Title: Shoulder to shoulder: Musculoskeletal function of the amniote pectoral girdle and the foundations of the mammalian forelimb
Abstract: Before mammals could swing from the trees, return to the oceans, or take to the skies using modified forelimbs, they first had to get up off the ground. Mammals are the surviving branch of a group of vertebrates called synapsids, which split from their common ancestor with lizards, snakes, birds, crocodiles, and turtles more than 300 million years ago. Part of that distant amniote heritage is a wide, low stance frequently described as “sprawling” posture, where the limbs are extended to either side and the elbows are bent as if doing a push-up. This is a far cry from the “upright” posture considered a hallmark of quadrupedal mammals, where the limbs are carried underneath the body, the elbows are tucked back against the ribcage, and the body is supported high off the ground. How, when, and why our synapsid ancestors transitioned from sprawling to upright posture is an enduring question in vertebrate palaeontology. The non-mammalian synapsid fossil record is rich but morphologically ambiguous, and over a century of debate has failed to result in a clear consensus on how to interpret the skeletons of these enigmatic animals.
Lying at the intersection of vertebrate palaeontology, comparative anatomy, and locomotor biomechanics, this dissertation aims to flesh out our picture of the past by considering how muscles and bones work together in animals that are alive today. At its core is an inferential tool known as “extant phylogenetic bracketing”, where data from the living descendants and distant cousins of a fossil animal are used to reconstruct aspects of its biology that are not preserved with the skeleton. Using tegu lizards and Virginia opossums as an extant phylogenetic bracket for extinct synapsids, I apply computational tools and recent imaging advances to study the intricately connected muscles and bones that make up the shoulder joint of the forelimb, in order to parameterize digital models that will provide fresh insight into how posture evolved throughout synapsid evolutionary history. Chapter 1 presents a reconstruction of a fossil non-mammalian cynodont (Massetognathus pascuali), and demonstrates that synapsids had already evolved flexible shoulders and almost a full set of mammalian muscles by the late Triassic (~200-220 MYA). In Chapter 2, I dissect tegus and opossums to reveal that the biomechanical properties of their shoulder muscles have remained largely unchanged since their divergence. Chapter 3 goes a step deeper in investigating integrated shoulder function across this extant phylogenetic bracket, combining three-dimensional X-ray motion capture and machine learning to create a map of joint mobility and muscle leverage in tegu and opossum cadavers. In Chapter 4, I probe the fundamental differences between sprawling and upright shoulder function by experimentally manipulating the posture of living tegus and opossums. By exploring how the motions and forces of locomotion change with increasing uprightness, and placing these shifts in the context of changing muscle leverage, I outline an updated scenario of how morphological transformation of the musculoskeletal system may have underpinned the synapsid sprawling to upright transition, and propose a new lens for looking at very old bones.
Committee: Stephanie Pierce, Co-Advisor and Andy Biewener, Co-Advisor