A Carnivore Conundrum

Many of South America’s most characteristic mammals are members of the order Carnivora, the group that include cats, dogs, bears, and their close relatives. Notable Neotropical representatives of this group include the stilt-legged maned wolf, the curious coati, the powerful and iconic jaguar, and the reclusive spectacled bear.

From a paleontological perspective, however, these aren’t South American mammals at all, at least not in the sense that armadillos, sloths, and opossums are. The ancestors of these carnivorans dispersed from North America relatively recently, and their remains are extremely rare at South American fossil sites that are more than a few million years old. It was only after the formation of a land bridge between the Americas that cats, dogs, and kin became integral members of South American mammal communities, as they are today.

So what types of mammals occupied their ecological niches millions of years earlier?

Sparassodonts – more or less.

Fig. X Borhyaena tuberata.jpg
Reconstruction of the early Miocene sparassodont Borhyaena tuberata from Croft (2016). Illustration by Velizar Simeonovski. Copyright D. Croft.

If you’ve never heard of a sparassodont, you are not alone. Most people have not. This is mainly due to the fact that these mammals only lived in South America, and they went extinct several million years ago. Sparassodonts belong to the marsupial rather than the placental side of the mammal family tree, and although their evolutionary relationships aren’t precisely known, paleontologists agree that they are not closely related to Australian meat-eating marsupials such as the Tasmanian devil and the recently-extinct thylacine. Sparassodonts evolved to fill meat-eating ecological niches in South America independently of their distant Australian cousins.

The reason I say that sparassodonts “more or less” filled the ecological niches of modern South American carnivorans is based on a study several co-authors and I recently published on the ecological diversity of these ancient meat-eating marsupials. In short, we closely examined the teeth of all presently-known sparassodonts and found that nearly all of them had teeth most similar to living mammals that feed almost exclusively on meat, which are technically known as hypercarnivores. In other words, as a group, they seem to have occupied a relatively narrow ecological niche.

Fig. 2 Arctodictis.jpg
Lateral view of the left lower jaw of Arctodictis sinclairi (MLP 85-VII-3-1), a large early Miocene sparassodont. Note the tall, pointed cusps of the molar at the back of the jaw (the tooth furthest to the right) and the v-shaped meat-slicing trough in between the cusps. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

In contrast to sparassodonts, not all members of the cat-dog-bear group are hypercarnivores. Wolves are hypercarnivores, but foxes, jackals, and most other dogs are mesocarnivores: they feed on insects and fruit in addition to meat, though meat still usually comprises most of their diet. There are also carnivorans like raccoons and most bears that eat very little meat. As a consequence, they are classified as hypocarnivores. Bear in mind (so to speak) that being a hypocarnivore does not make a mammal any less dangerous! It just means that beef is seldom what’s for dinner.

Most modern mammal communities include species of all three categories of carnivores: some hypercarnivores, some mesocarnivores, and some hypocarnivores. Within each of these categories, species tend to vary from one another in size, where and how they hunt, and other features that serve to minimize competition among them. As a result, more than two dozen species of carnivorans can belong to the same ecological community. Collectively, such species comprise what is known as the carnivore guild.

In addition to examining sparassodont diets and variations in diversity through time, our study looked at how mammal carnivore guilds vary in the numbers and proportions of species in the dietary categories mentioned above (hypercarnivore, mesocarnivore, hypocarnivore). We collected data from several modern sites (Yellowstone National Park, Serengeti National Park, and Malaysia) and nearly a dozen Northern Hemisphere fossil sites and compared them to a very well-sampled, 18 million-year-old site in southern Argentina known as Santa Cruz. What we found was remarkable: no other modern or fossil guild we examined was composed entirely of hypercarnivores like that of Santa Cruz.

Fig. 3 Guild Pie Charts.jpg
Pie charts illustrating the proportion of species in each carnivore dietary category at the fossil site of Santa Cruz, Argentina (left) and at three modern sites on different continents. Note the absence of mesocarnivores and hypocarnivores at Santa Cruz. N equals the total number of species at each site. The lower number of species identified at Santa Cruz is typical for fossil sites. Data from Croft et al. (2018:table S9). Figure by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

What does this mean for understanding the paleoecology of these ancient meat-eating marsupials and the ecological communities in which they lived?

The big take-home message is that some aspects of ancient mammal communities are completely unlike what we see in modern mammal communities today. Modern terrestrial carnivore guilds include a mix of species filling different dietary niches. By contrast, Santa Cruz’s 11 species of sparassodonts all seem to have been dedicated meat-eaters. How were they able to co-exist? Size differences would have allowed them to feed on different types of prey, as would differences in the amount of time they hunted on the ground or in the trees. Nevertheless, these factors are also at work in modern carnivore guilds, so they aren’t sufficient to explain why sparassodonts apparently stuck to an all-meat diet. Perhaps sparassodonts lived at lower densities then placental carnivorans and therefore encountered one another less frequently. That might have allowed more species to live on the same foods than would be expected otherwise. This could also explain the rarity of sparassodont remains at South American fossil sites, a phenomenon I pointed out in a previous study of sparassodont diversity. Clearly much more work needs to be done to address this question.

There is also an alternative scenario: perhaps sparassodonts actually DID vary in their diets but those differences aren’t reflected in their teeth. If true, this finding would challenge a fundamental principle of paleoecological reconstruction: the idea that mammals that eat the same types of foods evolve similar-looking teeth. This principle is well-established among meat-eating mammals of many kinds; the more dedicated a mammal becomes to a carnivorous lifestyle, the more specialized its teeth become for slicing meat. Nevertheless, marsupials are a bit different from placentals in this regard and have to make some evolutionary compromises in their dentition due to the way they replace their teeth. (If you want the details, see this classic paper by Lars Werdelin.) Given this constraint and the fact that we can’t examine a living sparassodont to see what types of teeth go with what types of diets, we can’t be as confident about how much meat a particular sparassodont species ate as we can with an extinct species of dog or raccoon. A recent study that reconstructed sparassodont diets using lower jaw shape rather than tooth shape concluded that some species may have been mesocarnivorous or hypocarnivorous rather than hypercarnivorous. Could it be that their teeth are leading us astray?

What we are left with is a paleoecological Catch-22: either our predictions about sparassodont diets are correct, and ancient terrestrial carnivore guilds of South America weren’t following the rules that govern carnivore guilds elsewhere, or these carnivore guilds followed the same basic rules as all others, and well-established form-function relationships are providing wrong information about what some sparassodonts were eating. My gut tells me the second scenario is correct, but unfortunately I don’t have any strong data to back that up!

The bottom line is that we often fall into the trap of assuming that modern mammals and their ecological communities are “typical” and that extinct mammals and their communities must fall somewhere within the range of variation of modern ones. That is not always the case, as illustrated by the splendidly isolated paleofaunas of South America.

And that’s one more reason why I love being a paleomammalogist.

References:

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