The double-fanged adolescence of saber-toothed cats

The fearsome, saber-like teeth of Smilodon fatalis -- California's state fossil -- are familiar to anyone who has ever visited Los Angeles' La Brea Tar Pits, a sticky trap from which more than 2,000 saber-toothed cat skulls have been excavated over more than a century.

Though few of the recovered skulls had sabers attached, a handful exhibited a peculiar feature: the tooth socket for the saber was occupied by two teeth, with the permanent tooth slotted into a groove in the baby tooth.

Paleontologist Jack Tseng, associate professor of integrative biology at the University of California, Berkeley, doesn't think the double fangs were a fluke.

Nine years ago, he joined a few colleagues in speculating that the baby tooth helped to stabilize the permanent tooth against sideways breakage as it erupted. The researchers interpreted growth data for the saber-toothed cat to imply that the two teeth existed side by side for up to 30 months during the animal's adolescence, after which the baby tooth fell out.

In a new paper accepted for publication in the journal The Anatomical Record, Tseng provides the first evidence that the saber tooth alone would have been increasingly vulnerable to lateral breakage during eruption, but that a baby or milk tooth alongside it would have made it much more stable. The evidence consists of computer modeling of saber-tooth strength and stiffness against sideways bending, and actual testing and breaking of plastic models of saber teeth.

"This new study is a confirmation -- a physical and simulation test -- of an idea some collaborators and I published a couple of years ago: that the timing of the eruption of the sabers has been tweaked to allow a double-fang stage," said Tseng, who is a curator in the UC Museum of Paleontology. "Imagine a timeline where you have the milk canine coming out, and when they finish erupting, the permanent canine comes out and overtakes the milk canine, eventually pushing it out. What if this milk tooth, for the 30 or so months that it was inside the mouth right next to this permanent tooth, was a mechanical buttress?"

He speculates that the unusual presence of the baby canine -- one of the deciduous teeth all mammals grow and lose by adulthood -- long after the permanent saber tooth erupted protected the saber while the maturing cats learned how to hunt without damaging them. Eventually, the baby tooth would fall out and the adult would lose the saber support, presumably having learned how to be careful with its saber. Paleontologists still do not know how saber-toothed animals like Smilodon hunted prey without breaking their unwieldy sabers.

"The double-fang stage is probably worth a rethinking now that I've shown there's this potential insurance policy, this larger range of protection," he said. "It allows the equivalent of our teenagers to experiment, to take risks, essentially to learn how to be a full-grown, fully fledged predator. I think that this refines, though it doesn't solve, thinking about the growth of saber tooth use and hunting through a mechanical lens."

The study also has implications for how saber-toothed cats and other saber-toothed animals hunted as adults, presumably using their predatory skills and strong muscles to compensate for vulnerable canines.

Beam theory

Thanks to the wealth of saber-toothed cat fossils, which includes many thousands of skeletal parts in addition to skulls, unearthed from the La Brea Tar Pits, scientists know a lot more about Smilodon fatalis than about any other saber-toothed animal, even though at least five separate lineages of saber-toothed animals evolved around the world. Smilodon roamed widely across North America and into Central America, going extinct about 10,000 years ago.

Yet paleontologists are still confounded by that fact that adult animals with thin-bladed knives for canines apparently avoided breaking them frequently despite the sideways forces likely generated during biting. One study of the La Brea predator fossils found that during periods of animal scarcity, saber-toothed cats did break their teeth more often than in times of plenty, perhaps because of altered feeding strategies.

The double-fanged specimens from La Brea, which have been considered rare cases of individuals with delayed loss of the baby tooth, gave Tseng a different idea -- that they had an evolutionary purpose. To test his hypothesis, he used beam theory -- a type of engineering analysis employed widely to model structures ranging from bridges to building materials -- to model real-life saber teeth. This is combined with finite element analysis, which uses computer models to simulate the sideways forces a saber tooth could withstand before breaking.

"According to beam theory, when you bend a blade-like structure laterally sideways in the direction of their narrower dimension, they are quite a lot weaker compared to the main direction of strength," Tseng said. "Prior interpretations of how saber tooths may have hunted use this as a constraint. No matter how they use their teeth, they could not have bent them a lot in a lateral direction."

He found that while the saber's bending strength -- how much force it can withstand before breaking -- remained about the same throughout its elongation, the saber's stiffness -- its deflection under a given force -- decreased with increasing length. In essence, as the tooth got longer, it was easier to bend, increasing the chance of breakage.

