Lichens are common composite organisms that can be found in many diverse areas, including the most inhospitable environments on earth. The approximately 15,000 described species inhabit everything from lush forests to deserts to arctic tundra, exploiting resources wherever possible. Lichens are not a single organism, but rather an association between a particular species of fungi and a particular species of algae or cyanobacteria. They usually form symbiotic relationships, with the fungi providing a safe, moist substrate of hyphae while the algae/cyanobacteria provide a source of food via photosynthesis.
Many different fungi, algae, and cyanobacteria can be combined in numerous ways, resulting in many different associations and growth forms. Once in a while you may look at a lichen and experience pareidolia, the psychological phenomenon where a random object appears to resemble something significant. The lichens in these photos seemed to resemble a human skull. A find made up of pure chance, to be sure, but amusing nonetheless.