Creationism in Crisis - How Sabretoothed Cats Were Avoiding Broken Teeth - Before 'Creation Week'

Saber-toothed cat. Smilodon fatalis

Massimo Molinero

Smilodon fatalis skull

The double-fanged adolescence of saber-toothed cats | Berkeley

As an example of daft, Heath-Robinson design, the teeth of the North American sabre-toothed cat, Smilodon fatalis is a good as it gets. Obviously, having huge canine teeth with which to rip the throat out of large prey and so subdue it quickly, has some advantages, but the trouble is that the longer they are, the more likely they are to break, and broken teeth for a Smilodon could well have meant starvation and death. There needs to be a trade-off between ever-bigger teeth and death due to breakages.

And typical of creationism’s daft designer is the Heath-Robinson solution to a simple problem - you've guessed it, another layer of complexity. Instead of losing their 'milk teeth' like many mammals do as their head and mouth grows, Smilodon kept its milk teeth to act as a sort of splint for the adult Sabre tooth, reducing the lateral strain on the adult teeth until the adult was about 30 months old, by which time it had probably learned how to minimise lateral stress on its teeth.

This was discovered by Paleontologist Jack Tseng, associate professor of integrative biology at the University of California, Berkeley, who has published his findings in an open access paper in the journal The Anatomical Record. It is also explained in a University of California news release by Robert Sanders:

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.

A portion of the right maxilla of a saber-toothed cat, Smilodon fatalis, showing a fully erupted baby saber tooth with the adult tooth just erupting. Based on Tseng's tooth eruption timing table, he estimates that the animal was between 12 and 19 months of age at the time of death. The fossil is from the La Brea Tar Pits and is housed at the Natural History Museum of Los Angeles County.

Jack Tseng, UC Berkeley

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. 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?

Professor Jack Tseng, author
Curator, Museum of Paleontology.
University of California, Berkeley, CA, USA.

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. 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.

Professor Jack Tseng

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.

The complete cranium of a Smilodon with fully-erupted sabers. The left saber, seen behind, is broken. Based on the condition of the fully adult saber, Tseng estimates that the animal was at least 3 years old when it died. The fossil is from the La Brea Tar Pits and is housed at the Natural History Museum of Los Angeles County.

Jack Tseng, UC Berkeley

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.

A finite element model of an adult saber tooth indicating saber bending stress. The warmer the color, the higher the stress and the more likely failure will occur in a particular area of the tooth model. The red dot near the tip is where the force was applied to measure the sideways bending stress.

Jack Tseng, UC Berkeley

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. 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.

Professor Jack Tseng

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. 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.

Professor Jack Tseng

Postdoctoral fellow Narimane Chatar tests the bending strength of a resin model of an adult saber tooth.

Jack Tseng, UC Berkeley

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.

Professor Jack Tseng

Related information:

• Bending performance changes during prolonged canine eruption in saber-toothed carnivores: A case study of Smilodon fatalis (The Anatomical Record)
• Using a Novel Absolute Ontogenetic Age Determination Technique to Calculate the Timing of Tooth Eruption in the Saber-Toothed Cat, Smilodon fatalis (PLOS ONE, 2-15)
• A jaw-dropping conundrum: Why do mammals have a stiff lower jaw? (2023)
• Was this hyena a distant ancestor of today’s termite-eating aardwolf? (2022)
• Young T. rexes had a powerful bite, even if only one-sixth that of their parents (2021)
• Jack Tseng's website

More technical detail is given in Professor Tsang’s paper:

Abstract

The canine of saber-toothed predators represents one of the most specialized dental structures known. Hypotheses about the function of hypertrophied canines range from display and conspecific interaction, soft food processing, to active prey acquisition. Recent research on the ontogenetic timing of skull traits indicates the adult canine can take years to fully erupt, but the consequences of prolonged eruption on inferences of canine functional morphology are missing from current discourse and have not been quantified. Here I evaluate hypotheses about adult canine bending strength and stiffness, respectively, during eruption in the felid Smilodon fatalis. Simulated eruption sequences of three adult canines were generated from specimen models to assess shifting cross-sectional geometry properties, and bending strength and stiffness under laterally directed loads were estimated using finite element analysis. Consistent with beam theory expectations, S. fatalis canine cross-sectional geometry is optimized for increased bending strength with increased erupted height. However, canine cross-sectional geometry changes through eruption exaggerate rather than minimize lateral deflection. Spatial constraint for maximum root length from adjacent sensory structures in the maxilla and the recently identified universal power law are hypothesized to limit the growth capacity of canine anteroposterior length and, consequently, maintenance of bending stiffness through eruption. Instead, the joint presence of the deciduous and adult canines for >50% of the adult canine eruption period effectively increases canine mediolateral width and brings bending strength and stiffness estimates closer to theoretical optima. Similarly prolonged retention of deciduous canines in other sabretooths suggests dual-canine buttressing is a convergently evolved strategy to maximize bending strength and stiffness.

