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Researchers study owl fossils by looking at the shapes of their bones, including their tarsometatarsus, Smith said in an interview with LiveScience.

A mammalian convergence on the avian tarsometatarsus.

In penguins, the tarsometatarsi is very distinct; it is extremely short and broad, as if someone took a heron’s tarsometatarsus, cut out the long bit in the middle and glued the top and bottom together again.

But even within penguins, the shape of the tarsometatarsus varies.

This shows that the typical penguin tarsometatarsus morphology evolved more rapidly than we thought.