Fossil Brains Revisited

From the history of the brain in fish, amphibians, and reptiles as available from the fossil record, the most unusual inference is that these can all be treated as lower vertebrates in encephalization (Fig. 5). The exceptions are sharks and, perhaps, the ostrich-like dinosaurs (ornithomimids). Here is a list of a few more outstanding discoveries.

First, from the evidence of small (<15 cm long) Carboniferous (350 Ma) fossil fish, the diversity in living teleosts was probably foreshadowed by some of the earliest bony fish. They had optic lobes enlarged in ways comparable to those of living fish, such as trout, that feed at or near the surface of the water and rely on visual information.

Second, although sharks and other cartilaginous fish are often considered primitive, they are not primitive with respect to their brains. Many sharks are big-brained, overlapping the distributions of relative brain size of the lower vertebrates, on the one hand, and of birds and mammals, on the other hand. There is one uncrushed endocast known in a Paleozoic shark, the earliest evidence of encephalization beyond the grade of living bony fish. The species was comparable in both brain and body size to the living horned shark (Heterodontus).

Third, dinosaurs continue to be libeled with the walnut-size brain label despite evidence to the contrary. Their brains were within the expected size range for reptiles. The Tyrannosaurus 404-ml endocast implies a brain in the size range of those of living deer—small for an elephant-sized mammal but impressive for a reptile. The basic data are shown in Fig. 6.

Fourth, the major transition from water to land in the amphibians more than 350 Ma was accomplished without enlargement of the brain, and there has been no enlargement since if one compares present amphibians with their fossil ancestors. Of course, there was and is, considerable diversity in relative brain size within each of the classes of living lower vertebrates, just as there is in birds and mammals, and there has been significant reorganization of the brain across species of lower vertebrates, especially between classes but also within classes. By any standard, one must recognize the brain in living reptiles as more specialized than that of fish or amphibians.

Finally, of the major lateralization of function in the living human brain, and the recognized lateralization in the brains of many other living species, there is little or no evidence at a gross level. There is no good fossil evidence for such lateralization, although some has been claimed and evidence may be forthcoming. The problem is that asymmetries are difficult to establish, even in living brains, and are almost impossible to establish in fossils, which are often asymmetrically distorted and partially crushed.

These statements sum up the evidence on brain evolution in about three-fourths of the vertebrate species. There remains the story of about 10,000

10-1 10° 101 102 103 104 105 106 107 108 Body Size (Grams)

Figure 6

Data from Fig. 5 for mammals, birds, reptiles, and three dinosaurs (Al, Allosaurus; Ty, Tyranosaurus; and Br, Brachiosaurus), showing extension of reptilian minimum convex polygon by adding dinosaur data.

species of birds and 5000 species of mammals, in which encephalization is a major feature. As background, and for a better sense of the potential and limitations of the method, I discuss the relevant data from Fig. 5, namely, those on reptiles, birds, and mammals. These are presented in Fig. 6, in which distracting information from the three classes of fish were removed and to which I have added data on three dinosaurs.

The three added species are the large well-known carnivorous dinosaurs, Tyrannosaurus and Allosaurus, and the largest of all dinosaurs in which there are reliable body size estimates, Brachiosaurus. To illustrate the method and its use, I will review the status of the ''ostrich dinosaurs'' from this perspective. One of these is a relatively small late Cretaceous carnivorous dinosaur, Troodon, which has been described as encephalized as large living birds and in the range of encephalization of living mammals. The analysis depends on estimating body size as well as brain size. There is a reliable estimate of Troodon's body size as about 45 kg. Its brain size was first estimated by inspection and by perceived similarity to an ostrich's brain size of 40 g. However, with the help of a computer program recently developed and a qunati-tative analysis of its brain size, I now estimate its brain as about 20 ml in volume. This is somewhat smaller than the albatross's 27-ml brain as determined by the same computer analysis. If Troodon had a 20 ml brain in a 45 kg body, it would fit within the reptile polygon of Fig. 6. More analysis is needed, but it is evident that

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reports of the past 30 years of these large-brained dinosaurs need to be reevaluated. Other dinosaur data, such as those in Fig. 6, appear to be reasonably reliable, and provide an acceptable basis for evaluating dinosaur brain evolution. The ostrich that provided data for Fig. 6 weighed 133 kg and had a 42 g brain. It is the end-point of the avian polygon, which falls somewhat below the corresponding point on the polygon for living mammals. I have not entered data points for albatross at a body size of 12 kg, but it would fit comfortably within the avian polygon. Troodon fits within the reptilian polygon as extended in Fig. 6 to include a few dinosaurs. Troodon was indeed among the larger brained dinosaurs, but it probably did not reach either the mammalian or avian polygon.

The analysis with convex polygons is, in short, based on inclusion or exclusion of a particular species within the taxon described by a particular polygon. In Fig. 5, it was this approach that demonstrated the anomalous situation for living electric fish and cartilaginous fish and the "normal" position for agnathans. In Fig. 6, it indicates that dinosaurs were reptilian rather than avian in relative brain size.

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