Acarospora obnubila ("cloudy cobblestone lichen") is a dull brown squamulose areolate crustose lichen that may grow up to 4 mm in diameter or with squamules scattered among other lichens. They are common in Arizona, southern California, and Baja California. They grow by themselves on acidic rock in full sunlight. Squamules measure up to 2 mm in diameter and are round to irregular, with have a stipe. They may also grow on members of the Aspicilia genus (lichenicolous). Competition for space with other lichens stimulates longer stipes to develop, whereby the squamules may overlay other lichens.