Osteological Correlates of Fascicle Length and Gape in Carnivora

Abstract

Previously, our lab has studied the scaling and dietary correlates of masticatory muscle fiber architecture separately in felids, musteloids, canids, and ursids as well as across the carnivoran order as a whole. In some of these lineages, fascicle lengths (FLs; a correlate of gape) scale with relative diet size, while across the order and divisions thereof, correlates of bite force (muscle mass and physiological cross‐sectional area; PCSA) predominantly scale with body size. In a subsequent recent analysis of 40 taxa spanning eight families from this broader sample, we found that volumetric estimators (e.g., the commonly used “dry‐skull” method described by Thomason) can be used to closely estimate muscle mass and PCSA from osteological remains alone. In the current investigation, we are using the same sample to evaluate osteological correlates of dissection‐based FL measurements. In a preliminary analysis of 14 musteloid taxa representing mustelids, mephitids, and procyonids, chosen because musteloid FL correlated best with diet size, we measured 13 origin to insertion distances to compare to the FLs of the masticatory adductors of these taxa. Raw individual and combined osteological distances were then regressed against FL for each individual muscle as well as average adductor FL using RMA regression. Residuals of same variables relative to body mass and cranial size were also compared. The most highly correlated raw osteological measurement for each muscle resulted in r2 values of 0.92‐0.65 for the best correlate of each adductor individually and their combination. However, correlations dropped substantially (r2=0.69‐0.29 for temporalis, masseter, and all adductors combined) when body size was accounted for, and the medial pterygoid osteological FL proxies accounted for almost none of the variation in measured FL (r2=0.08 for its highest proxy). Although the temporalis is the largest mandibular adductor, the best osteological proxies for relative FL were found for the masseter muscles (r2=0.69 vs. r2=0.29 for temporalis) – perhaps because the temporalis origin is affected by the neuro‐protective and sensory functions of the cranium, while the origin to insertion region of the masseter is almost entirely influenced by masticatory function. Combining these findings with measures of jaw leverage, it is possible to reconstruct not only FL in fossil musteloids but also gape – a method that may give new insight into relative diet sizes consumed by these taxa. Expanding our methods across the other members of the order will allow for the theoretical ex vivoapproximation of gape in modern taxa for which in vivo data have not been systematically collected and also myological approaches to gape estimation in extinct taxa like Smilodon and other sabertooths.