The Origins of Music: Preface - Preface

for mammals (figure 12.4).(Readers can see the original of the specimen

for views of many mammalian brains at http://www.neurophys.wisc.edu/

brain/paleoneurology.html).The species was a small ungulate,an “ore-

odont”that lived thirty-five mya in what is now the Big Bend area of the

Rio Grande river in Texas.It is described in detail at the internet site and

is also illustrated in other publications (Jerison 1990,1991).The endo-

cast shows many major structures of the brain,such as the olfactory

bulbs,cerebral hemispheres,and cerebellum,as well as the marginal

sulcus and longitudinal sulcus of the neocortex.One can use the position

of the rhinal fissure,which is the boundary between six-layer neocortex

and paleocortex,to analyze the evolution of neocortex.In the figure I

have named a few neocortical convolutions using the same criteria as in

living ungulates as presented in standard brain atlases.

The most important conclusion from such evidence may be to show

that mammalian neocortex evolved to larger relative size during the past

sixty million years (Jerison 1990).This verified the frequently stated

hypothesis,based on comparisons among living species,that “neocorti-

calization”(an aspect of encephalization) occurred during mammalian

evolution.From expected structure-function relationships,this implies

190 Harry Jerison

Figure 12.4

Tracing from dorsal (top) and lateral (bottom) views of the fossil brain of Bathygenys

reevesi,an upper Eocene oreodont (Merycoidodontidae,Artiodactyla).A view of the orig-

inal in color is on the Internet at http://neurophys.wisc.edu/brain/paleoneurology.html.

Olfactory bulbs (OB),cerebral cortex (CX),cerebellum (CL),and medulla (M) are

labeled,as are several cortical sulci:the longitudinal sulcus (l.s.) separating the left from

the right hemisphere,marginal sulcus (mar),and rhinal fissure (r.f.).

Fig.12.4