Comparative Biomechanical Modeling of Metatherian and Placental Saber-Tooths: A Different Kind of Bite for an Extreme Pouched Predator

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’.

Muscle moment arm analyses applied to vertebrate paleontology: a case study usingStegosaurus stenopsMarsh, 1887

“…The corollary of grouping muscles by function, therefore, is that it may be necessary to model a single muscle as two separate functional units, particularly when osteological evidence suggests the presence of two When subdividing fan-shaped muscles, the criteria for selecting the number and location of multiple lines of action are not always made clear, and may be related to position ('superficial' vs. 'deep'), perceived function, or selected in order to capture differing fiber directions within the muscle. The practice of partitioning muscles with large attachment sites is common in human biomechanical modelling (Delp et al, 1990;van der Helm and Veenbaas, 1991;Holzbaur et al, 2005;Chadwick et al, 2009;Arnold et al, 2010;Webb et al, 2012) and is increasingly applied to other extant taxa, particularly in MBDA of skull function (Wroe et al, 2007(Wroe et al, , 2013Gröning et al, 2013;Watson et al, 2014). Modern studies benefit from the incorporation of dissection data and MRI-based imaging of in-situ 3D muscle geometry when subdividing muscles for this purpose.…”

“…However, this only provides a first approximation of the wide range of possible scapula motions, and again, we know of no paleontological musculoskeletal model in which translation/rotation of the scapula is included. Similarly, biomechanical models of feeding in fossil taxa often assume a simplified hinge for the jaw joint (e.g., Bates and Falkingham, 2012;Wroe et al, 2013;Lautenschlager et al, 2015; but see Snively et al, 2015) despite some degree of translation and/or long-axis rotation being present in both mammal (Noble, 1973;Terhune et al, 2011) and reptile (Jones et al, 2012) jaws.…”

Extinction of South American sparassodontans (Metatheria): environmental fluctuations or complex ecological processes?

“…This metatherian was also one of the last sparassodontans to become extinct, surviving until the end of the Pliocene (Wroe et al . ).…”

Moa diet fits the bill: virtual reconstruction incorporating mummified remains and prediction of biomechanical performance in avian giants

“…Prediction of muscle forces was scaled according to body mass following a two-thirds power relationship, whereby muscle force is proportional to cross-sectional area while body mass is proportional to volume [56]. All else being equal, larger animals have relatively less available muscle force in proportion to body mass compared with smaller animals [57].…”