By adding a supportive baby tooth in the beam theory model, however, the stiffness of the permanent saber kept pace with the bending strength, reducing the chance of breaking.

"During the time period when the permanent tooth is erupting alongside the milk one, it is around the time when you switch from maximum width to the relatively narrower width, when that tooth will be getting weaker," Tseng said. "When you add an additional width back into the beam theory equation to account for the baby saber, the overall stiffness more closely aligned with theoretical optimal."

Though not reported in the paper, he also 3D-printed resin replicas of saber teeth and tested their bending strength and stiffness on a machine designed to measure tensile strength. The results of these tests mirrored the conclusions from the computer simulations. He is hoping to 3D-print replicas from more life-like dental material to more accurately simulate the strength of real teeth.

Tseng noted that the same canine stabilization system may have evolved in other saber-toothed animals. While no examples of double fangs in other species have been found in the fossil record, some skulls have been found with adult teeth elsewhere in the jaws but milk teeth where the saber would erupt.

"What we do see is milk canines preserved on specimens with otherwise adult dentition, which suggests a prolonged retention of those milk canines while the adult tooth, the sabers, are either about to erupt or erupting," he said.

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Did sabertooth tigers purr or roar?

When a sabertooth tiger called out, what noise did it make -- a mighty roar or a throaty purr? A new study from North Carolina State University examined the data behind the arguments for each vocalization and found that the answer was more nuanced than they thought -- and that it could depend on the shape of a few small bones.

Modern cats belong to one of two groups: either the pantherine "big cats," including the roaring lions, tigers and jaguars; or Felinae "little cats," which include purring cats like lynxes, cougars, ocelots and domestic cats.

"Evolutionarily speaking, sabertooths split off the cat family tree before these other modern groups did," says Adam Hartstone-Rose, professor of biological sciences at NC State and corresponding author of the research. "This means that lions are more closely related to housecats than either are to sabertooths.

"That's important because the debate over the kind of vocalization a sabertooth tiger would have made relies upon analyzing the anatomy of a handful of tiny bones located in the throat," Hartstone-Rose says. "And the size, shape and number of those bones differ between modern roaring and purring cats."

Although vocalization is driven by the larynx and soft tissue in the throat, not bones, anatomists noticed that the bones responsible for anchoring those tissues in place -- the hyoid bones -- differed in size and number between roaring and purring cats.

"While humans have only one hyoid bone, purring cats have nine bones linked together in a chain and roaring cats have seven," says Ashley Deutsch, a Ph.D. student at NC State and lead author of the research. "The missing bones are located toward the top of the hyoid structure near where it connects to the skull."

"Because sabertooth tigers only have seven bones in their hyoid structure, the argument has been that of course they roared," Hartstone-Rose says. "But when we looked at the anatomy of modern cats, we realized that there isn't really hard evidence to support this idea, since the bones themselves aren't responsible for the vocalization. That relationship between the number of bones and the sound produced hasn't ever really been proven."

The researchers looked at the hyoid structures of four species of roaring cats: lions, tigers, leopards and jaguars; and five species of purring cats: cougars, cheetahs, caracals, servals and ocelots. They compared these to 105 hyoid bones from the iconic sabertooth tiger Smilodon fatalis.

"You can argue that since the sabertooths only have seven bones they roared, but that's not the whole story," Hartstone-Rose says. "The anatomy is weird. They're missing extra bones that purring cats have, but the shape and size of the hyoid bones are distinct. Some of them are shaped more like those of purring cats, but much bigger."

According to the researchers, if the missing bones (called epihyoid bones) were key to different vocalizations, the bones most closely connected to them should look different between the two groups. However, those bones looked very similar in shape whether they came from purring or roaring cats.

In fact, the researchers saw more shape variation in the bones closer to the vocal apparatus; i.e., the thyrohyoid and basihyoid bones. The uniformity of the upper bones between the two groups suggests that if the hyoid structure plays a role in vocalization, the lower bones are more important than the upper ones. So having these key hyoid bones shaped like those of purring cats could indicate that they purred rather than roared.

"We found that despite what history has told us about the number of bones in the hyoid structure, no one has validated the significance of that difference," Hartstone-Rose says. "If vocalization is about the number of bones in the hyoid structure, then sabertooths roared. If it's about shape, they might have purred. Due to the fact that the sabertooths have things in common with both groups, there could even be a completely different vocalization."

