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Sabertooth predators have inspired awe in humans since time immemorial, and are reminders that Hollywood-worthy carnivores once stalked the earth alongside our ancestors. Despite our fascination with these prehistoric icons, however, they are often misunderstood. Popular media commonly refers to these ancient predators as “sabertooth lions” or “sabertooth tigers,” yet in reality they were never part of the lineage that led to lions and tigers: many were not even cats at all.

Sabertooth morphology appeared independently in several branches of the mammalian family tree: the Felidae (although not the Felinae, the subfamily that includes modern lions, tigers, and other big cats), the Nimravidae, and the Barbourofelidae. Two different metatherian lineages also included sabertooth predators: the marsupial Thylacosmilidae and the Borhyaenidae, which belong to a group called the Sparassodonts that are now thought to be a sister taxon to “true” marsupials. Saberteeth were quite the fad amongst many different mammalian families for tens of millions of years.

The last of the sabertooth predators—felids known as machairodonts—died out around 12,000 years ago, and our mammalian predators have possessed relatively modest dentition ever since. The only modern carnivore with skull features that even approach sabertooth proportions is the clouded leopard, Neofelis nebulosa (Christiansen 2006). Several aspects of the clouded leopard’s cranial morphology are comparable to those of later machairodonts. In addition to having canine teeth that are very long in proportion to overall skull length, the angle of the clouded leopard’s jaw joint is intermediate between those of modern cats and sabertooths, and its facial bones are rotated towards the posterior, in classic sabertooth fashion. Another trademark feature of sabertooth morphology was an incredibly wide gape, and Christiansen (2006) demonstrated that the clouded leopard does have—by far—the widest gape of any extant felid.

The clouded leopard is so rare and enigmatic that little data is available regarding its hunting and killing behavior, however, so it is impossible to tell whether it uses its large canines in the same fashion that the sabertooths did. Given this uncertainty about hunting technique and the fact that most of the clouded leopard’s features are intermediate between modern cats and sabertooths, it is likely only a loose analogue to actual ancient sabertooth predators.

We tend to automatically look to large, charismatic species such as cats for modern equivalents to the sabertooth “tigers.” This may be a mistake. Some of the best-known sabertooth predators were marsupials, such as the infamous Thylacosmilus, which had the most extreme sabertooth morphology discovered so far. There are still quite a few marsupial predators around today, yet most of these species are relatively small-bodied and have been largely overlooked in studies of predator skull morphology. That appears to be changing, however. A recent paper in the Journal of Zoology begins to fill in this gap by revealing sabertooth morphology in a seemingly unlikely candidate: a small (~100g) South American opossum, Monodelphis dimidiata (Blanco et al. 2013)


Thylacosmilus skull

This miniature marsupial boasts longer canines—relative to body size—than any other extant marsupial, including carnivorous species such as the Tasmanian devil. Analyses of an array of skull measurements showed that M. dimidiata‘s impressive teeth, in combination with other cranial adaptations, put the tiny animal well within the morphometric range that is characteristic of extinct sabertooth predators. Its index of canine tooth length actually exceeds those of large sabertooths such as Dinictis and Homotherium. Monodelphis dimidiata’s cranial morphology is even more similar to the extinct sabertooths than that of the clouded leopard, meaning that a mouse-sized marsupial actually usurps the majestic cat as being the closest modern equivalent to a group of iconic prehistoric predators.


Monodelphis dimidiata

The data behind this discovery are more complex than simply revealing an overlooked sabertooth marsupial. Monodelphis dimidiata is distinguished from modern marsupials by its combination of long canines, short jaws, beefy forelimb bones, and the mandibular shape and cranial muscles that enable an extended gape width. There is a caveat, however: not all of M. dimidiata’s skull measurements show sabertooth adaptations. Skull proportions such as the lever arms of the masseter and temporalis and the mandibular length index are not significantly different from other marsupials.

Monodelphis dimidiata’s combination of sabretooth and “unspecialized” traits illustrates an important evolutionary concept: mosaic evolution. Natural selection and evolutionary adaptations most often happen in a “mosaic,” patchwork pattern, meaning that not all of an organism’s features will change at the same pace. Selection works at a very fine scale, and the idea that all of an organism’s traits evolve in lockstep is a widely held misperception. This means that some parts of a given species’ skull could display characteristic sabertooth features to a more extreme degree than others at a given point in time. We have seen this with hominid fossil remains too. For example, Australopithicines such as the famed “Lucy” had lower limbs that were capable of bipedalism like modern humans, despite retaining ape-like brain capacities.

Paleontologists already had a hunch that sabertooth morphology evolved bit by bit, rather than being strictly tied to other cranial-mandibular adjustments. The mosaic pattern of adaptation seen in M. dimidiata is consistent with previous analyses of sabertooth fossils (Christiansen 2011). The opossum’s mixture of specialized and unspecialized features is highly consistent with those of “primitive” sabertooths, such as the nimravid Dinictis and the creodont Machaeroides, although it is not as extreme as that of more highly derived sabertooths.

