The morphological basis of hallucal orientation in extant birds
Abstract
The perching foot of living birds is commonly characterized by a reversed or opposable digit I (hallux). Primitively, the hallux of nonavian theropod dinosaurs was unreversed and lay parallel to digits II-IV. Among basal birds, a unique digital innovation evolved in which the hallux opposes digits II-IV. This digital configuration is critical for grasping and perching. I studied skeletons of modern birds with a range of hallucal designs, from unreversed (anteromedially directed) to fully reversed (posteriorly directed). Two primary correlates of hallucal orientation were revealed. First, the fossa into which metatarsal I articulates is oriented slightly more posteriorly on the tarsometatarsus, rotating the digit as a unit. Second, metatarsal I exhibits a distinctive torsion of its distal shaft relative to its proximal articulation with the tarsometatarsus, reorienting the distal condyles and phalanges of digit I. Herein, I present a method that facilitates the re-evaluation of hallucal orientation in fossil avians based on morphology alone. This method also avoids potential misinterpretations of hallucal orientation in fossil birds that could result from preserved appearance alone.
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This would be particularly acute in non-avian theropod like Microraptor, given the lack of a reversed hallux to aid in securing a purchase.
“This would be particularly acute in non-avian theropod like Microraptor, given the lack of a reversed hallux to aid in securing a purchase [19], [20].”
From Assessing Arboreal Adaptations of Bird Antecedents: Testing the Ecological Setting of the Origin of the Avian Flight Stroke (2011) · cites this paper as [19]
A reversed hallux is crucial for arboreal locomotion in extant birds because they cannot rotate their ankles.
“and is crucial for arboreal locomotion in extant birds [19], [25] because they cannot rotate their ankles.”
From Assessing Arboreal Adaptations of Bird Antecedents: Testing the Ecological Setting of the Origin of the Avian Flight Stroke (2011) · cites this paper as [19]
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- Authors
- Middleton, K. M.
- Affiliations
- Brown University (US)
- Corresponding author
- Middleton, Kevin M.
- ORCID
- Middleton, Kevin M.
- Venue
- Journal of Morphology
- Publisher
- Wiley
- Published
- 2001-09-21
- Volume
- 250
- Issue
- 1
- Pages
- 51-60
- ISSN
- 0362-2525 (ISSN-L)
- 1097-4687
- OpenAlex
- W2032929085
- Cited by
- 43
- References
- 40
- Citations by year
- 2026: 2 · 2025: 1 · 2024: 1 · 2023: 7 · 2021: 1 · 2020: 2 · 2018: 4 · 2017: 3 · 2016: 3 · 2015: 4 · 2014: 1 · 2013: 2
- FWCI
- 0.34
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Journal of Morphology (publishedVersion)
- PubMed (publishedVersion)
- Topics
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Evolutionary Biology
- Health Sciences › Medicine › Urology › Comparative Animal Anatomy Studies
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Evolution and Paleontology Studies
- Keywords
- tarsometatarsus, grasping, fossil birds, theropod dinosaurs, digital innovation
- MeSH terms
- Animals, Birds, Foot, Fossils, Metatarsal Bones, Posture, Tarsus, Animal, Hand Strength
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