The Diets of Parallel Pigs

Peccaries from South America

The word "ungulate" refers to a broad category of mammals that are generally large and herbivorous. They form a natural evolutionary group, although only if you include the cetaceans, which are descended from vaguely hippo-like ancestors. The word comes from the Latin word for "hoof", and the great majority of living ungulates have hooves of one kind or another. Even ignoring the cetaceans there are exceptions, but it doesn't take a great stretch of the imagination to see that camels, for instance, are at least the same general kind of animal as deer or cows.

All living ungulates can be placed into one of two mammalian orders: the artiodactyls, which includes the cloven-footed kinds, and the perissodactyls, which includes, among others, the horses. If you start looking at fossils, however, it becomes clear that other ungulate orders once existed. Most were very early, dying out in the first half of the Age of Mammals, representing early branches in the great ungulate family tree that ultimately proved unsuccessful.

Not so the "South American Native Ungulates", known as SANUs for short, or by the more technical term "meridiungulates". The ancestors of this group were isolated in South America, which was an island continent for much of the Age of Mammals. Without serious competition, they thrived there, becoming some of the dominant animals on the continent. While the primitive groups died out, they continued to evolve, surviving right through until South America finally collided with the north just before the Ice Ages. At which point, deer and llamas headed south, eventually extinguishing their distant relatives.

Prior to this, however, there was a remarkable degree of diversity among the SANUs. They included small to giant forms, sufficiently varied that we traditionally group them into no fewer than five different orders, albeit only two of them survived to reach the Ice Ages. Many were radically different from their counterparts on other continents, but others were less so, examples of parallel evolution where two different groups of mammals evolved similar solutions to similar problems.

One such example is that of the megalodolodines. These lived during the mid to late Miocene epoch between around 15 and 5 million years ago, and are placed within the proterothere family in the litoptern order - the second most diverse of the SANU orders, and one of the ones that came closest to surviving into modern times. 

The proterotheres were medium-sized herbivorous animals with limbs that were, in most species, adapted to fast running, presumably to escape predators. Like many litopterns, most had three toes on each foot, having lost the two at the sides, but, unusually, the second and fourth toes were also reduced, which likely helped boost their running speed. Indeed, the most extreme of the proterotheres, Thoatherium, had completely lost these toes too, leaving it with just one toe on each foot - a pattern parallel to that seen in the evolution of horses.

The megalodolodines were not that extreme, still retaining the typical three-toed pattern, and they were somewhat stockier and short-limbed than was typical for the family; they likely represent the oldest known branch within it, diverging somewhat from their kin. If other proterotheres paralleled early horses, and perhaps resembled gazelles in size and form, the megalodolodines were something else. In fact, while their heads would have been smaller, with rounded snouts, their general body form would have been closer to that of pigs.

The resemblance doesn't necessarily end there. The molar teeth of proterotheres were brachydont, something we typically wouldn't see in grazing animals. A brachydont tooth is one that has a distinct crown, covered in enamel, with the root of the tooth instead lined with cementum, anchoring it into the jaw. Human teeth are brachydont, so it's the normal pattern you'll see if you look for pictures of "tooth anatomy" online. It's less effective for grinding up tough and gritty plant matter, but perfect for most other purposes.

Where the megalodolodines differ from the other protherotheres is that their cheek teeth are not only brachydont, but also large and bunodont. The latter is a fancy way of saying 'blunt' and, once again, it's true of human teeth (as opposed to, say, those of dogs). In fact, the large molar teeth of megalodolodines most closely resemble those of pigs. To further enhance the pig-like appearance, two of the three known species had tusks, albeit these were formed from the incisors, not the canine teeth as they are in true pigs.

Because of this, the suggestion has been that the megalodolodines had a diet heavy in fruit, and perhaps were unusually omnivorous for ungulates, much as pigs are. But, beyond inferring diet from the general shape of the teeth, how could we know? Well, it turns out that there is more that we can learn from fossil teeth than merely their shape and structure.

Teeth, especially those of herbivores that are constantly grinding up food, rather than just biting into it, naturally wear down over time. By analysing the exact pattern in which this happens, we can learn about what kind of foods an animal has been eating. The techniques for doing this can vary, not least with the precise details of the animal we're looking at, but they fall broadly into two main types: microwear and mesowear analysis. 

The former looks at microscopic pitting in the tooth enamel and reveals details of what the animal was eating in the last few weeks of its life. While it's extremely useful for many purposes, it is relatively difficult and expensive, and requires well-preserved specimens. Mesowear, in contrast, requires a magnifying glass at best, rather than a microscope, and can generally be done with the naked eye. It's much easier, assesses diet over a longer period of the animal's life, and doesn't require such high-quality fossils. (Macrowear analysis, if you're wondering, assesses the age of an animal by how much its teeth have worn down and is not directly related to diet).

The basis of mesowear analysis is that different diets will wear down the teeth in different ways. Grazing wears down teeth because of the tough nature of the food. There is some dispute as to how much this is due to the nature of the grass itself or to the fact that it inevitably comes with particles of soil and grit when you try to eat it from the ground; the reality is likely somewhere in between. Either way, however, browsing animals eat much softer food, such as fresh leaves or fruit, and this means that the main wear on their teeth is instead caused by grinding against the teeth in the other jaw. Fine-tuning this further gives us a picture of what a herbivore has been eating and allows us to compare fossil animals with living ones whose diets we already know.

A recent study examined the teeth of fossil specimens of the three known megadolodid species, including Megadolodus, for which the group is named. About the size of a warthog, this lived in what are now Colombia, Venezuela, and Peru about 13 million years ago, at a time when the local habitat probably consisted of forested wetlands. The study compared the wear patterns on the fossil teeth with those of a variety of living species of pig and peccary, to see which they most resembled.

The nature of the wear patterns on the teeth of the living species varied along a clear range according to their diet. At one extreme was the collared peccary, which primarily eats soft fruit, then the bushpig, which prefers leaf litter and roots, followed by omnivorous animals such as wild boar, with the grass-eating warthogs showing the strongest wear patterns at the opposite end of the chart.

All three megalodolodines, with samples taken from multiple different fossils, showed that whatever they were eating was generally more abrasive than that of most pigs or peccaries, but well short of the grass-rich diet of warthogs. Unless their food was unusually gritty across the whole of their range, this implies that they ate a lot more grass than had previously been assumed based purely on the shape of their teeth. 

But not necessarily...

This is because the closest match was with the teeth of babirusas. Unfortunately, as I mentioned when I discussed these animals a few years ago, the details of babirusa diet are something of a mystery, beyond the fact that they seem more herbivorous than the omnivorous wild boars and their relatives. However, it's thought that fruit with tough seeds are a significant element, especially pangi - these contain multiple large seeds, and are inedible to humans unless cooked and treated properly because they contain high levels of cyanide when raw. 

If the megalodolids had a similarly poisonous diet, there is no way we would know, but crunching on tough fruit seeds is entirely plausible. It's also notable that babirusas have a less pronounced nose-disk than other pigs, and so don't root around in the ground. While we don't have a complete enough snout from any of the fossil to know with certainty, it seems unlikely that parallel evolution between pigs and megadolodines went that far, which again enhances the similarity between them and babirusas.

This could be another example of parallel evolution between pig-like animals millions of years and half a planet apart.

[Photo by Mike Peel, from Wikimedia Commons.]