Mosaic evolution may partially explain why the sabertooth form was able to develop independently in multiple mammalian lineages throughout evolutionary history. While it would be unlikely for natural selection to repeatedly produce the entire suite of extreme sabertooth features in synchrony, the appearance of these traits would be more probable if the cranial adaptations develop piecemeal, through successive responses to biomechanical pressures brought on by specific adaptations, such as out-sized teeth. This pattern would gradually produce animals that fall along a spectrum of “sabertoothness,” as we see in the fossil record.

This leads us to the second evolutionary concept illustrated by M. dimidiata: exaptation. Nature loves to be thrifty, and organisms can often employ specialized features for more than one purpose as time goes on. Gould and Vrba (1982) coined a term form this phenomenon: “exaptation.” Exaptations, or “preadaptations,” are features that initially evolved in response to one set of selective pressures, but have been co-opted for another purpose as well. For example, it is now widely believed that feathers initially developed for uses ranging from thermoregulation to visual display to tools for catching insects. These early feathers must have been awfully handy, but were aerodynamically useless and only became useful for flight many generations later (Norell & Xu 2005). The origins of key features may not be as obvious as they seem at first glance, and it takes careful consideration of all that is known about an organism’s life history patterns to fully understand its evolutionary history.

In the case of M. dimidiata, Blanco and colleagues showed that only males have highly specialized sabertooth cranial morphology. What selective pressures could cause that disparity? The answer, as with many things in biology, is sex. Monodelphis dimidiata is a semelparous species, meaning it only survives for a single breeding season (Vilela, et al. 2010). The short-lived creatures only have one chance to reproduce before they die, and competition between males is urgent and fierce (Gonzalez & Claramunt 2000). Enlarged canines and robust forelimbs are highly advantageous during showdowns over mate acquisition, thus increasing a male’s reproductive fitness. High levels of male-male competition provides selective pressure for long teeth, a wide gape, and a strong grip. These features enable the type of killing behavior displayed by ancient sabertooths—creating a prime opportunity for the traits to be “exapted” for predation. If possessing sabertooth morphology means that a male is both better at acquiring mates and has the added benefit of being better at acquiring food, it is not hard to see why the traits would be selected for and enhanced over the course of many generations.

By revealing a previously unknown—if diminutive—sabertooth predator in our midst, this study drives home the fact that our planet’s biodiversity still holds a rich trove of new discoveries and insights, just waiting to be revealed. Monodelphis dimidiata may not be quite as regal as the roaring sabertooth “tigers” we have seen in so many books and museum dioramas, but it shows that certain evolutionary pressures and outcomes defy scale. These forces are timeless, occurring over and over across the eons—from the nimravids that paced the earth 40 million years ago to a mouse-sized marsupial making its quiet living in modern South America. If a tiny opossum can beat out large, majestic carnivores for top sabertooth billing, what else might we find, if we just pay attention?


Anne-Marie Hodge is currently working on her doctoral degree at the University of Wyoming. She graduated from Auburn University in 2009 with a bachelor’s degree in Zoology, including a concentration in Conservation/Biodiversity and a minor in Anthropology. During her years at Auburn, Anne-Marie was a founding member of Alabama’s first chapter of the Society for Conservation Biology. She completed a Master of Science in Biology at the University of North Carolina-Wilmington in 2012, and has participated in field research expeditions in the southwestern U.S., Mexico, Belize, Ecuador, and Kenya. When she is not chasing carnivores at the equator, Anne-Marie blogs at Endless Forms on the Nature Network and is a frequent contributor to Ecology.com.

References

Blanco, R.E., Jones, W.W. & Milne, N. (2013) Is the extant southern short-tailed opossum a pigmy sabretooth predator? Journal of Zoology, n/a–n/a.

Christiansen, P. (2006) Sabertooth characters in the clouded leopard (Neofelis nebulosa Griffiths 1821). Journal of Morphology, 267, 1186–1198.

Christiansen, P. (2011) A dynamic model for the evolution of sabrecat predatory bite mechanics. Zoological Journal of the Linnean Society, 162, 220–242.

Gonzalez, E.M. & Claramunt, S. (2000) Behaviors of captive short-tailed Opossums, Monodelphis dimidiata (Wagner, 1847) (Didelphimorphia, Didelphidae). Mammalia, 64, 271–286.

Norell, M.A. & Xu, X. (2005) Feathered Dinosaurs. Annual Review of Earth and Planetary Sciences, 33, 277–299.

Vilela, J.F., De Moraes Russo, C.A. & De Oliveira, J.A. (2010) An assessment of morphometric and molecular variation in Monodelphis dimidiata (Wagner, 1847) (Didelphimorphia: Didelphidae). Zootaxa, 2646, 26–42.

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Published On: July 30, 2013

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