Dino brains

How do we know the size and shape of a dinosaur brain? Casts can be obtained of the interior of the braincase. To do this, latex is painted onto the inside of a well-preserved braincase that was not crushed during fossilization. When the latex has dried (and is flexible), it can be peeled off the inside ofthe braincase, and pulled through the foramen magnum ("big hole"), the opening through which the spinal cord entered the skull in life. The result is a three-dimensional cast of the region occupied by the brain (see Figures 5.9 and 12.3; Box 12.4). Such casts give some inkling about the shapes and sizes of brains. Unfortunately, observations made of the brains of living lizards, snakes, and crocodilians show that these brains take up somewhat less room within the braincase than do those of mammals or birds. Researchers have long suspected that the brains of dinosaurs should be similarly smaller than the entire volume ofthe braincase and have provided correction factors, which are reflected in calculations of dinosaurian gray matter.

From such brain/body studies, it is now clear that, as animals get bigger, their brains also get larger, but not in proportion to their increase in body size. Even for large-brained mammals like ourselves, as we reach maturity and stop growing, our brains have grown proportionately less than have our bodies. The same pattern applies to shrews and elephants, as well as to lizards and crocodilians. And presumably to extinct dinosaurs as well, for we know that the same pattern is found in the living variety: birds.

This relationship between estimated dinosaurian brain size (calculated from the expected brain size of lizards, snakes, and crocodilians scaled up to dinosaur size) and dinosaurian body size has been used to make comparisons with measured brain size in dinosaurs (see Box 12.4).

range of behaviors, would have had time to stop and smell the roses . . . had there been any roses!1

Social lives ofthe enigmatic. We don't have much of an idea about the social behavior of stegosaurs or know much about their life histories. No nests, isolated eggs, eggshell fragments, or hatchling material is yet known for any stegosaur. In fact, only a few juvenile and adolescent stegosaur specimens can tell us anything about the lives of subadult stegosaurs.

Among fully adult individuals, it appears that there was some sexual dimorphism; that is, differences between the sexes. This shows up in, of all places, the number of ribs that contribute to the formation of the pelvis. But whether it is the male or the female that has the greater number of ribs is anybody's guess. Sex-based differences in the size and shape of the spines and/or plates might be predicted if only we had better samples.

Little is also known about the degree of sociality among stegosaurs. The mass accumulation of disarticulated, yet associated, Kentrosaurus material from Tendaguru in Tanzania (see Chapter 14) provides us with a hint that Kentrosaurus was gregarious (exhibited herding and other social behaviors). In other genera, however, we have no such information. The fossil record is simply silent on this issue - so far.

Spines and plates. Whether or not stegosaurs were gregarious, there are still some features of these animals that give clues about behavior: the spines and plates. As we have learned, the majority of stegosaurs had at least one row of osteoderms along the dorsal margin of each side of the body. And these osteoderms generally take the form of spines, spikes (Figure 5.10), blunt cones, or plates. In all cases, at the end of the tail were pairs of long spines. All of these, like all osteoderms, were embedded in the skin (Figure 5.11). What purpose might they have

1. Roses didn't appear until after the last stegosaur went extinct - see Chapter 13.

Figure 5.10. The skeleton of Kentro-saurus, a spiny stegosaur from the Late Jurassic of Tanzania.

Plates

Right humerus

Plates

Right humerus

Stegosaurus Skeleton

Figure 5.11. Diagram of one ofthe best skeletons of Stegosaurus as it was found in the field. Note that the plates do not articulate directly with the vertebrae.

Figure 5.11. Diagram of one ofthe best skeletons of Stegosaurus as it was found in the field. Note that the plates do not articulate directly with the vertebrae.

■ Left front limb served? Originally, the idea was that they were all about protection and defense. But defense, if it is any part of the story, isn't the whole story.

The shapes and patterns of plates and spines in stegosaurs are nearly always species specific; that is, diagnostic for a particular species (in this case stegosaurs). Moreover, they have their maximum visual effect when viewed from the side. The osteoderms, therefore, might have served a display function - both for predators and for other stegosaurs. If intraspecific display (display among members of a species) was involved, it is likely that individual stegosaurs would have used these structures not only to tell each other apart, but also to gain dominance in territorial disputes and/or as libido-enhancers during the breeding season. But that's yet another question we can't answer: the stegosaur fossil record just isn't rich enough.

Hot plates. The surfaces of the plates of Stegosaurus are covered with an extensive pattern of grooves, while the insides are filled with a honeycomb of channels (Figure 5.12). These external grooves and internal channels most likely formed the bony walls for an elaborate network of blood vessels. With such a rich supply of blood from adjacent regions of the body, could the plates have been used to cool the body by dissipating heat as air passed over them,

Stegosaurus Dermal Plate
Figure 5.12. Lateral view of one of the dermal plates of Stegosaurus. Note the great number of parallel grooves, presumably conveying blood vessels across the outer surface of the plate.

or to warm the body by absorbing solar energy? In short, could the plates have been used for thermoregulation (temperature control)? As a test of this idea, paleontologist J. O. Farlow and colleagues tested the ability of the plates to radiate (or absorb) heat. Arranged in symmetrical pairs, as they are in life, the plates provided significant heat dissipation, suggesting that thermoregulation may also have been a role of the plates.

The few juvenile stegosaurs known appear not to have had large spines or plates on their backs. The absence of these features in small, sexually immature individuals suggests that only when maturity was reached did looking big and sexy acquire importance. Likewise, thermoregulation may have only been important to adult stegosaurs.

The role of the long, pointed tail spikes is less ambiguous. These were likely slashed from side to side on the powerful tail. While older reconstructions show these spikes as pointing primarily upward, recent discoveries suggest that the spikes actually splayed out to the sides, producing a much more effective defensive weapon.

So stegosaurs appear as a mass of contradictions: chewers that may not have chewed that well; thermoregulators that appear to have moved slowly and lacked the sophisticated neural controls usually associated with thermoregulation; and animals in which supposed sexual display functions were prominent but in which there is little evidence for gregarious behavior. Until very recently, we didn't even know the positions of the plates in Stegosaurus, the best-known genus. So much of what is apparently contradictory about stegosaurs is likely due to how little we know about them - a situation that we'd like to see changed!

Ankylosaurs were masters of the art of defense-by-hunkering. As their name implies, anky-losaurs were encased in a pavement of bony plates and spines - each embedded in skin and interlocked with adjacent plates - that formed a continuous shield across the neck, throat,

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  • Linda
    How would we even know the size and shape of a dinosaur brain?
    9 years ago
  • priamus boffin
    How do we know the size and shape of dinosaur brains?
    9 years ago

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