Senter, P. 2009. Pedal function in deinonychosaurs (Dinosauria: Theropoda):
a comparative study. Bulletin of the Gunma Museum of Natural History
13:1-14.

ABSTRACT: Members of the Cretaceous theropod clade Deinonychosauria have a
highly modified second toe with an enlarged, recurved ungual. Here I present
the first comparative study of pedal function in deinonychosaurs and other
theropods to test hypotheses of function for this toe. I manually
manipulated bones and casts to determine pedal range of motion in the
non-deinonychosaurian theropods Dilophosaurus, Allosaurus, Mononykus, and
Chirostenotes; and the deinonychosaurs Troodon, Neuquenraptor, Rahonavis,
Bambiraptor, Deinonychus, and Dromaeosaurus. I also used movies of walking
bird feet for comparison. The results of the study show that: (1) contrary
to previous suggestions, the distodorsal eminence of pedal phalanx II-1 of
deinonychosaurs is not correlated with greater hyperextensibility of phalanx
II-2 than in other theropods; (2) the proximoplantar process of phalanx II-2
of deinonychosaurs limits flexion, as previously suggested (3) movement of
the second toe is sub-orthal in dromaeosaurids, the toe remains adducted
(angled toward the animal's midline) through hyperextension and flexion in
Troodon, and in other theropods the second and fourth toes diverge during
hyperextension and converge during flexion; (4) pedal phalanx I-1 is
immobile in Chirostenotes and the examined deinonychosaurs but has a wide
range of motion in other theropods; (5) contrary to a previous suggestion,
the hallux of Rahonavis was not retroverted; (6) range of motion in the
second toe is consistent with opening tough insect nests in dromaeosaurids
but not Troodon; (7) the deinonychosaurian second toe was hyperextended, not
flexed, during locomotion.