A relative of the rhinos, Brontops belonged to an ancient group of mammals called the Brontotheres. Standing up to eight feet tall at the shoulder, Brontotheres were the largest mammals ever to walk on North American soil. Despite the massive size of the Brontops skull, its brain cavity is no bigger than the size of an average person’s fist.

CASE STUDY
The White River Formation of the Badlands is a spectacular burial ground for mammals that lived during the Oligocene Epoch, about 25-35 million years ago.  Spreading across parts of Nebraska, South Dakota, Wyoming, and Colorado, this region was once filled with lush forests and pond-strewn savannas. Thanks to this wet environment, skeletons from many Oligocene animals were buried quickly and preserved, so scientists can now reconstruct this ancient ecosystem in amazing detail.

Q: What can small fossil fragments tell you about an ancient ecosystem?

CT: Tiny bits of bone represent the reality of the fossil record ­ scientists rarely find entire skeletons, or even entire skulls, intact. Teeth are the most likely part of a vertebrate skeleton to survive over time. Luckily, their shape and size provide valuable clues to both an individual’s identity and diet and to an ecosystem’s food web.

Dr. Carol Tang
Senior Science Educator and Research Associate
Department of invertebrate Zoology & Geology

Q: Why are teeth more durable than other bones?

PR: Tooth enamel is the hardest tissue in the mammalian body. Unlike other bone, there is almost no space between the mineral deposits in enamel, so it takes much longer for teeth to be worn down or splintered by water, salts, fluctuating temperatures, or gnawing animals.

Dr. Peter D. Roopnarine
Assistant Curator and Department Chair
Department of Invertebrate Zoology & Goelogy

Q: Which herbivores were most common in the White River Formation?

CT: This area supported a wide variety of browsers and grazers with characteristic grinding molars, including several rhinoceros species and their relatives, early horses and camels, the first true tapirs, and a number of extinct sheep-like animals called oreodonts. Early squirrels and rabbits, with their distinctive gnawing incisors, were also common inhabitants.

Dr. Carol Tang
Senior Science Educator and Research Associate
Department of invertebrate Zoology & Geology

SKULL FACT
Oreodonts were the most common mammals in the Badlands ­ they roamed both the woods and the plains in great herds. This diverse group of even-toed ungulates included about 50 different species globally.  Although their teeth are all adapted for a diet of leafy vegetation, differences in the cusp patterns can be used to distinguish the species.

Q: Does this Leptomeryx skull contain fossilized brain material?

CT: Although nothing remains of the brain itself, this specimen includes an endocast of the brain ­ a cast for which the cranial cavity served as a mold. During fossilization, sediments washed into the skull and formed a hard cast of the brain shape. Endocasts like this are used to study the evolution of the brain.

Dr. Carol Tang
Senior Science Educator and Research Associate
Department of invertebrate Zoology & Geology

SKULL FACT
Although rhinos live only in Africa and Asia today, they are believed to have originated in America. Three rhinoceros groups thrived in the Badlands, including amynodonts (aquatic rhinos), hyracodonts (long-legged running rhinos), and several Subhyracodon species (the ancestors of today’s rhinos). North American rhinos became extinct about five million years ago.

SKULL FACT
A relative of the rhinos, Brontops belonged to an ancient group of mammals called the Brontotheres. Standing up to eight feet tall at the shoulder, Brontotheres were the largest mammals ever to walk on North American soil. The impressive pair of horns on this specimen provides a large clue to its sex ­ Brontops males had bigger horns than females, which they probably used both to ward off predators and to compete with one another.

SKULL FACT
Despite the massive size of the Brontops skull, its brain cavity is no bigger than the size of an average person’s fist. Like most mammals with large horns or antlers, the bones of the brain case are especially thick, which would have helped Brontops to withstand the shock of impact during head butting matches. These bones are lightened by an intricate system of interconnecting air-filled cavities, so this ancient giant would have been able to hold up its head without too much effort.

