A worldwide community photographing and learning about wildlife
Montagnea arenaria
Desert Inkcap fungi sprouting up from a dry area in the Rio Grande riverbed, spotted in Big Bend National Park, TX.
I'm not at all good with mushroom ID, however from the location and the black crusty edge when it is dry this looks like the most likely candidate. On the reference page in the suggestion click on the Desert Inkcap for info and pics.