Abstract
Questions surrounding the dramatic morphology of saber-tooths, and the presumably deadly purpose to which it was put, have long excited scholarly and popular attention. Among saber-toothed species, the iconic North American placental, Smilodon fatalis, and the bizarre South American sparassodont, Thylacosmilus atrox, represent extreme forms commonly forwarded as examples of convergent evolution. For S. fatalis, some consensus has been reached on the question of killing behaviour, with most researchers accepting the canine-shear bite hypothesis, wherein both head-depressing and jaw closing musculatures played a role in delivery of the fatal bite. However, whether, or to what degree, T. atrox may have applied a similar approach remains an open question. Here we apply a three-dimensional computational approach to examine convergence in mechanical performance between the two species. We find that, in many respects, the placental S. fatalis (a true felid) was more similar to the metatherian T. atrox than to a conical-toothed cat. In modeling of both saber-tooths we found that jaw-adductor-driven bite forces were low, but that simulations invoking neck musculature revealed less cranio-mandibular stress than in a conical-toothed cat. However, our study also revealed differences between the two saber-tooths likely reflected in the modus operandi of the kill. Jaw-adductor-driven bite forces were extremely weak in T. atrox, and its skull was even better-adapted to resist stress induced by head-depressors. Considered together with the fact that the center of the arc described by the canines was closer to the jaw-joint in Smilodon, our results are consistent with both jaw-closing and neck musculature playing a role in prey dispatch for the placental, as has been previously suggested. However, for T. atrox, we conclude that the jaw-adductors probably played no major part in the killing bite. We propose that the metatherian presents a more complete commitment to the already extreme saber-tooth ‘lifestyle’.
Highlights
Saber-tooth morphology has a deep history, having independently evolved at least twice in the Permo-Triassic among nonmammalian cynodonts and at least five times among Cenozoic mammals, i.e., within the creodont, nimravid, barbourofelid and machairodontine placental, and sparassodont metatherian clades [1,2,3,4]
Maximal 2D gape angles measured between the upper mesial incisor, jaw-joint, and lower mesial incisor were 87.1u for S. fatalis, 105.8u for T. atrox, and 72.6u for P. pardus
These results were 2.5–12.5u less than determined in previous studies for S. fatalis [21,58], slightly higher than the figure of 102u previously suggested for T. atrox [9], and, within the 65–70u range predicted for extant felids for P. pardus [58]
Summary
Saber-tooth morphology has a deep history, having independently evolved at least twice in the Permo-Triassic among nonmammalian cynodonts and at least five times among Cenozoic mammals, i.e., within the creodont, nimravid, barbourofelid and machairodontine placental, and sparassodont metatherian (a sister group to marsupials) clades [1,2,3,4]. Saber-toothed taxa have long figured prominently in analyses and discussions of adaptive convergence. Of all saber-toothed species, representatives of the felid subfamily Machairodontinae are the best known. There are two widely recognized morphotypes, dirk- and scimitar-tooths. Scimitar-toothed taxa, e.g., Homotherium serum, are characterized by shorter, broader upper canines, longer limbs and more gracile physiques. Dirk-tooths, which include the iconic Smilodon fatalis, possess longer, more laterally compressed upper canines, and are typically much more robust, with shorter legs and lumbar regions [5]. A third morphotype based on a single machairodontine species has been proposed, incorporating a combination of features [6]
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