Sunday 21 November 2021

Brains of the Monster-Teeth

Dissopsalis, another member 
of the "monster-tooth" family
Among the distinguishing features of mammals, as compared with other vertebrates are the larger relative size and greater structural complexity of the brain. It's worth pointing out at the outset that many birds are as capable as mammals of complex behaviour despite not possessing the brain structures we'd otherwise associate with such things so that we can't solely use brain anatomy as a proxy for things like 'intelligence'. But, if we're looking only at mammals, brain anatomy can tell us quite a bit and it's probably fair to say that it does distinguish us from, say, reptiles.

The key feature here is the neocortex. This forms the outer layer over much of the two main hemispheres of the brain. It's usually defined by the fact that it consists of six sub-layers allowing for particularly complex function and, by this definition, it's unique to mammals. How much of the brain volume it occupies varies considerably between different species, and, over the course of evolution, there's a tendency for it grow larger more rapidly than the rest of the brain (or, indeed animal) does. As it does so, its surface becomes increasingly folded, giving it a greater surface area without occupying more space inside the skull. In humans, the neocortex occupies a whopping 76% of the brain's volume sitting on top of the deeper structures that do things such as regulating the heartbeat - something no more essential in humans than it is in, say, fish.

Quite how the shape of these folds is determined isn't entirely clear, and there are at least two competing theories, both of which probably have some truth to them. However, whatever the reason, the folds are not random, and their general pattern has been conserved between different mammalian groups. This allows us to distinguish different parts of the brain which, from various studies, we know have particular functions, such as the occipital lobe at the back of the brain containing the main visual processing centres. In theory, this should give us some idea of what a particular mammal species might be good at by comparing the relative size of its brain areas - although, in practice, this isn't always as simple as one might think.

Significantly, one can even do this for mammals that we know only from fossils.

This is because the brain fits fairly tightly inside the skull so that the shape of the skull interior gives us a good idea of the shape of the brain itself - although the details of the finer folds are likely to be lost. Sometimes, when an animal dies, the hollow skull may fill up with sediment after the brain rots away, and if the sediment subsequently turns into rock, it can produce a natural endocast that roughly mirrors the shape of the brain. If a fossil skull is comparatively intact, it may also be possible to use liquid rubber to create a hollow artificial endocast that can be pulled out through the base or - as is more common in the 21st century - to use a CT scanner to build up a 3D image of the inside without risking damage.

It's fair to say that most interest in this field has been focussed on fossil primates, and especially our own ancestors. But it can be applied more widely. In fact, the first endocast to be described was all the way back in 1804 by George Cuvier, when he examined the skull of Palaeotherium, an animal now known to be related to horses. 

Apart from the primates, another group of land-based mammals with relatively large brains are the carnivorans, the group including cats and dogs. The study of carnivoran endocasts also dates back to the 19th century, but it took off in the 1970s when Leonard Radinsky studied a number of living and fossil carnivorans, showing that, for example, the brains of sabretooth cats were remarkably similar in structure to those of cats living today.

He also managed to bust a myth about the rise of the modern carnivorans. It had been thought that the ancestors of today's cats and dogs triumphed over the other carnivorous mammals of the day because they had larger brains and were more intelligent. But that, it turns out, isn't so - it's true that modern carnivorans have larger brains than their ancient competitors, but that's a change that's happened in the time since, and there was no such difference when they still lived alongside one another.

What competitors were these? It used to be thought that the order of extinct mammals most closely related was a group called the "creodonts", similarly carnivorous animals lacking some of the distinct specialisations of their modern relatives. Although there remains some debate on the matter, it's entirely possible that the creodonts were not a single group of animals and should instead be divided into multiple different orders. If so, which among those orders was the most closely related to today's carnivorans is far less certain, but the hyaenodonts are strong contenders.

These were mostly long-headed, flat-footed animals with sharp, flesh-tearing, teeth and they survived well into the Miocene before modern carnivorans finally replaced them. We may not know how they related to other fossil groups with any great certainty (although they were certainly more closely related to carnivorans than to anything else alive today) but there is an increasing consensus on the families within the group.

Until recently, we had information on the brain structures of all of the major hyaenodont families, with just one exception. The exception were the teratodontids (literally "monster-teeth"), a family that aren't as well-known as they might be. That's probably because they were almost entirely restricted to Africa, although a few are known from elsewhere. 

Earlier this year, that gap was filled in.

This was due to the discovery of a fossil skull from a site in Kenya that dates to about 17 million years ago, during the Early Miocene, about 4 million years before the hyaenodonts as a whole died out. The new animal has been named Ekweeconfractus, which translates from a combination of Latin and Turkana as "broken fox". However, while we know it only from the one skull, making some reasonable assumptions about its body shape it was probably slightly larger even the largest of red foxes and certainly much larger than (say) a kit fox.

Although the fossil consists only of the skull and part of the snout and the cheekbones are badly damaged, the cranium is largely complete making it possible to do a CT scan and, allowing for some distortion after millions of years compressed in rock, determine some details about the size of the brain and how much of it must have been composed of neocortex.

Assuming that the calculations about the size of the animal are correct, it had an EQ of around 0.5. This is a measure of how much larger the brain is than we would expect for an animal of its size. 0.5 is, by definition low - it means that the brain is only about half the size we would "expect". But that's not staggeringly low, because there are many living mammals that score much lower and, in fact, it's about the same as a cow.

Which is admittedly low compared with living carnivores - cats score around 1 - but isn't bad for something that lived 17 million years ago. Indeed, it's fairly typical for African hyaenodonts, suggesting that, since it was one of the last ones, brain size didn't increase much (relative to the rest of the animal) through the course of their evolution. This stands in contrast with the Eurasian/American hyaenodonts, where the EQ ranged from 0.4 in the early Proviverra to 0.85 in the much later Hyaenodon - a clear increase that could, conceivably, reflect a more complex social life or sophisticated hunting tactics.

But that doesn't mean that the African species stood still when it came to brain development. Although we don't have endocasts from early African hyaenodonts, we do have for Tritemnodon, an early offshoot of the branch of the order that led to the African families, but which itself lived in North America over 45 million years ago. This had a brain almost exactly the same size as Ekweeconfractus, but the endocasts show that much less of that brain was composed of neocortex.

While there are were number of smaller groups, especially early on, the two main branches of the hyaenodont order were the Eurasian/American group (technically, the hyaenodontoids) and the African group (hyailurodontoids). The discovery of this new animal, and the resulting analysis, fills in a gap in our understanding of the latter group. It shows that, while the Eurasian/American hyaenodontoids grew increasingly large and more complex brains as time went on, the African ones developed what appear to be more sophisticated brain structures without the brain itself getting any larger. The two developments must have occurred independently, since the groups split from one another very early on indeed, but it's interesting to note that they happened in different ways.

We can't really say from this how complex the life of Ekweeconfractus was, let alone how that complexity may have shown itself. It's unlikely to have been as intelligent as a modern wolf but it was likely an improvement on its own ancestors. Dogs and the like eventually displaced it but it must have had something going for it. After all, it survived at least as long as the potentially more intelligent Hyaenodon did and its closest relatives lived even longer.

[Illustration by Edwin Harris Colbert, in the public domain.]

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