Abstract
The musculoskeletal configuration of the mammalian pectoral limb has been heralded as a key anatomical feature leading to the adaptive radiation of mammals, but limb function in the non-mammaliaform cynodont outgroup remains unresolved. Conflicting reconstructions of abducted and adducted posture are based on mutually incompatible interpretations of ambiguous osteology. We reconstruct the pectoral limb of the Triassic non-mammaliaform cynodont Massetognathus pascuali in three dimensions, by combining skeletal morphology from micro-computed tomography with muscle anatomy from an extended extant phylogenetic bracket. Conservative tests of maximum range of motion suggest a degree of girdle mobility, as well as substantial freedom at the shoulder and the elbow joints. The glenoid fossa supports a neutral pose in which the distal end of the humerus points 45° posterolaterally from the body wall, intermediate between classically 'sprawling' and 'parasagittal' limb postures. Massetognathus pascuali is reconstructed as having a near-mammalian complement of shoulder muscles, including an incipient rotator cuff (m.subscapularis, m.infraspinatus, m.supraspinatus, and m.teres minor). Based on close inspection of the morphology of the glenoid fossa, we hypothesize a posture-driven scenario for the evolution of the therian ball-and-socket shoulder joint. The musculoskeletal reconstruction presented here provides the anatomical scaffolding for more detailed examination of locomotor evolution in the precursors to mammals.
Highlights
Today’s mammals inhabit disparate ecological niches, comprising cursorial, fossorial, aquatic, and even volant forms (Vaughan et al 2013; Hildebrand, 1989; Fischer et al 2002)
The osteology of the cynodont pectoral girdle and forelimb is well known from the fossil record, and does not appear to have been disparate; in a series of papers, Jenkins (1970a, 1971a) reconstructions of the cynodont pectoral limb have been advanced, drawing on skeletal morphology (Watson, 1917; Jenkins, 1970b, 1971a; Kemp, 1980a, 1980b; Oliveira & Schultz, 2016) as well as muscle anatomy as inferred from osteology and homology to extant taxa (Gregory & Camp, 1918; Romer, 1922)
As we were unable to locate all of the fragments of the interclavicle, we modeled missing segments based on the preserved cranial portion, other interclavicles in the MCZ collections, and Jenkins’ (1970b, 1971a) description of the same element in other cynodonts
Summary
Today’s mammals inhabit disparate ecological niches, comprising cursorial, fossorial, aquatic, and even volant forms (Vaughan et al 2013; Hildebrand, 1989; Fischer et al 2002) These varied lifestyles are supported by modifications of the pectoral limb into anatomical structures as diverse as wings and flippers. The evolution of the mammalian-style pectoral limb—mobile scapula, ball-and-socket glenohumeral joint, “parasagittal” limb posture—has been suggested as a key innovation leading to the adaptive radiation of the clade (Polly, 2007). Morphological diversification of this anatomical module began early on in mammalian evolution (Meng et al 2006; Ji et al 2006), predating the emergence of the crown group (Luo, 2007). Multiple reconstructions of the cynodont pectoral limb have been advanced, drawing on skeletal morphology (Watson, 1917; Jenkins, 1970b, 1971a; Kemp, 1980a, 1980b; Oliveira & Schultz, 2016) as well as muscle anatomy as inferred from osteology and homology to extant taxa (Gregory & Camp, 1918; Romer, 1922)
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