The Evidence A Fossil Brains

The fossil record of the brain is from casts (''endo-casts'') that are molded by the cranial cavity of fossil skulls. Natural endocasts are made by the replacement of soft tissue in the skull by sand and other debris that eventually fossilizes. Artificial endocasts can be made by cleaning the cavity and filling it with a molding compound such as latex, from which plaster casts can be made. Errors in identifying "brain" areas in endocasts of birds and mammals are likely to be about the same as in brains when superficial markings rather than histological or physiological evidence are the basis for the identification. Endocasts of some fossil animals are so brainlike in appearance (Fig. 1) that they are often referred to as fossil brains.

Figure 1 presents lateral views of the endocast and brain from the same domestic cat, Felis catus (Figs. 1A and 1B), and a copy of a natural endocast from a fossil sabretooth (Fig. 1C). The sabretooth is Hoplophoneus primaevus, which lived in the South Dakota Badlands during the Oligocene epoch of the Tertiary Period, about 30 Ma. Although no more than a piece of rock, its endocast is unmistakably a picture of its brain as it was in life; it was clearly appropriate to name its parts as brain areas in Fig. 1D following the nomenclature for cat brains.

There are several lessons to be learned from Fig. 1. First, an endocast can provide an excellent picture of the whole brain. This is evident when comparing Fig. 1A with Fig. 1B: The endocast of the domestic cat provides an excellent picture of external features of its brain and correctly estimates its size. (The estimation from the endocast is as "correct" as that from the brain, which is probably slightly shrunken by fixation.) Second, the convolutional pattern in an endocast may be fairly constant in related species. Thus, despite their separation by 30 million years of felid evolution, the endocasts of the living cat and of the sabretooth (Figs. 1A and 1C) are clearly similar. This lesson is especially important because convolutions map the way a brain is organized, at least in a general way. A third lesson, therefore, is that the felid brain of 30 Ma was probably organized in a way similar to that of living felids. Finally, as counterpoint to the lesson of uniformity of organization, there is a lesson of diversity: Two gyri, the coronal and sigmoid gyral complexes, are differentiated in living domestic cats but are undifferentiated in the sabretooth. Increases such as these in the apparent complexity of the brain in felid evolution may be related to increases in information processing in the expanded areas in later species compared to earlier species.

The quality of an endocast as a model of the brain differs in different taxa. Endocasts from fish, amphibians, and reptiles (except in very small specimens) are poor models, useful mainly to estimate total brain size after suitable corrections, because the brain fills only a

Figure 1 Brain and endocasts of felids. (A) Endocast of domestic cat (volume = 30 ml). (B) Brain of same cat (weight = 29.1 g). (C) Endocast of Oligocene sabretooth, Hoplophoneus primaevus, of 30 million years ago (volume = 50 ml; Specimen No. USNM 22538 at the United States National Museum, Smithsonian Institution). (D) Tracing of the endocast of Hoplophoneus with labels for several structures: Cb, cerebellum; E, ectosylvian gyrus; L, lateral gyrus; R, rhinal fissure; S, suprasylvian gyrus; S-C, sigmoid and coronal gyral complex (undifferentiated). Both endocasts are rotated about the anterior-posterior axis, exposing the longitudinal fissure (heavily inked in D). Olfactory bulbs (OB) are sketched in on the basis of more complete endocasts (e.g., AMNH 460 at the American Museum of Natural History). The unlabeled gyrus above the lateral gyrus is the lateral gyrus of the right hemisphere.

Figure 1 Brain and endocasts of felids. (A) Endocast of domestic cat (volume = 30 ml). (B) Brain of same cat (weight = 29.1 g). (C) Endocast of Oligocene sabretooth, Hoplophoneus primaevus, of 30 million years ago (volume = 50 ml; Specimen No. USNM 22538 at the United States National Museum, Smithsonian Institution). (D) Tracing of the endocast of Hoplophoneus with labels for several structures: Cb, cerebellum; E, ectosylvian gyrus; L, lateral gyrus; R, rhinal fissure; S, suprasylvian gyrus; S-C, sigmoid and coronal gyral complex (undifferentiated). Both endocasts are rotated about the anterior-posterior axis, exposing the longitudinal fissure (heavily inked in D). Olfactory bulbs (OB) are sketched in on the basis of more complete endocasts (e.g., AMNH 460 at the American Museum of Natural History). The unlabeled gyrus above the lateral gyrus is the lateral gyrus of the right hemisphere.

fraction of the cranial cavity. In mammals and birds, on the other hand, the brain actually helps shape the cranial cavity during development, and endocasts are usually excellent pictures of the outside of the brain. Olfactory bulbs, forebrain, and hindbrain are readily identifiable, as are most of the cortical gyri and sulci that are seen when a brain is first removed from the skull. Certain large-brained living mammals—namely, cetaceans, elephants, great apes, and humans—are exceptions to this rule, with little or no impression of their convolutions on their endocasts; even the boundary between cerebrum and cerebellum may be unclear.

Fossils provide other information for understanding brain evolution and for extrapolations to behavior. Body size, for example, estimated from postcranial skeletal data, is used to analyze encephalization of fossil vertebrates. Details of structure, such as the shape of teeth, forelimbs, and hindlimbs, can enable one to analyze feeding habits, gait, and other behavior. There is even fossil evidence on social behavior, for example, in dinosaurs, which has been reconstructed from the aggregation of fossils, their eggs, and their foot and tail prints. Perhaps most important for an analysis of brain evolution, there is a fossil record of sensory structures that is useful in reconstructing the information available to fossil animals. Olfactory bulbs, of course, are visible on the endocast, and their size is related to the evolution of the sense of smell. There are fossil middle ear bones and cochlea important for the analysis of the evolution of hearing; the orientation of the orbits of the eye provides evidence on the evolution of binocular vision, and the placement of the hyoid bones on fossil humans has been the basis for speculations on the evolution ofthe voice box and of articulated speech.

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