unknown shrub with spines in Pennsylvania
This shrub completely stumped two professors of ecology and several officers in the Philadelphia Botanical Club, but I bet someone here will be able to identify it.
Besides the photo below, I have posted larger photos.
The shrub was growing wild on an island in a river that runs through Philadelphia. It's not something I've ever seen in the wild before; it may be a garden escape.
The shrub was around 6' tall. As you can see in the photo, it has sharp spines, but is otherwise rather nondescript. It didn't have any flowers or seeds to help with identification.