Q: What did Brontops eat?

PR: Like the Subhyracodon teeth, Brontops molars have sharp crests, which they could use to cut through tough leafy vegetation. Archaeotherium had a much more general diet.  This early giant pig ate roots, grasses and leaves as well as meat, so its molars have a more multipurpose shape.

Dr. Peter D. Roopnarine
Assistant Curator and Department Chair
Department of Invertebrate Zoology & Goelogy

Q: What types of predators hunted the White River herbivores?

PR: One of the earliest known dogs, Hesperocyon, evolved in the Badlands during the Oligocene. Several saber tooth cat species also prowled the plains, along with Hyaenodons - hyena-like members of a separate carnivorous group called the creodonts.

Dr. Peter D. Roopnarine
Assistant Curator and Department Chair
Department of Invertebrate Zoology & Goelogy

Q: How can you distinguish between the skulls of true carnivores and creodonts?

PR: Both groups of carnivorous animals evolved sharp, enlarged cheek teeth called carnassials, which slide against one another to shear meat. However, they developed these teeth in different parts of the jaw. In creodonts, the carnassials are always farther back in the mouth.

Dr. Peter D. Roopnarine
Assistant Curator and Department Chair
Department of Invertebrate Zoology & Goelogy

SKULL FACT
Like modern cats, the Oligocene Hoplophoneus had binocular, or forward-facing eye sockets. This placement allowed its eyes to take in overlapping fields of vision, providing it with excellent depth perception ­ an important adaptation for pouncing predators.

Q: Why did saber tooth cats develop such long canines?

CT: These teeth were specialized for stabbing and slicing ­ they allowed saber tooth cats to penetrate the thick skins of even large prey and reach their vital organs. Smilodon, a cat that lived over 25 million years later than Hoplophoneus, developed even longer canines.

Dr. Carol Tang
Senior Science Educator and Research Associate
Department of invertebrate Zoology & Geology

SKULL FACT
Fossil lizard skulls are rare finds ­ the individual bones in their skulls are not tightly fused together, so they fall apart easily. This lizard was probably killed during a flood, mudslide, or other rapid burial event, which helped its skull to remain intact. The bony plates that cover its cranium are called dermal bone, because they grow out of the skin.

Q: What is the most common cause of death for California sea lions?
RB: It is often difficult to determine the cause of death from a skull, but sea lion bones do bear the marks of two frequent killers – sharks and humans. Wounds from shark teeth, gunshots and gill nets can all be seen in their skulls.

Q: Why does the Academy have so many sea lion skulls?
RB: We use these skulls to address questions about California sea lions, ecology and history. The more specimens we have, the more accurate our answers will be. Large collections allow scientists to study even those aspects of a population that are uncommon, such as disease or developmental abnormalities.

Raymond Bandar
Field Associate
Department of Ornithology & Mammalogy

SKULL FACT
The sea lion wall in the Skulls exhibit holds 860 California sea lion skulls. More staggering still, it contains less than half of the Academy’s California sea lion skull collection. Ray Bandar, field associate in the Academy’s Ornithology and Mammalogy department, single-handedly acquired and prepared over two thirds of this colossal collection — the largest group of Zalophus californianus skulls in the world.

Q: What have scientists learned from these sea lion skulls?
DL: Researchers have worked with this collection to study everything from the effects of environmental toxins and dental disorders to a 19th century Russian colony on the Farallon Islands. You can read some of their findings in the "case study" captions along this wall.

Dr. Douglas J. Long
Collections Manager and Acting Department Chair
Department of Ornithology & Mammalogy

CASE STUDY
Could California sea lions on either side of the Baja peninsula have become separate subspecies in recent history? Academy scientists Robert Orr and Jacqueline Schonewald studied skulls from both coasts to find out. Their analysis of the skulls showed that no consistent differences have developed between the two populations, so although the two groups are largely separated geographically, they are probably still interbreeding to some extent.

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© 2002 California Academy of Sciences