 Morphological similarities between trilobites and earlier arthropod-like creatures such as "Spriggina", "Parvancorina", and other "trilobitomorphs" of the Ediacaran period of the Precambrian are ambiguous enough to make a detailed analysis of their ancestry complex.Morphological similarities between early trilobites and other Cambrian arthropods (e.g.the Burgess shale fauna and the Maotianshan shales fauna) make analysis of ancestral relationships difficult as well.