The first complex multicellular animals appeared in the fossil record around 590-600 million years ago, with simple cup-shaped fossils found in Canada and China. The Cloudina fossil, made up of calcite cones, is another example of an ancient complex animal, with uncertain classification. The earliest true fossils of complex animals are around 575 million years old, with Charnia wardi being the most ancient, but its classification remains uncertain.
The first multicellular complex animals of any kind appear in the fossil record about 600 to 590 million years ago, particularly in the Twitya Formation in the Mackenzie Mountains of Canada. These are simple cup-shaped animals that appear in fossil form as simple disc and ring imprints. They are thought to correspond to simple cnidarians (relatives of jellyfish) or sponges. Because they are so simple, these fossils provide little information about the animals that created them. However, they are still considered “complex” due to their very large size (about an inch in diameter) compared to the simpler microscopic single-celled fossils before and next to them.
The next oldest complex animals preserved as fossils appear in the Doushantuo Formation in south-central China. A wide variety of fossil embryos are preserved, as well as the earliest known bilateral animal, the 0.1 mm (about 590-600 mya) Vernanimacula, the “little vernal animal”, a tiny ball-shaped animal with what appears to be a fossil gut, as well as surface pits that may be sensory structures. Small fossil embryos of complex animals, such as cnidarians, are preserved with an unprecedented level of detail in the Doushantuo Formation, providing paleontologists with important information about the earliest known embryos.
Another ancient fossil among the complex animals is the enigmatic Cloudina, a segmented fossil made up of calcite cones layered on top of each other. Measured in millimeter increments, these fossils are among the earliest shells known in the fossil record. They show buds, which suggests that they reproduced asexually, as well as boring predators, which shows that there were predators even at the dawn of known multicellular life. Like many early fossils, there is a lack of uncertainty as to exactly what Cloudina was: current opinion is divided between the idea that the specimen is a stem-group annelid and that any classification, even at the phylum level, she is not wise. They could be archaeocyathids, ancient sponges that built reefs hundreds of millions of years before coral evolved.
The earliest true fossils of “complex animals” (larger than 1 mm, not cnidarians or corals) are about 575 million years old and are found in the Mistaken Point assemblage in Newfoundland, Canada. The most ancient of all is the Charnia wardi, a frond-shaped organism with alternating right and left crests. Because these ridges are not fully symmetrical about the central axis, Charnia is not a true bilateral organism and can only be described as semi-bilateral. Initially described as an early relative of sea pens, scientists now simply have no idea how to classify Charnia.
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