"It is perhaps most likely that the size of the hyoids plays a role in the pitch of vocalization," says Deutsch. "Although Smilodon wasn't quite as big as the largest modern cats, its hyoid bones are substantially larger than those of any of their living relatives, so potentially they had even deeper vocalizations than the largest tigers and lions."

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Fossils of a saber-toothed top predator reveal a scramble for dominance leading up to 'the Great Dying'

A tiger-sized saber-toothed creature called Inostrancevia has previously only been found in Russia. But scientists have discovered its fossils in South Africa, suggesting that it migrated 7,000 miles across the supercontinent Pangaea during the world's worst mass extinction 252 million years ago. Heading to South Africa allowed it to fill a gap in a faraway ecosystem that had lost its top predators.

Two hundred and fifty-two million years ago, Earth experienced a mass extinction so devastating that it's become known as "the Great Dying." Massive volcanic eruptions triggered catastrophic climate change, killing off nine out of every ten species and eventually setting the stage for the dinosaurs. But the Great Dying was a long goodbye -- the extinction event took place over the course of up to a million years at the end of the Permian period. During that time, the fossil record shows drama and upheaval as species fought to get a foothold in their changing environments. One animal that exemplifies this instability was a tiger-sized, saber-toothed creature called Inostrancevia: a new fossil discovery suggests that Inostrancevia migrated 7,000 miles across the supercontinent Pangaea, filling a gap in a faraway ecosystem that had lost its top predators, before going extinct itself.

"All the big top predators in the late Permian in South Africa went extinct well before the end-Permian mass extinction. We learned that this vacancy in the niche was occupied, for a brief period, by Inostrancevia," says Pia Viglietti, a research scientist at the Field Museum in Chicago and a co-author of the new study in Current Biology.

The prehistoric creature looked the part of "top predator." "Inostrancevia was a gorgonopsian, a group of proto-mammals that included the first saber-toothed predators on the planet," says Viglietti. It was about the size of a tiger and likely had skin like an elephant or a rhino; while vaguely reptilian in appearance, it was part of the group of animals that includes modern mammals.

Prior to this new paper, Inostrancevia had only ever been found in Russia. But while examining the fossil record of South Africa's Karoo Basin, Viglietti's colleague Christian Kammerer identified the fossils of two large predatory animals that were different from those normally found in the region. "The fossils themselves were quite unexpected," says Viglietti. It's not clear how they made it from what's now Russia, or how long it took them to cross Pangaea and arrive in what's now South Africa. But being far from home was just one element of what made the fossils special.

"When we reviewed the ranges and ages of the other top predators normally found in the area, the rubidgeine gorgonopsians, with these Inostrancevia fossils, we found something quite exciting," she says. "The local carnivores actually went extinct quite a bit before even the main extinction that we see in the Karoo -- by the time the extinction begins in other animals, they're gone."

The arrival of Inostrancevia from 7,000 miles away and its subsequent extinction indicates that these top predators were "canaries in the coal mine" for the larger extinction event to come.

"This shows that the South African Karoo Basin continues to produce critical data for understanding the most catastrophic mass extinction in Earth's history," says co-author Jennifer Botha, director of GENUS Centre of Excellence in Palaeosciences and professor at the Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg.

"We have shown that the shift in which groups of animals occupied apex predator roles occurred four times over less than two million years around the Permian-Triassic mass extinction, which is unprecedented in the history of life on land. This underlines how extreme this crisis was, with even fundamental roles in ecosystems in extreme flux," said Christian Kammerer, the study's first author and a research curator of paleontology at the North Carolina Museum of Natural Sciences and research associate at the Field Museum.

The vulnerability of these top predators matches what we see today. "Apex predators in modern environments tend to show high extinction risk, and tend to be among the first species that are locally extirpated due to human-mediated activities such as hunting or habitat destruction," says Kammerer. "Think about wolves in Europe or tigers in Asia, species which tend to be slow to reproduce and grow and require large geographic areas to roam and hunt prey, and which are now absent from most of their historic ranges. We should expect that ancient apex predators would have had similar vulnerabilities, and would be among the species that first go extinct during mass extinction events."

In addition to shedding new light on the extinction event that helped lead to the rise of the dinosaurs, Viglietti says that the study is important for what it can teach us about the ecological disasters the planet is currently experiencing.

