[Iwonda (OP)]

What is the oldest mummified lifeform that has been found and are they only found in amber?

[evan_au]

I agree that there are spectacular fossils embedded in amber. With the introduction of CAT scanning, even more are being revealed in samples of opaque amber.

Limestone is a mummified creature - the original coral has been preserved (but the normally-invisible organic polyps are lost).

The oldest, at 1 billion years are probably stromatolites, where you can see the layers of silt built up by cyanobacteria into a dome. The normally-invisible organic algae are no longer present.
See: https://en.wikipedia.org/wiki/Stromatolite#Fossil_record

But if you are looking for something that retains the organic content, coal is a mummified lifeform: the original carbon is retained, as are remains of some of the chemicals that made up the original plants. A lot of coal deposits date from the Carboniferous era, around 300 million years ago (before land animals came along to eat the plants).
See: https://en.wikipedia.org/wiki/Carboniferous

You may need to define "mummified" more closely.

[Iwonda (OP)]

I think your reply very much covered what I was looking for. I find the part about coal very interesting.