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One of the simplest known seems to be: en.wikipedia.org/wiki/Trichoplax
www.u-tokyo.ac.jp/focus/en/articles/a_00220.html "The simplest multicellular organism unveiled" from 2013 mentions Tetrabaena socialis.
Then of course: Caenorhabditis elegans is a relatively simple and widely studied model organism.
Video 3. Nicole King (UC Berkeley, HHMI) 1: The origin of animal multicellularity by iBiology (2015) Source.

Ancestors