I actually just attended a colloquium presentation on a topic related to this. The earliest fossils of multicellular animals were Rangeomorphs
. These were fractal organisms, to 4 fractal iterations:
A collection of populations of these were preserved in a very fine-mud turbidite nearly 600 million years ago, in rocks that are now a part of Newfoundland here in Canada (the turbidite was buried shortly afterward by directly-dateable volcanic ash):
They were buried in-situ
, preserving excellent detail and allowing scientists to see where members of these species were placed in relation to each other. This information allows us to get an idea of what that pre-Cambrian ecology must have looked like:
These organisms would have been absorbtion and/or suspension feeders, much like today's sponges and corals but without the same degree of tissue or structural specialization, and certainly no vascular system
. Their fractal shape allowed their cells to remain sessile
while maximizing their cellular colony's surface area.
Taxonomically, they fall somewhere between fungi and sponges, and are the earliest examples of animal life ever discovered. They are also an evolutionary dead-end
, as no subsequent organisms show any evidence of descent from these ones. But they're still the first multicellular life ever found, and the benefits of multicellularity are evident from what we've learned about their structure and mode of life.