Observed this object today (early October) in Sabino Canyon, on the north side of Tucson AZ, in a riparian area. It's attached to a mesquite tree (Prosopis spp). It's attached to one side of the branch, it does not wrap around the branch. About 1.5 - 2 cm along the maximum dimension.
Looks like some kind of insect egg case and I'm curious if anyone can confirm and knows the insect.