Hop, hop and away: On the take-off leap of Archaeopteryx

Abstract

Where take-off is a crucial phase of flight requiring an energy-intensive motion to accelerate into the air, how Archaeopteryx, the first bird, took to the air is a subject of great debate. Although it is known that extant land birds take off by leaping, generating the initial take-off velocity primarily from the hindlimbs, the biomechanics of such leaps remain largely unknown. Understanding the detailed musculoskeletal mechanics associated with an extant avian jumping take-off could provide key insight into the evolution of avian flight. As a basis for further analyses, this thesis first developed a computational biomechanical model of a passerine bird (magpie, Pica pica), a representative of a class of birds that includes over half of all extant bird species, to quantify the functional hindlimb anatomy in leaping birds. Comprehensive analyses considering key sources of uncertainty provided robust estimates for the moment-generating capacity of its pelvic muscles and demonstrated substantial capability for internal/external rotation as well as flexion/extension, revealing that avian hip muscle function is not limited to the sagittal plane. Informed by these new insights, a computational musculoskeletal model of the zebra finch, Taeniopygia guttata, hindlimb was developed and driven with previously published take-off ground reaction forces and 3D kinematics. This first biomechanical model to study the internal biomechanics associated with a take-off leap used an inverse dynamics approach to calculate the external moments at the ankle, knee, and hip joints and contrasted these to the cumulative capacity of the hindlimb muscles to balance these moments across a range of take-off conditions. We report substantial external moments at the hip and ankle joints, reaching magnitudes of about two times values previously reported during the running of a flightless bird. Having confirmed the capability of the computer model to determine the hind limb biomechanics during a successful take-off leap in an extant bird, this thesis proceeded to test the leaping ability of Archaeopteryx to become airborne. By carefully adapting a published model of Archaeopteryx to reflect the novel understanding of avian hindlimb kinematics and kinetics developed here, we confirm the capability of Archeopteryx to leap and determine robust estimates of the maximum take-off velocity powered by their hind limbs. Using a conservative approach to integrate contributions of hindlimbs and wings we then show that Archaeopteryx, taking successive leaps like a living bird, could use its hindlimbs to generate sufficient velocity to reach the minimum sustainable flight speed within two to three hops. The state-of-the-art biomechanical analyses developed in this thesis thus provide new quantitative evidence in support of a ground-up leaping mechanism for the evolution of avian flight and offer a methodological framework for rigorous biomechanical hypothesis testing to expand our understanding of the evolution of avian flight.

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Identifiers

ORCID for Erik Anthony Meilak: orcid.org/0000-0003-4806-9448

ORCID for Markus Heller: orcid.org/0000-0002-7879-1135

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