Skeletal element that forms superficially in the organism, usually in association with the ectoderm[VSAO]. [ http://purl.obolibrary.org/obo/go/references/0000034 http://dx.plos.org/10.1371/journal.pone.0051070 https://github.com/obophenotype/uberon/issues/267 VSAO:0000130 ]