Mystery, Solved!
We have a winner! Thanks to LilyB, who recommended the Wildflowers and Trees of Western Pennsylvania website, we have determined the common identity of the yellow flowers!
Our answer comes from the gracious Bob Zuberbuhler, author of Wildflowers and Trees of Western Pennsylvania. Here is what Bob had to say:
“I think the flowers you photographed are Lesser celandines, Ranunculus ficaria, in the buttercup family. I've never found one in Western PA, but understand they are more common in the eastern part of the state. The 9 petals are consistent with a buttercup, and the leaves look a lot like those of the Lesser celandine.
Please don't hesitate to send on any further unknowns; I enjoy trying to figure them out.
Hope you enjoy PA.
Bob”
Thanks for all your help Bob, and thank you to all the Arborealists who offered suggestions, thoughts, and other ideas! I appreciate everyone’s input with our ongoing discoveries.