The answer to that question has been that it is not a trunk, it is a fish that the bear has caught and is holding in its mouth. That would certainly explain it, but what is a fishing bear doing in the desert outside of Moab, Utah? Alaska yes, British Columbia yes, even the Pacific Northwest, but in Moab, Utah? In their February, 2006, issue, National Geographic magazine included a photograph by Steve Winter showing an Alaskan brown bear with a fish in its mouth. When I saw this photograph the plausibility of the explanation of the petroglyph being a bear with a fish in its mouth had been strengthened.
The real question is what kind of fish in the Colorado River would be that big? One possible candidate is the endangered Colorado Pikeminnow (Ptychocheilus lucius, formerly known as the squawfish). Reports of individuals of this species have (according to Wikipedia) ranged up to 6 feet (1.8 meters) long and weighing over 100 pounds (45 kilograms).
So let’s give credit where credit is due – while we may not have a petroglyph of an extinct mastodon, we just might have a petroglyph of an endangered fish.