August 01, 2011

Ask the Burke: Bird or Dinosaur?

This Ask the Burke is based on a question from Adena Mooers, a reader from Bellingham. Her question was:

“When a three toed footprint fossil is found in sandstone in Washington, how can you tell if it is a dinosaur/lizard or a large bird? We have recently had Diatryma footprints found in Whatcom County. They look very similar to the dinosaur footprint we saw at the Burke last Friday.”

Burke Museum Paleontology experts answered:

Great question. If you find a fossil footprint and want to determine whether it was a bird or a dinosaur, here are a few tips:

We all know what a typical bird’s foot looks like. Most birds have four toes. And in most species, the first toe (digit I) points backwards. The second, third and fourth toes are counted from the inside of the foot (digits II, III and IV). Chicken feet are probably the most familiar example.

Birds and dinosaurs are closely related, so it may be difficult to them apart. The currently recognized distinction is that “dinosaurs” include two groups: the birds, or avian dinosaurs, and the “non-avian dinosaurs.” But, many non-avian dinosaurs have feet that are very similar to typical bird feet.

And we have all seen bird tracks. How do bird tracks differ from non-avian dinosaur tracks?

For most birds, flying is the principal means of locomotion. So bird feet have long and slender toes that consist mostly of bone, tendon and scaly skin. For the non-avian dinosaurs, their mode of travel is by foot and their feet are relatively more robust. Comparing animals of the same size, the non-avian dinosaur foot will be more heavily built.

Fossilized heron tracks from approximately 50 million years ago, on display in the
Burke Museum’s Life and Times of Washington State exhibit.

Bird footprints are characterized by: slender digit impressions, impressions of three front digits (digits II, III and IV) and a fourth posterior digit (digit I), the front digit impressions (digits II, III and IV) converge, and lastly, there is usually a wide angle between digits II and IV.

In most cases, bird footprints are quite distinctive. However, as you can tell, determining a species from a fossilized track is an exercise in deductive reasoning.

The Burke Museum partners with the Seattle PI's Big Blog to answer commonly asked questions about the natural and cultural history of our region. This post originally appeared on the Big Blog on July 27, 2011.

Got a question for next time? Send it here!