1 INTRODUCTION

Saber-like canine teeth evolved convergently in multiple vertebrate clades, including five times within mammals (Lautenschlager et al., 2020). Despite the ongoing debate regarding predatory behavior implied by the morphologically specialized canine teeth of saber-toothed predatory mammals (machairodontine felids, nimravids, barbourofelids, thylacosmilids, creodonts) (e.g., Andersson et al., 2011; Antón & Galobart, 1999; Anyonge, 1996; Biknevicius et al., 1996.1; Deng et al., 2016; Domínguez-Rodrigo et al., 2022; Emerson & Radinsky, 1980; Figueirido et al., 2018; Martin, 1980.1; McCall et al., 2003; Meachen-Samuels, 2012; Wroe et al., 2008, 2013), one consistent theme in sabertooth bite mechanics research is the emphasis on fully erupted adult (permanent) canines (Wysocki, 2019). Although fully adult canines represent the most impressive and oft-discussed versions of the elongate teeth, the observation that subadult individuals of the extinct felid Smilodon fatalis and other sabertooths spent a substantial amount of time living with partially erupted canines suggests that the months to years leading up to full adult canine eruption may be crucial ones for understanding sabertooth function in adult individuals. Wysocki et al. (2015) estimated that the maximum timespan required for adult canines to fully erupt in S. fatalis may have been just shy of 30 months. Thus, sub-adult sabertooths would have had to adjust to a continuously shifting set of dental weaponry for more than 2 years during their ontogeny. The importance of such acclimation to a shifting dentition is underscored by observations that S. fatalis from the Rancho La Brea Tar Pits exhibit higher incidents of tooth breakage than other commonly found predators in the region such as dire wolves (Binder & Van Valkenburgh, 2010; van Valkenburgh & Hertel, 1993).

The cross-sectional geometry of canine crowns has been shown to play an important part in determining the fracture resistance (Freeman & Lemen, 2007a; Soukup et al., 2015.1) as well as the functional specialization and ecological correlation (Freeman & Lemen, 2007.1b; Pollock, Hocking, & Evans, 2022.1; Pollock, Panagiotopoulou, et al., 2022.2) of canine teeth in extant carnivoran mammals. It has long been observed that the hypertrophied canines of sabertooth carnivorans such as S. fatalis exhibit varied cross-sectional geometry along the height axis of the tooth (Merriam & Stock, 1932; Tseng et al., 2010.1; Wysocki, 2019). However, the resulting changes in mechanical performance of the saber-like teeth through various stages of eruption have not been explicitly quantified.

Prior research on the functional morphology of sabertooth canines have firmly established the utility of beam theory in clarifying patterns of functional variation across adult canines of different sabertooth taxa (Christiansen, 2007.2; Van Valkenburgh & Ruff, 1987) as well as mandibles (Therrien, 2005). The bending strength of a beam-like canine structure around its anteroposterior axis (S_AP) is defined using beam theory as: $$$$ {S}_{\mathrm{AP}}=\frac{I_{\mathrm{AP}}}{hy} $$$$ where h is the canine height, y is the mediolateral width of the canine, and I_AP the second moment of area (a measure of how material is distributed in the cross-section of the beam-like structure) around the bending axis, defined as: $$$$ {I}_{\mathrm{AP}}=\frac{\pi x{y}^3}{4} $$$$ where x is the anteroposterior length of the canine, and y as above. If we substitute the formula for second moment of area into the bending strength formula above, S_AP becomes: $$$$ {S}_{\mathrm{AP}}=\frac{\pi x{y}^2}{4h} $$$$ Based on this equation, it is expected that for bending strength of saber-like canines that exhibit beam-like behavior to stay constant with increasing crown height, xy² should increase linearly with h. See Figure 1h for visual definitions of the terms used in the equation.

