moved elsewhere or labeled “indeterminate.” Until we find a skeleton attached to a jaw identical to the holotype, we can’t add any more information to Megalosaurus.
Like with any closet, sometimes we just have to clean house. It may create the illusion of finding new dinosaurs without new fossils or some kind of new extinction, but it’s just scientists refining their ideas as new information comes along. —Jeff Bond
Pictured below: the only confirmed fossil of Megalosaurus, and the first fossil of a dinosaur to be scientifically described.
For Further Reading:
Species: foxii
Range: Early Cretaceous (Hauterivian-Barremian, 130-125 MYA) of Great Britain, possibly France and Spain
Length: 5-6 feet, 22-45 lbs
Discovery: Aldous Huxley, 1869
Classification: dinosauria, ornithischia, genasaura, ornithopoda, hypsilophodontidae
When threatened by predators, this dinosaur’s best defense lay in its speed. Long legs, the hip design unique to dinosaurs, and an efficient breathing system helped this animal run at high speed. Scientists do not know exactly how fast it could go, however. Hypsilophodon’s special tooth design made it an efficient chewer as well. Wrinkles on the surface of its teeth helped them keep their grinding edge even as they wore down.
As one of the early dinosaur discoveries, Hypsilophodon has spent its share of time as a so-called “wastebasket taxon.” For many years it was the only small plant-eating dinosaur known. Scientists often referred poorly preserved fossils to this genus because they didn’t have anywhere else to put them. They also grouped any small plant-eating dinosaur in a family named after Hypsilophodon. Later studies have proven that the "hypsolophodontids" are not closely related after all. Fortunately, Huxley based Hypsilophodon’s original description on a well-preserved set of fossils. As a result, scientists have had an easier time determining its relationships to other dinosaurs.
The only definitive Hypsilophodon fossils occur on the Isle of Wight in Great Britain. Fragmentary fossils may indicate its presence elsewhere in western Europe. A land bridge also may have connected North America, Europe, and Africa as late as the Early Cretaceous epoch. If so, Hypsilophodon or its close relatives may await discovery in any of those continents. However, the small, lightly-built bones of these creatures do not preserve well. Scientists may have to continue to rely on fragments to hypothesize on the extent of Hypsilophodon’s range.