Paleontologists struggle to classify some fossils, including the Vendozoa and Cloudina from the Ediacaran period, and Anomalocaris from the Cambrian period. The Ediacaran biota is particularly mysterious, with conflicting interpretations of its organisms’ affinity. Many organisms become easier to classify after the Cambrian period.
Sometimes, paleontologists dig up hard-to-classify fossils. Some fossils are so cryptic that there’s not even any agreement as to which animal phylum they belong to. Some, such as the Vendozoa, mysterious quilted “mattresses” and bags from the Ediacaran period, about 600 to 542 million years ago, have been tentatively assigned their own phyla. Others remain misplaced, with conflicting interpretations of their affinity. A fossil whose classification is unknown is referred to as incertae sediis, Latin for “uncertain location.”
Hard-to-classify fossil organisms are most numerous from the dawn of multicellular life, during the Edicaran and Cambrian periods. Multicellular life emerged in the Ediacaran about 600 million years ago. The fossils of this era are so primitive that it’s hard to tell what they are. One of these fossil organisms is Cloudina, one of the first biomineralizing organisms, of which only a calcareous tube remains. Various fossil organism experts call it a primitive sponge, a coral-like cnidarian (relative of jellyfish), or even an early segmented worm.
The fossil organisms of the Ediacaran biota are among the most mysterious, because there are so many of them and there is little consensus on the classification of any but a few. The Ediacara biota comprised the earliest known multicellular ecosystems and, as stated earlier, consisted of various fluid-filled sacs, fronds, and mattresses with a distinctive tufted pattern. An iconic member of the Edicaran biota, Charnia somewhat resembles a modern cnidarian, the marine pen, leading some scientists to classify the Ediacaran biota as stinging-cell-free cnidarians, but other researchers call them lichens, fungi, giant protists, basal metazoans, bacterial colonies and more. The question is still unresolved.
Other fossil organisms that have been difficult to classify come from the Cambrian period, which occurred immediately after the Ediacaran. One of the most famous is the predatory crayfish-like creature Anomalocaris, the constituent pieces of which were once thought to be three distinct organisms. One of its detached “arms” was initially thought to belong to a crustacean, its mouth, shaped like a slice of pineapple, was mistaken for a jellyfish, and its body was classified as a sponge. Only in 1985 did scientists realize that these were parts of a predatory proto-arthropod, the largest known Cambrian predator up to a meter long.
Other fossil organisms that have proved difficult to classify include the Cambrian Wiwaxia, Opabina, Odontogriphus, and many others. After the Cambrian, many organisms settle into well-established phyla and become easier to classify.
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