of Psychiatry, Univ.of California, Los Angeles
collaboration with the National Museum of Health and Medicine
BRAIN (ENDOCAST) OF BATHYGENYS REEVESI
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.
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.
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.
Age: Age: Chadronian (Upper Eocene): 35-37 million
The Big Bend area of the Rio Grande in Texas, U.S.A., North
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.
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.
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
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.
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
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.
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
Analysis (3D) of Fossil Brains
for Primary School, Middle School, High School, and College:
RJG & Long, MA, Mammal Evolution: An illustrated guide.
British Museum (Natural History), London, 1986.
for College and for specialists:
JA (1971). "Early Tertiary vertebrate faunas, Vieja
Group: Trans- Pecos Texas: Agriochoeridae and Merycoidodontidae."
Bulletin 18, Texas Memorial Museum, Pp. 1-83.
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:
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.
H.J. Jerison, University of California Los Angles, firstname.lastname@example.org
Harry J. Jerison, Professor Emeritus, UCLA.
More to come.
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