Erik Brians found a fungal-type growth under some garden plants in Sonora. He brought a sample back.
I removed one of the tubes, and cut it open, as you can see in a series of images below. It was tough and fibrous.
I passed the photos along to Dr. David William Fischer, Mycologist, at American Mushrooms®. He believes it is a type of bird’s nest mushroom, but long dead. You can see photos of living specimens here. Thanks Dr. Fischer!