FIGURE 1
Canine eruption and beam models of Smilodon fatalis analyzed in the study. (a) Canine eruption sequence models based on AMNH FM55576, (b) Beam models based on AMNH FM55576, (c) Canine eruption sequence models based on LACM RLB P23-4739, d. Beam models based on LACM RLB P23-4739, (e) Canine eruption sequence models based on KU S2255, (f) Beam models based on KU S2255. (g) Approximate positions of two different canine eruption stages to demonstrate the eruption sequence tested in the study. Silhouette modified from phylopic.org (credit: Steven Traver, CC0 license). (h) The axis system, dimensional terms, and the specific mode of bending tested using beam theory equations in this study. Lighter blue outlines in panels (a–f) represent cross-sectional geometries; darker blue shapes represent lateral views of models. Canine model sizes are at the same scale, showing variation in the size and height of adult canines.

Furthermore, it can also be shown from beam theory principles that if both the load placed on a beam-like cantilever structure such as a sabertooth canine and its elastic modulus (a material property that describes stiffness) are assumed to stay the same, then the total mediolateral deflection (δ_ML, the weakest axis of the saber-like canines) of the canine from a given load would be proportional to: $$$$ {\delta}_{\mathrm{ML}}\propto \frac{h^3}{{\mathrm{I}}_{\mathrm{AP}}}=\frac{h^3}{\frac{\pi x{y}^3}{4}}=\frac{4}{\pi}\times \frac{1}{x}\times {\left(\frac{h}{y}\right)}^3 $$$$ Therefore, to keep deflection (which is inversely proportional to bending stiffness) of the canine about the anteroposterior axis constant, xy³ should increase linearly with h³. These two strength and deflection expectations are the minimum beam theory-based requirements for maintaining saber mechanical performance through the eruption period.

Given (1) the expected cross-sectional geometry changes predicted by the beam theory equations shown above, (2) the prior observation that fully erupted adult canines in S. fatalis tend to have higher bending strength than expected from estimated masticatory forces (Christiansen, 2007), and (3) the fact that laterally compressed canines such as those found in saber-toothed predators are mechanically weakest in the mediolateral bending direction, I hypothesize that S. fatalis exhibit increased canine mediolateral bending strength and maximum deflection (stiffness) changes through their prolonged eruption period in order to maintain, if not improve, sabertooth mechanical performance. Namely,

H1.The eruption sequence of S. fatalis adult canines is characterized by the maintenance or an increase in bending strength, specifically by following a cross-sectional geometry where xy² increases linearly with regard to h, and/or.

H2.The eruption sequence of S. fatalis adult canines is characterized by the maintenance or a decrease in maximum deflection, specifically by following a cross-sectional geometry where xy³ increases linearly with regard to h³.

If both H1 and H2 are supported, it would suggest that the canine eruption period in S. fatalis was evolutionarily optimized for maintaining (or increasing) strength and stiffness throughout the ontogenetic transition towards fully erupted adult canine morphology. If only one of the hypotheses is supported, it would suggest that sabertooth canines were only partially optimized, for either strength or stiffness but not both. If neither hypothesis is supported by data, then it would suggest that sabertooth canines are not optimized for strength or stiffness during eruption, and that either the adult canines are minimally load-bearing relative to less erupted versions of themselves, and/or that there are musculoskeletal and/or behavioral mechanisms, extrinsic to canine morphology, for preventing high loads along the weakest, mediolateral axis of the saber-like teeth. I test these hypotheses using a combination of beam theory analysis and finite element simulations, which have been previously shown to capture broad patterns of canine functional morphology across mammalian carnivores (Pollock, Panagiotopoulou, et al., 2022.2).

Designing large 'sabre' teeth which break because of their size, then needing to design a work-around to solve the problem, is not the act of an intelligent, omniscient designer. It is, however, what we expect of a mindless utilitarian process working without a plan and with no regard to the suffering of the animal dying of starvation because its essential teeth eventually break, so long as it allows them to produce more offspring while it still has teeth, than its ancestors without sabre teeth could have done in an entire lifetime.

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