[Iwonda (OP)]

The fossil record shows that in the past complex lifeforms for some reason grew to be much larger than what we see today. When we think of microscopic life today we think of single cell organisms such as microbes. Is there anything from the fossil record or any studies that suggest that complex lifeforms in the past were also abundant at the microscopic level?

[evan_au]

Some microscopic lifeforms produce macroscopic results:
- The white cliffs of Dover are hundreds of meters thick, and are formed from the skeletons of microscopic plankton.
- The red iron oxide deposits of Western Australia were formed when oxygen was being produced by microscopic plankton.
- Marine organisms sometimes capture layers of dirt and sand in their biofilms, and build up a recognisable mound, a stromatolite. It is thought that some examples may be over 3 billion years old.
See: https://en.wikipedia.org/wiki/Stromatolite#Fossil_record.

[Iwonda (OP)]

Hmmm, I guess that microscopic marine organisms would come close to what I'm asking about but what I really would like to know is, were there any microscopic plants, insect or even animals in the past?

[chiralSPO]

There were microscopic plants and animals in the past, and there still are plenty today!
Plants:
https://en.wikipedia.org/wiki/Green_algae

Animals:
https://en.wikipedia.org/wiki/Nematode
https://en.wikipedia.org/wiki/Tardigrade
https://en.wikipedia.org/wiki/Demodex

There are whole ecosystems comprised of microscopic critters, with predators and scavengers and grazers!

[Iwonda (OP)]

Well, it would appear that complex lifeforms did indeed conquer both ends of the size spectrum, both jumbo and micro. When you  think of micro lifeforms the first thing that comes to mind is microbes or single cell so it's very amazing to find out that there are also many complex micro lifeforms.
Cool, thanks...

[chiralSPO]

You're welcome!