April 4, 2022 Scott Kraus

Ornitholestes is a Small Bipedal Carnivore

Ornitholestes, pronounced “or-NITH-o-less-teez”, was a small but fast bipedal dinosaur that lived in the areas of modern day Wyoming and Utah. It was a hunter of small game, like lizards, and its name comes from the Greek words meaning “bird robber”. This theropod lived in the late Jurassic period, about 154 million years ago. Paleontologists believe that ornitholestes is closely related to the line from which the birds originated – this dinosaur was able to tuck its hands up close to its body just like how a modern bird holds its wing.

Quick Facts

What did Ornitholestes look like?

You might not know by looking at Ornitholestes that it was a capable hunter. It was 6 feet long, but only about 3 feet tall and most of its length was in its long tail. This dinosaur’s body was lightly built, only weighing about 25 pounds, and its head was small. Paleontologists think this carnivore was fast and agile and capable of deftly chase its prey and grabbing it with its long-clawed fingers. Ornitholestes legs and arms were long, although its small head was proportionately smaller in relation to its body than most carnivores. This dinosaur’s skull was strong and tough with a sturdy jaw and its orbits, or eye sockets, were proportionately large, at over 25% of the skull’s size. This feature indicates keen eyesight. Ornitholestes also had sharply serrated back teeth that were characteristic of other theropods, most of which are meat eaters. The front teeth however, were round and less serrated .

Physical Attributes of Ornitholestes

Ornitholestes was a small bipedal meat eater. So, despite its slight build, it had the physical attributes of a carnivore, from very sharp teeth to long clawed fingers. It was once thought by scientist Gregory S. Paul to have a nasal horn from a broken skull specimen that seemed to bulge upward, but two other scientists, Oliver W.M Rauhut and Kenneth Carpenter, disagreed. They pointed out that the upward bulging was due to post-mortem damage to the skull and Paul later amended his writing to reflect that new interpretation. The most noticeable part of Ornitholestes is probably its tail, which was long, flexible, and made up over half of the body length.

Guessing about Ornitholestes Skeletal Remains

The case of the possible crest on the small theropods head was not the only time that paleontologists have guessed about the dinosaur’s skeletal structure, since fossils for this dinosaur have been very few and far between. Regarding the vertebrae, paleontologists know that it had an s-shaped curve in its neck and a long flexible tail, but the number of vertebrae has been debated since not all of them were preserved. In 1917, paleontologist Osborn estimated 9 or 10 vertebrae in its neck, thirteen back vertebrae, four hip vertebrae, and 39 to 44 tail vertebrae. In 2005, paleontologist Carpenter determined that the specimen actually had 5 hip vertebrae. More guessing has been done regarding lack of preservation of its third claw bone. Paleontologists extrapolated from the closest relatives of the dinosaur and now believe that it was shorter than the first two claw bones that were well preserved. Even more debate has occurred, centering on the nature of the claw on its second digits, the length of the dinosaur, and the weight.

What did Ornitholestes Eat?

Ornitholestes was a carnivore, so it ate meat despite its small stature. Paleontologist Henry Fairfield Osborn, who named Ornitholestes in 1903, first noted the large teeth, its strong hands, and its tail that would have been used to balance while running. He concluded from these adaptations that the dinosaur could have preyed on contemporary birds, hence the “bird robber” meaning behind its name. However, he later went back on this idea and said in 1917 that the dinosaur was in the early transition from carnivore to herbivore. At the same time, Charles R. Knight had drawn a popular illustration depicting Ornitholestes chasing Archaeopteryx, operating under the same theory as Osborn’s first one about the small theropod. That idea eventually waned among paleontologists and most now think the dinosaur ate small animals like mammals, lizards and hatchling dinosaurs. Some paleontologists go even further, suggesting that its strong jaws would have allowed it to prey on larger animals, especially if it was a pack hunter.

Did Ornitholestes have Feathers?

The theory that Ornitholestes could have had feathers was first proposed by Percy Lowe, an ornithologist, in 1944. This interpretation was rejected for over 50 years, as the consensus was that theropod dinosaurs, like Ornitholestes, were more like reptiles than birds and had scaly skin. That consensus began to change in 1986 with Robert T Bakker’s “The Dinosaur Heresies” who stated that dromaeosaurids had feathers and would prove to be correct. He also argued that all dinosaurs were endothermic, although he didn’t include feathers on the illustration of Ornitholestes. Then the Sinosauropteryx was found in China in 1996. It was a well-preserved fossil proving the dinosaur had a coat of feathers. Now paleontologists believe that all coelurosaurs, like Sinosauropteryx, had insulating feathers or were related to ancestors that had them. John Foster suggested that Ornitholestes would have had primitive feathers, as a relative to other bird ancestors. He said of the feathers, “[they] would have covered the body except the legs in a short coat, perhaps with longer feathers lining the top of the skull or neck and the back edge of the forearms.” The feathers would have been used for brooding eggs and insulation.

How did Ornitholestes Move?

Ornitholestes was bipedal, so it moved on two legs on the ground. Paleontologists believe it was a fast runner and it is estimated to have been able to run as fast as an ostrich, up to 43 miles per hour. Its long tail was very flexible and would have been used to balance and aid in agility when chasing prey.

In 2006 a study was done on articulated casts of Ornitholestes’ right arm to find out its range of motion. The dinosaur was found to be able to swing freely within a 95° range. When the arm was bent inward at the elbow as much as it could, the upper arm and lower arm formed a 53° angle. The dinosaur’s ability to bend its forearm farther than  90° is typical of more advanced theropods and not existent at all in primitive theropods. When the arm was straightened at the elbow, there wasn’t a straight angle and scientists concluded it couldn’t pronate, or twist its arm with the palm of its hand downward. That means its forearms moved inward when the elbows were bent and it could have used that ability to hold prey with both hands at once.

The Discovery of Ornitholestes

The first specimen of Ornitholestes was discovered in July of 1900 in the Bone Cabin Quarry of Wyoming. The excavation was conducted by Peter C. Kaisen, Paul Miller and Frederick Brewster Loomis who represented the American Museum of Natural History. The specimen was only a partial skeleton with the skull, many parts of the vertebrae, the arms, pelvis, and legs. Henry Fairfield Osborn named and described the skeleton in 1903, using a name meaning “bird robber” suggested by Theodore Gill, as the dinosaur was originally thought to prey on birds.

Ornitholestes Tentative Hand Discovery

A partial hand specimen was originally thought to belong to Ornitholestes as described by Osborn in 1903, but Gregory S. Paul noted in 1988 that the preservation was too poor in the specimen to be considered anything but “tentative” in the association with the dinosaur. In 2005, a new small theropod was found near the hand specimen and the match was much better than with Ornitholestes, so it was reassigned.

The Importance of Ornitholestes

Unfortunately, the fossil evidence of Ornitholestes is very scarce and the specimen that is available is incomplete. Paleontologists have had to infer a lot about the exact physical specifications of Ornitholestes, not to mention its behavior. But it does have the distinction of being the first theropod discovered in the 20th century. Ornitholestes is also considered to be one of the distant ancestors of modern birds, which helps paleontologists understand how animal life has transitioned from so many million years ago, to what it is today.