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How the 'marsupial sabertooth' thylacosmilus saw its world

A new study investigates how an extinct, carnivorous marsupial relative with canines so large they extended across the top of its skull could hunt effectively despite having wide-set eyes, like a cow or a horse. The skulls of carnivores typically have forward-facing eye sockets, or orbits, which helps enable stereoscopic (3D) vision, a useful adaptation for judging the position of prey before pouncing. Scientists from the American Museum of Natural History and the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales in Mendoza, Argentina, studied whether the "marsupial sabertooth" Thylacosmilus atrox could see in 3D at all. Their results are published today in the journal Communications Biology.

Popularly known as the "marsupial (or metatherian) sabertooth" because its extraordinarily large upper canines recall those of the more famous placental sabertooth that evolved in North America, Thylacosmilus lived in South America until its extinction about 3 million years ago. It was a member of Sparassodonta, a group of highly carnivorous mammals related to living marsupials. Although sparassodont species differed considerably in size -- Thylacosmilus may have weighed as much as 100 kilograms (220 pounds) -- the great majority resembled placental carnivores like cats and dogs in having forward-facing eyes and, presumably, full 3D vision. By contrast, the orbits of Thylacosmilus, a supposed hypercarnivore -- an animal with a diet estimated to consist of at least 70 percent meat -- were positioned like those of an ungulate, with orbits that face mostly laterally. In this situation, the visual fields do not overlap sufficiently for the brain to integrate them in 3D. Why would a hypercarnivore evolve such a peculiar adaptation? A team of researchers from Argentina and the United States set out to look for an explanation.

"You can't understand cranial organization in Thylacosmilus without first confronting those enormous canines," said lead author Charlène Gaillard, a Ph.D. student in the Instituto Argentino de Nivología, Glaciología, y Ciencias Ambientales (INAGLIA). "They weren't just large; they were ever-growing, to such an extent that the roots of the canines continued over the tops of their skulls. This had consequences, one of which was that no room was available for the orbits in the usual carnivore position on the front of the face."

Gaillard used CT scanning and 3D virtual reconstructions to assess orbital organization in a number of fossil and modern mammals. She was able to determine how the visual system of Thylacosmilus would have compared to those in other carnivores or other mammals in general. Although low orbital convergence occurs in some modern carnivores, Thylacosmilus was extreme in this regard: it had an orbital convergence value as low as 35 degrees, compared to that of a typical predator, like a cat, at around 65 degrees.

However, good stereoscopic vision also relies on the degree of frontation, which is a measure of how the eyeballs are situated within the orbits. "Thylacosmilus was able to compensate for having its eyes on the side of its head by sticking its orbits out somewhat and orienting them almost vertically, to increase visual field overlap as much as possible," said co-author Analia M. Forasiepi, also in INAGLIA and a researcher in CONICET, the Argentinian science and research agency. "Even though its orbits were not favorably positioned for 3D vision, it could achieve about 70 percent of visual field overlap -- evidently, enough to make it a successful active predator."

"Compensation appears to be the key to understanding how the skull of Thylacosmilus was put together," said study co-author Ross D. E. MacPhee, a senior curator at the American Museum of Natural History. "In effect, the growth pattern of the canines during early cranial development would have displaced the orbits away from the front of the face, producing the result we see in adult skulls. The odd orientation of the orbits in Thylacosmilus actually represents a morphological compromise between the primary function of the cranium, which is to hold and protect the brain and sense organs, and a collateral function unique to this species, which was to provide enough room for the development of the enormous canines."

Lateral displacement of the orbits was not the only cranial modification that Thylacosmilus developed to accommodate its canines while retaining other functions. Placing the eyes on the side of the skull brings them close to the temporal chewing muscles, which might result in deformation during eating. To control for this, some mammals, including primates, have developed a bony structure that closes off the eye sockets from the side. Thylacosmilus did the same thing -- another example of convergence among unrelated species.

This leaves a final question: What purpose would have been served by developing huge, ever-growing teeth that required re-engineering of the whole skull?

"It might have made predation easier in some unknown way," said Gaillard, "But, if so, why didn't any other sparassodont -- or for that matter, any other mammalian carnivore -- develop the same adaptation convergently? The canines of Thylacosmilus did not wear down, like the incisors of rodents. Instead, they just seem to have continued growing at the root, eventually extending almost to the rear of the skull."

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