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How did the wrist joints of theropod dinosaurs resemble those of birds?

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A zebra finch (Taeniopygia guttata) with a bright orange beak, gray head, orange cheek patch, and black-and-white striped tail perches on a branch. — Bird A brightly colored zebra finch (*Taeniopygia guttata*) perched on a branch.

Small theropod dinosaurs and birds share a unique wrist joint structure that allows the hand to flex sideways rather than up and down. This specific wrist motion is virtually identical to the movement used by birds during flight.1

While this motion is essential for avian flight today, paleontologists believe that in these small theropod dinosaurs, the primary function of this wrist flexibility was likely for catching prey. This anatomical similarity is one of the key pieces of evidence supporting the scientific consensus that birds evolved from small theropod dinosaurs.12