Shake a tail feather: the evolution of the theropod tail into a stiff aerodynamic surface

PLoS One. 2013 May 15;8(5):e63115. doi: 10.1371/journal.pone.0063115. Print 2013.


Theropod dinosaurs show striking morphological and functional tail variation; e.g., a long, robust, basal theropod tail used for counterbalance, or a short, modern avian tail used as an aerodynamic surface. We used a quantitative morphological and functional analysis to reconstruct intervertebral joint stiffness in the tail along the theropod lineage to extant birds. This provides new details of the tail's morphological transformation, and for the first time quantitatively evaluates its biomechanical consequences. We observe that both dorsoventral and lateral joint stiffness decreased along the non-avian theropod lineage (between nodes Theropoda and Paraves). Our results show how the tail structure of non-avian theropods was mechanically appropriate for holding itself up against gravity and maintaining passive balance. However, as dorsoventral and lateral joint stiffness decreased, the tail may have become more effective for dynamically maintaining balance. This supports our hypothesis of a reduction of dorsoventral and lateral joint stiffness in shorter tails. Along the avian theropod lineage (Avialae to crown group birds), dorsoventral and lateral joint stiffness increased overall, which appears to contradict our null expectation. We infer that this departure in joint stiffness is specific to the tail's aerodynamic role and the functional constraints imposed by it. Increased dorsoventral and lateral joint stiffness may have facilitated a gradually improved capacity to lift, depress, and swing the tail. The associated morphological changes should have resulted in a tail capable of producing larger muscular forces to utilise larger lift forces in flight. Improved joint mobility in neornithine birds potentially permitted an increase in the range of lift force vector orientations, which might have improved flight proficiency and manoeuvrability. The tail morphology of modern birds with tail fanning capabilities originated in early ornithuromorph birds. Hence, these capabilities should have been present in the early Cretaceous, with incipient tail-fanning capacity in the earliest pygostylian birds.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Biological Evolution*
  • Dinosaurs / anatomy & histology*
  • Dinosaurs / classification
  • Feathers / anatomy & histology*
  • Phylogeny
  • Principal Component Analysis

Grants and funding

The authors are grateful for support from the University College London Graduate School Research Projects Fund and Student Conference Fund, University College London Earth Sciences, The Doris O. and Samuel P. Welles Research Fund, The Jurassic Foundation, The Gloyne Outdoor Geological Research Fund of the Geological Society of London and The Jackson School of Geosciences Student Member Travel Grant. Additional support was provided by the Department of Veterinary Basic Sciences (The Royal Veterinary College) and Natural Environment Research Council grant NE/G005877/1 to JRH, awarded in 2009. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.