The morphological basis of hallucal orientation in extant birds.

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. Copyright 2001 Wiley-Liss, Inc.