The Effect of Cartilage Thickness on Reconstructions of Range of Motion and Muscle Leverage in Extinct Tetrapods

Abstract

Investigating structure‐function relationships in the locomotor system requires an understanding of the complex interactions between bones, cartilage and muscles. This is especially true when examining how these musculoskeletal structures evolved over time and across different environments in both extant and extinct species. We aimed to quantify the effect of different assumptions about cartilage thickness on reconstructions of femur length, hip range of motion (ROM), and muscle leverage in Acanthostega gunnari, one of the earliest known tetrapods and an important species in the vertebrate water‐land transition. We began by examining the hindlimbs of two extant salamanders, Dicamptodon ensatus and Ambystoma mexicanum, using contrast‐enhanced micro‐CT in order to extrapolate the degree to which cartilage thickness might affect femur length in a fossil taxon with salamander‐like morphology. Data visualization software was used to measure the distal and proximal cartilage caps of the femora in each of the extant species. Three‐dimensional modeling software was then used to estimate the center of rotation of the hip joint of Acanthostega. Then, cartilage correction factors were applied based on the measurements from the extant taxa. The limit of ROM was defined as the point at which either the femur contacted the pelvis, or less than 50% of the femoral head remained within the acetabulum. Finally, we used biomechanical modeling software to map muscles onto the bones of Acanthostega and plotted their leverage to examine how assumptions about cartilage affect reconstructions of muscle leverage. Cartilage caps on the proximal end of the femur in Dicamptodon and Ambystoma measured 36% and 14% of femur length, respectively. Taking the most conservative estimate, we modeled Acanthostega with cartilage caps of 0, 7.5 and 13% femur length. The greatest variation in muscle leverage occurred in muscles whose axes of movement were nearly perpendicular to the femur (70–120% of the mean moment arm), as opposed to those muscles whose axes were nearly parallel (less than 10%). We also found that assuming the highest percentage of cartilage increased ROM in protraction and retraction by 15–20° and in elevation and depression by 15–30° but had little to no effect on long‐axis rotation. These results will help to predict the effects of unpreserved soft tissues in future studies that reconstruct ROM and muscle leverage in extinct animals.Effect of different cartilage correction factors on reconstructed hip muscle leverage in the early tetrapod Acanthostega gunnari.The y‐axis shows coefficient of variation of muscle leverage (standard deviation/mean moment arms) across cartilage correction factors of 0, 7.5 and 13% femur length. Moment arms were measured in 3 axes of movement for muscles that cross the hip joint. The greatest effects occurred in muscles with lines of action nearly perpendicular to the femur.Figure 1