From the Introduction: "The division of functions betwen cell types is a hallmark of an-
imal multicellularity. 1–3 Specialized cell states result from the
differential co-regulation of functional gene modules, such
as actomyosin contractility, ciliary, or pre-synaptic scaffold
components. 3,4 Through comparative genomics, we have a
detailed understanding of the evolutionary histories of the
constituents of these modules, with many key animal genes
predating multicellularity. 2,5 In contrast, we lack a detailed un-
derstanding of when these genes assembled into co-regulated
modules, how they are deployed in different cell types, and
what their evolutionary dynamics are. The comparative study
of cell diversity and genome regulation in early-branching,
non-bilaterian animals (sponges, cnidarians, ctenophores,
and placozoans) offers the opportunity to address these funda-
mental questions and reconstruct the evolutionary emergence
of major cell type programs.
Placozoans are millimeter-sized, flat animals that employ
ciliary beating and mucus secretion to glide over surfaces. 6–9
These marine animals feed on algae and other microbial eukary-
otes by extracellular lysis of prey, phagocytosis, and intracellular
digestion. 10 Placozoans emerged 750–800 mya "

Uncalibrated molecular dating has inherent large uncertainties, but it's
safe to say that the clade originated in the Ediacaran, if not Neo-proterozoic. Unfortunately, the genome gives no hint of what it "looked like".

An Ediacaran animal, Dickinsonia (see Wiki entry) has the following traits
in common with placozoa: it was mobile and fed by engulfing algae on the sea floor. Dickinsonia has mostly been discussed as a possible stem bilaterian, and is much more complex than placozoans, having a consistent body plan and developement, but simplification of genomes over hundreds of millions of years is plausible. Dickinsonia disappeared before the end of the Ediacaran, probably the victim of greatly increased predation from the "small shelley" biota. It has been suggested as a stem placozoan, not far removed from stem bilaterian.

On Friday, November 3, 2023 at 2:44:00 PM UTC-7, erik simpson wrote:
> From the Introduction: "The division of functions betwen cell types is a hallmark of an-
> imal multicellularity. 1–3 Specialized cell states result from the
> differential co-regulation of functional gene modules, such
> as actomyosin contractility, ciliary, or pre-synaptic scaffold
> components. 3,4 Through comparative genomics, we have a
> detailed understanding of the evolutionary histories of the
> constituents of these modules, with many key animal genes
> predating multicellularity. 2,5 In contrast, we lack a detailed un-
> derstanding of when these genes assembled into co-regulated
> modules, how they are deployed in different cell types, and
> what their evolutionary dynamics are. The comparative study
> of cell diversity and genome regulation in early-branching,
> non-bilaterian animals (sponges, cnidarians, ctenophores,
> and placozoans) offers the opportunity to address these funda-
> mental questions and reconstruct the evolutionary emergence
> of major cell type programs.
> Placozoans are millimeter-sized, flat animals that employ
> ciliary beating and mucus secretion to glide over surfaces. 6–9
> These marine animals feed on algae and other microbial eukary-
> otes by extracellular lysis of prey, phagocytosis, and intracellular
> digestion. 10 Placozoans emerged 750–800 mya "
>
> Uncalibrated molecular dating has inherent large uncertainties, but it's
> safe to say that the clade originated in the Ediacaran, if not Neo-proterozoic. Unfortunately, the genome gives no hint of what it "looked like".
>
> An Ediacaran animal, Dickinsonia (see Wiki entry) has the following traits
> in common with placozoa: it was mobile and fed by engulfing algae on the sea floor. Dickinsonia has mostly been discussed as a possible stem bilaterian, and is much more complex than placozoans, having a consistent body plan and developement, but simplification of genomes over hundreds of millions of years is plausible. Dickinsonia disappeared before the end of the Ediacaran, probably the victim of greatly increased predation from the "small shelley" biota. It has been suggested as a stem placozoan, not far removed from stem bilaterian.

I screwed up. The reference in Cell is https://www.cell.com/cell/pdf/S0092-8674(23)00917-0.pdf