Spinosaur brain scans reveal possible new insights into their senses

It may have been a case of matter over mind for dinosaurs which lived around water.

Skull scans of the early semi-aquatic spinosaurs Baryonyx and Ceratosuchops suggest that their brains weren't that different from their relatives. Instead, the evolution of their crocodile-like snouts may have been enough to allow them to begin hunting fish and other aquatic prey, as their brains were already up to the task.

The scientists behind the research, published in the Journal of Anatomy, described the results as 'surprising.' This is because it might have been suspected that the brains and senses of early fish-eating spinosaurs would have been different to their land-living relatives.

Lead author Chris Barker, a PhD student at the University of Southampton, says, 'Despite their unusual ecology, it seems the brains and senses of these early spinosaurs retained many aspects in common with other large-bodied theropod dinosaurs.'

'There is no evidence that their semi-aquatic lifestyles are reflected in the way their brains are organised.'

The research adds to ongoing debates over the exact nature of spinosaurs, which remain controversial following claims that they could have spent much of their lives in water.

Spinosaur senses

While studies on the brains of spinosaurs have taken place before, Baryonyx and Ceratosuchops are among the oldest known members of the group. By studying these dinosaurs, researchers can compare their brains to their close relatives and descendants to find out more about their evolution.

Based on scans of their skulls, this latest study suggests that the brain size of these dinosaurs was broadly similar to other theropod dinosaurs. When adjusted for size, only some tyrannosaurs had values that were significantly different to the others.

While the link between overall brain size and cognition is controversial, it could indicate a broadly comparable level of intelligence between spinosaurs and their relatives. But narrowing the comparisons to smaller areas of the brain can give further clues about the dinosaurs' lifestyle.

For example, the region of the brain that processes smell, known as the olfactory bulb, was about the same size in Ceratosuchops as expected for a dinosaur of its size.

This is a surprise, as some researchers have suggested that these olfactory bulbs shrunk as the spinosaurs became increasingly aquatic. If this did happen, then it only occurred later in the evolution of these dinosaurs.

Another sense that appears to have changed during the evolution of spinosaurs was hearing. The cochlear duct, the part of the inner ear which converts sound into brain signals, was found to be slightly elongated in the early spinosaurs and therefore specialised for lower frequencies. This is also seen in other carnivorous dinosaurs such as abelisaurs and allosaurs.

But later spinosaurs such as Irritator evolved to hear higher pitches, suggesting that these sounds were more important for them as they increasingly adapted for an aquatic environment.

Irritator also appears to have had larger floccular lobes, a region of the brain which helps to keep the eyes steady. This could have allowed Irritator to stabilise its vision to snatch fish out of the water from above, whereas Baryonyx may have used a different way of catching prey, perhaps even in the water itself.

That said, the researchers note that these differences could be caused by a difference in age between the specimens at the time of death.

There is also uncertainty over whether there was any visual overlap between the eyes of Ceratosuchops, which would have given it better depth perception. The study suggests that more research on this recently discovered dinosaur could help to confirm this.

Studying dinosaur brains

The recent study is the latest in a long line of research which seek to understand the brains of extinct animals. As the brain and other soft tissues in the skull rot away after death, investigating the inside of the skull provides the next best evidence of what these organs may have been like.

Originally, these were investigated by making a physical cast of the skull's interior. The braincase itself can act as a mould for materials such as rubber or plaster which reveal its shape as they form what is called an endocast.  

In some very rare instances these casts are formed naturally. If mud or other soft sediment enters the skull shortly after death, then it can harden and fossilise along with the surrounding skeleton.

While these endocasts can be useful, they often aren't that detailed. More recently, CT scanning has allowed virtual casts to be created in incredible detail.

Professor Lawrence Witmer, who co-authored the research, says, 'This new research is just the latest in what amounts to a revolution in palaeontology due to advances in CT-based imaging of fossils.'

'We're now in a position to be able to assess the cognitive and sensory capabilities of extinct animals and explore how the brain evolved in behaviourally extreme dinosaurs like spinosaurs.'

Even with these scans, however, it is still difficult to know exactly how the brains looked. Unlike birds and mammals, the brains of reptiles don't generally fill their brain cavity, so researchers have to estimate how big this organ is based on known relationships to overall body size.

The uncertainty around the reliability of these estimates means that while the researchers did discover some potential differences between the shape and structures of Baryonyx and Ceratosuchops brains, there isn't enough evidence to suggest if they were differences between individuals or species.

Finding more, well-preserved specimens of these dinosaurs in the future could provide an answer, and offer a finer look at the brains of these extinct animals.