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The first multicellular complex animals of any sort appear in the fossil record about 600 to 590 million years ago, specifically 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 impressions. They are thought to correspond to simple cnidarians (relatives of jellyfish) or sponges. Because they are so simple, these fossils confer little information about the animals that made them. However, they are still considered "complex" because of their very large size (about an inch in diameter) compared to the simpler microscopic, unicellular fossils from before and alongside 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 Vernanimacula (about 590 to 600 mya), the "small springtime animal," a tiny sphere-shaped animal with what appears to be a fossilized gut, as well as surface pits that may be sensory structures. Tiny fossil embryos of complex animals, such as cnidarians, are preserved to an unparalleled degree of detail in the Doushantuo Formation, giving paleontologists important insight into the earliest embryos known.
Another early fossil among complex animals is the enigmatic Cloudina, a segmented fossil consisting of calcite cones layered on each other. Measured in increments of millimeters, these fossils are among the first known shells in the fossil record. They display budding, which suggests they reproduced asexually, as well as predatory boring, which shows that there were predators even at the very dawn of known multicellular life. Like many of the earliest fossils, there is a lack of uncertainty as to what exactly 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, is unwise. They may be archaeocyathids, ancient sponges that built reefs hundreds of millions of years before the evolution of coral.
The first truly "complex animals" (larger than 1 mm, not cnidarians or corals) fossils are about 575 million years old, and are found at the Mistaken Point assemblage in Newfoundland, Canada. The absolute oldest is Charnia wardi, a frond-shaped organism with left-and-right alternating ridges. Because these ridges are not completely symmetrical around the central axis, Charnia is not a true bilateral organism, and can only be described as semi-bilaterial. Initially described as an early relative of sea pens, scientists now simply have no idea how to classify Charnia.
@KoiwiGal - Actually microfossils are pretty common. I'm not sure if this is because a lot of the smaller animals had some kind of hard skeleton to change to rock or not. It does say in the article that some of them seemed to form coral-like groups.
I do know that diatomaceous earth is made up of microscopic fossils of algae. And that stuff is used in all kinds of applications, like in filters and to kill certain kinds of insects. If it was rare to find microscopic fossils (at least of that kind) they wouldn't be able to do that.
It is kind of weird to think you might be using the shells of animals that are extinct in your pool filter though.
@umbra21 - Well, if you've seen fossils of wood, you've likely seen the impressions that the leaves of those trees and plants leave behind surrounding the fossils.
Fossils aren't always an actual bone that's been left, sometimes they are only the impression of a bit of soft tissue that rotted before it could be replaced by stone, but after the ash and stone around it could form into its shape and record it.
This is why scientists have to guess at the composition of some of these fossils, but are fairly certain about others.
In some cases they only have a two dimensional snapshot of what the animal looked like.
It is amazing how much detail they can get from micro fossils though. If you are interested, there are some lovely photos of them online.
I had no idea that they were able to look at fossils of animals that lived so long ago. I mean, I guess I did know in a general kind of way, but I didn't realize that such simple creatures would leave fossils.
I thought it was only the hard parts of the animal which would leave a fossil, like the bones. Most of the fossils I've seen have been of wood or of bone, which was the stone replacing the hard material in an exact shape, preserving it.
I would have thought soft material would rot away too quickly for any of it to be preserved.
And the idea that microscopic soft bodies could be preserved in stone for hundreds of millions of years is absolutely amazing.
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