For many years after dinosaurs were first discovered and determined to be reptiles, scientists thought that they dragged their tails—much like living reptiles. That hypothesis was dispelled in 1970 by John Ostrom of Yale University. Ostrom found two new species of dinosaur from the Cloverly Formation of Montana, both of which possessed elaborate structural modifications of their vertebrae and muscles that prevented their tails from sagging or dragging. The two dinosaur species that provided this new information were Deinonychus antirrhopus and Tenontosaurus tilletti.
Deinonychus means “terrible claw,” referring to the slashing claw on the animal’s hind foot. Antirrhopus is Greek for “counterbalancing” and refers to the unusual adaptations of the tail vertebrae that keep the animal’s tail rigid.
The tail vertebrae of Deinonychus possess extremely long extensions, ossified (hardened into bone) tendons, aiding in creating a stiffened tail. These elongated outgrowths, called zygapophyses (ZIG-uh-POF-uh-seez), are the parts of the vertebrae that normally connect one vertebra to another. All vertebrae have four zygapophyses—two on the anterior (front end), and two on the posterior (back end). Normally, the connecting zygapophyses allow vertebrae to slide against one another, providing flexibility in the spine. In Deinonychus, however, the zygapophyses inhibit movement in any direction. As a result, Deinonychus could not swing its tail back and forth or up and down.
Tenontosaurus means “sinew reptile” and refers to the animal’s extensive network of ossified tendons that held its tail rigid. Tilletti refers to the Tillett family, whose ranch was near the site where the specimen was found.
Functional anatomical studies have shown that the upright, tail-dragging posture originally proposed for dinosaurs was incorrect. Instead, all dinosaurs had skeletal modifications that gave them rigid tails, meaning they maintained a horizontal vertebral column. Studies by former MSU PhD student Chris Organ have shown that rigid tails also aided in the movement of dinosaurs by helping to store elastic energy.
Case Contents: Tail section of Brachylophosaurus showing ossified tendons along
vertical spines and individual ossified tendons from Brachylophosaurus
DINOSAURS WERE WARM-BLOODED (Display Panel)
Thermophysiology is the study of how animals maintain their internal temperatures.
Temperature regulation is managed in a number of ways in living animals. When we call animals warm-blooded, we usually mean that they keep a steady, high internal temperature regardless of the outside temperature. When we say an animal is cold-blooded, we usually mean that it has a relatively low internal temperature that changes in response to the outside temperature. There are several distinctions inherent in each of these definitions: (1) How body temperature is regulated; (2) Whether the internal temperature is constant or varies;
(3) Whether the temperature and metabolic rates are high or low when the animal is at rest.
To describe the first distinction, scientists use the terms “endothermy” and ectothermy.” Endo- means “inside” and ecto- means “outside”; thermy comes from the Greek thermos, which means “heat” or “temperature.” An endotherm has an internal regulatory system that maintains its temperature at a constant level. An ectotherm has a body temperature that responds directly to
The second distinction has to do with the result of regulation—a constant high temperature or a normally low but widely varying temperature. “High” and “low” are relative terms. “High” generally refers to the range of resting body temperatures that are characteristic of birds and mammals, and “low” to the much lower resting temperatures that are characteristic of reptiles and amphibians. Maintenance of a constant high body temperature is called “homeothermy.” The wide variation of body temperature in response to environmental temperatures is called “poikilothermy.” The combination of homeothermy and poikilothermy is called “heterothermy.” Heterotherms have body temperatures that vary, but not to the extreme of the environment..
The third distinction has to do with metabolism. A simple definition of metabolism is that it consists of all the cellular activities and chemical reactions that transform food into energy and use that energy to run the body’s activities. Endotherms maintain their temperature at a high level by having a high metabolism; in other words, all that cellular and chemical activity occurs at a fast pace. Ectotherms have a much lower resting metabolism. Animals that keep their metabolic rate high all the time are “tachymetabolic,” and animals that have a slow, or low, resting metabolism are “bradymetabolic.”
Evidence suggests that dinosaurs were endothermic heterotherms. This physiology would have allowed them to reach larger sizes than mammals because they would have required less food to maintain their fluctuating body temperature.
EVIDENCE FOR ENDOTHERMY AND HETEROTHERMY (Display Panel)
RAPID GROWTH IN ALL SPECIES: Open spaces in bones are the channels for blood vessels and nerves. The faster a bone grows, the more blood vessels must be present. Histological evidence shows us that dinosaurs grew quickly, since many of these channels are present in their bones. Animals that grow quickly typically have high metabolisms, which is an endothermic quality. The display panel includes cross-section histology images. Visitors can compare the number of channels present in the ostrich, Maiasaura, and alligator cross-sections. The similarity between the ostrich and the Maiasaura tells us that dinosaurs, like birds, were endothermic heterotherms.
EGG INCUBATION IN THEROPODS: Only endothermic birds are known to brood (sit on) their eggs to incubate them. This Oviraptor specimen from Mongolia, another two specimens like it, and a Troodon specimen from the Two Medicine River Formation of Montana, indicate that some dinosaurs brooded their eggs.
FEATHERS ON SMALL SPECIES: Feathers on birds, like hair or fur on mammals, function to insulate the body so that it retains the heat produced by endothermy.
HETEROTHERMY: All birds are heterothermic. Because dinosaurs gave rise to birds, we can hypothesize that dinosaurs were heterothermic as well.