Chart depicting the relationship of brain and body weights

Relation between Brain Weight and Body Weight of Living Vertebrates

Fossil Endocasts reveal similar relationships:

Larger & more complexly evolved animals have larger brains

Prepared by

Harry J. Jerison

Department of Psychiatry, Univ.of California, Los Angeles

In collaboration with the National Museum of Health and Medicine


(WILSON, 1971)

What is an Endocast? An endocast is a casting for which the cranial cavity serves as a mold. In mammals, endocasts look remarkably brainlike, resembling a fresh brain with the dura mater in place. The pictured endocast of Bathygenys reevesi is a natural one, made at the time of fossilization by debris being washed into the cavity. Bathygenys reevesi The debris (mainly sand) filled the cavity tightly and completely, and, like the skull, became fossilized (mineralized), but it became even harder than the fossil bone. Millions of years later, when earth movements exposed the complete fossil skull, weathering occurred, which destroyed most of the skull but left the natural endocast. This exposure may have happened within the past few hundred years; had the cast remained at the surface, it too would have weathered away. As a natural endocast, this "fossil brain" of Bathygenys is unusually well preserved. Evidence of the evolution of the human brain is also based on endocasts, but none are quite as well preserved as Bathygenys.

Description of the specimen (and a short evolutionary speculation): Visible in this "fossil brain" are the olfactory bulbs at the left, cerebral hemispheres behind them, with the longitudinal fissure separating the right hemisphere from the left hemisphere, and cerebellum at the right. A bit of "medulla" is visible behind the cerebellum. This "medulla" is actually a cast of the region of the foramen magnum and is probably close to twice the diameter of the medulla of the brain. Can you guess an evolutionary reason for the difference between medulla and foramen magnum? One doesn't really know, but there is a mechanical explanation that makes evolutionary sense. The medulla is surrounded by the cisterna magna, which is a sack of blood and supporting tissue, that would cushion the medulla against shock if the brain bounces around a bit. The brain does do that when an animal runs or jumps. The cushioning would be important protection for the medulla from such shocks, because all sorts of vital functions depend on the medulla working properly when impulses are transmitted through it between brain and body. Natural selection would have favored individuals with better cushioning of the medulla, that is, with larger cisterns. On a technical note, if one analyzes brain/body relations (to determine an encephalization quotient, EQ, for example), the medulla of the brain can be used to represent body size. The "medulla" on an endocast, which is actually foramen magnum, works less well as a control for body size, because its size is determined by both brain and body size.

Bathygenys reevesi Age: Age: Chadronian (Upper Eocene): 35-37 million years ago.

From: The Big Bend area of the Rio Grande in Texas, U.S.A., North America.

Source of the original fossil: University of Texas, Texas Memorial Museum, Specimen Number UT 40209-431. It was collected by Dr. John A. Wilson of the University and is from the a ranch that belonged to John C. Dunagan and Mr. and Mrs. O. M. Reeves, in the Big Bend area of Texas.

Fossil record: Although fossil brains from several hundred different species of vertebrates have been described, this specimen is especially important, because it is one of a set of over 100 that have been found, all from the same fossil species. It is the finest of that set and its discovery enables one to analyze within-species variation in gross features of the brain.

Etymology: The name Bathygenys is from the Greek, meaning deep lower jaw and refers to the appearance of the skull of the fossil. The species name, reevesi, was in honor of the Reeves's who were very helpful in enabling the collection to be made. The taxonomic family name, "merycoidodon" means "ruminating tooth."

Classification: Order Artiodactyla (even-toed hoofed mammals), Family Merycoidodontidae (oreodonts). The oreodonts are entirely extinct. They resembled sheep and pigs in size and shape; Bathygenys reevesi was the smallest species, being about the size of house cat.

Adaptations: It is impossible to guess details of the lives of these herbivorous animals, though their herbivorous habit can be ascertained from the shape of their molars. These fossil remains were found in a small area and in large numbers, suggesting that they lived in herds, but the specific collection of Bathygenys fossils led its discoverer to "believe that it represents the refuse of a carnivore's lair." In life they may have moved in small groups like the small peccary, the javelina, that is presently in the region. Alternatively, the animal might have lived a near solitary life like that of the musk-deer, the smallest of living hoofed mammals.

Special features: The evidence of the brain is comparable to that on living species, such as the musk-deer. One can identify major brain areas such as olfactory bulbs, cerebral cortex and cerebellum. Within cerebral cortex major gyri (convolutions) can be identified and named as in living species: longitudinal gyrus and marginal gyrus are both visible on the endocast.

Remarks: From the endocast we recognize that Bathygenys, though smaller in body than any living even-toed (artiodactyl) mammal, was probably comparable to its living relatives in the way its brain was organized, that is, in the relations of the parts to one another. But taking body size into account, the brain of Bathygenys was only about 1/3 the expected size compared to living species. Rather than thinking about this as a measure of evolving "intelligence," one can be less anthropomorphic and identify the development in later mammal species, compared to Bathygenys, as an increase in the capacity to handle information about the outside world.

Computer Analysis (3D) of Fossil Brains

References for Primary School, Middle School, High School, and College:

Savage, RJG & Long, MA, Mammal Evolution: An illustrated guide. British Museum (Natural History), London, 1986.

References for College and for specialists:

Wilson, JA (1971). "Early Tertiary vertebrate faunas, Vieja Group: Trans- Pecos Texas: Agriochoeridae and Merycoidodontidae." Bulletin 18, Texas Memorial Museum, Pp. 1-83.

Jerison, HJ (1979). The evolution of diversity in brain size. In Hahn, ME; Jensen, C & Dudek, BC (eds.) "Development and Evolution of Brain Size." Pp. 29-57. New York: Academic Press.

Jerison, HJ (1990) Fossil evidence of the evolution of the brain. In Jones, EG & Peters, A (eds.) "Cerebral Cortex Vol 8A, Part 1: Comparative Structure and Evolution of Cerebral Cortex." Pp. 285-309. New York: Plenum Press.

Source: H.J. Jerison, University of California Los Angles, Harry J. Jerison, Professor Emeritus, UCLA.

More to come.

A translation of this page in Romanian can be found at

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