Reevaluating Maniraptoran Pelvic Musculature: Cursoriality, Convergence, and Caudal Decoupling

• Author / Creator

  Rhodes, Matthew M

• Maniraptora is a clade of non-avian theropod dinosaurs comprising avians and several “bird-like” groups with semilunate wrist bones. Maniraptorans tend to exhibit anterior migration in centre of mass, enlargement of the pectoral region, and progressive decrease in tail length and bulk. This culminates in birds with a habitually crouched posture, development of wings, and radical reorganization of leg and tail musculature that caused profound changes in locomotion. However, the evolution of these archetypal “avian” features is not entirely understood. Thus, a desire exists to characterize how and when these adaptations were acquired, and the resultant palaeobiological effects. Did these changes affect running ability (cursoriality)? Were similar adaptations gained independently (convergence)? Did disentanglement of tail from hip musculature (caudal decoupling) also proceed in a stepwise fashion, mirroring reduction of the tail as a whole? Previous studies addressing these questions have tended to focus strictly on osteology. However, soft tissue reconstruction affords a new perspective not possible using only skeletal. Furthermore, does soft tissue evidence corroborate adaptations inferred from osteology?
  Relatively recent fossil discoveries of caenagnathids, dromaeosaurids, therizinosaurians, and troodontids permit reconstruction of maniraptoran locomotory musculature to infer adaptations and compare to skeletal evidence. Major locomotory muscles moor to the pelvis, which is also a junction between the leg and the tail. Direct observation of fossil pelvic material for osteological correlates of muscle attachments was done with reference to the closest living relatives to infer the presence or absence of major locomotory muscles. Extant conditions were determined by dissection and supplemented by literature review on soft tissue morphology and homology. Relative sizes of muscles and muscle groups allowed qualitative comparison within and between theropod taxa.
  Skeletal and myological inferences are variably consistent. In the therizinosaurian Falcarius, muscular reconstruction reveals reduction in major leg extensors. This is consistent with therizinosaurian evolution but suggests that basal maniraptorans had a punctuated onset of caudal decoupling. In Caenagnathidae, moderate reduction of major leg extensors contradicts alleged high cursoriality inferred from limb proportions and metatarsus structure, but corroborates gradual, progressive caudal decoupling preceding the evolution of birds. Troodontid muscular reconstruction instead upholds postulated high cursoriality in derived members, an adaptation gained secondarily and predominantly owing to intrinsic hip muscles. In Dromaeosauridae, the microraptorine lateral pubic tubercle is considered homologous with other osteological correlates of the pubogastralial ligament. The relationship between the tubercle and the pubic apron, to which locomotory muscles attach, may explain why a pronounced tubercle or process is unique to Microraptorinae. Reconstruction of maniraptoran pelvic musculature reveals a complex relationship with skeletal data. However, this facilitates a reevaluation of locomotory adaptations such as cursoriality, convergence, and caudal decoupling in non-avian maniraptorans.
  

• Subjects / Keywords

  • caudal decoupling
  • myology
  • pelvis
  • locomotion
  • cursoriality
  • Maniraptora

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-c1fh-nq03

• License

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