What Is an Endocast? The Shape of a Brain That Rotted Away Long Ago

The braincase encloses the brain and the membranes and blood vessels around it, so the cavity is larger than the brain was and its shape is an approximation rather than a cast of the organ itself. How close an approximation depends on the group, since in birds and in some dinosaurs the brain fills the cavity closely while in crocodilians and many reptiles a substantial space separates them, which means the same technique gives more information for some animals than others and the uncertainty must be stated. Natural endocasts occur where sediment filled the braincase and lithified, and the specimen is then found as a rock in the shape of a brain, which is how the phenomenon was first noticed. Digital endocasts are made by computed tomography, scanning the skull and reconstructing the empty volume, and they are now standard because they are non-destructive, can be shared and measured precisely, and capture structures a physical cast cannot.

Several features carry information about how an animal lived:

• •Overall brain size relative to body mass, which is compared using an index and which correlates loosely with behavioural complexity while being a crude measure
• •The olfactory bulbs, whose size relative to the rest indicates how much of the brain was devoted to smell, which has been used to argue for keen scent in several predators
• •The optic lobes, which indicate visual capability and are large in animals relying on sight
• •The cerebellum and flocculus, which handle balance and coordination of eye and head movement, and whose enlargement is associated with agility and with flight
• •The inner ear labyrinth, preserved in the same scan, whose semicircular canals indicate head posture and agility and whose cochlear length indicates hearing range
• •Cranial nerve openings, which indicate what sensory equipment the head carried, including the pressure-sensing systems in some snouts

The technique has settled several questions and reopened others. Habitual head posture is inferred from the orientation of the horizontal semicircular canal, which animals hold roughly level, and applying that to dinosaurs and to early humans has revised how several are reconstructed as standing. Hearing range estimated from cochlear length indicates that many dinosaurs heard best at low frequencies, which fits the reconstructed sounds of the hollow-crested hadrosaurs. Olfactory bulb size in tyrannosaurs supports a strong sense of smell, which has been used in arguments about scavenging and hunting without settling them. Flight-related brain regions in early birds and in some dinosaurs bear on when the neural equipment for flight appeared. And in human evolution, endocasts are a principal source of evidence about brain size and asymmetry across the fossil record, including the question of when language-related regions became distinguishable, which remains contested precisely because the external shape constrains the interpretation loosely.

Endocasts support less than they appear to. They record external shape and not internal structure, so nothing about neuron density, connectivity or organisation is available, and those are what determine what a brain does. Brain size relative to body size is a weak predictor of anything specific, and body mass estimates for extinct animals carry large uncertainties which propagate directly into the index. Sulci and gyri, meaning the folds of the surface, leave impressions in some groups and not others, which limits what can be said about regional development. The gap between brain and braincase varies and is frequently unknown for an extinct species. And interpretations of function from proportions rest on comparison with living animals, which requires assuming the relationship held in the past. The field has become considerably more careful about these caveats, and published claims now generally state them, which is a change from earlier work that read behaviour confidently off a cast.

The braincase preserves the space a brain occupied, filled naturally by sediment or reconstructed digitally from a scan, and it records external shape rather than structure. Olfactory bulb and optic lobe proportions indicate which senses dominated, and the inner ear preserved in the same scan gives habitual head posture and hearing range. Nothing about neuron density or connectivity survives, which is what a brain's capability actually depends on.