Ediacaran Period

Cross references:    Eukaryote Colonies     Slime Molds      Neoproterozoic  
    Porifera       Placozoa       
Ediacaran Genomics   

Ediacaran (Wiki)

635 to 542 million years ago. 
The Ediacaran Period is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon.
"  "The Ediacaran Period represents the time from the end of global Marinoan glaciation to the first appearance worldwide of somewhat complicated trace fossils (Trichophycus pedum).

Ediacara Biota (Wiki) 
The Ediacara biota consisted of enigmatic tubular and frond-shaped sessile (stationary) organisms which lived during the Ediacaran Period (ca. 635-542 Ma). Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.[note 1] The Ediacara biota first appeared as the Earth thawed from the Cryogenian period's extensive glaciers, and largely disappeared contemporaneous with the rapid appearance of biodiversity known as the Cambrian explosion,
"  "Multiple hypotheses exist to explain the disappearance of this biota, including preservation bias, a changing environment, the advent of predators, and competition from other life-forms.

Determining where Ediacaran organisms fit in the tree of life has proven impossible. The morphology and habit of some taxa (e.g. Funisia dorothea suggest relationships to Porifera or Cnidaria.

Microbial mats are areas of sediment stabilised by the presence of colonies of microbes, which secrete sticky fluids or otherwise bind the sediment particles. They appear to migrate upwards when covered by a thin layer of sediment, but this is an illusion caused by the colony's growth; individuals do not, themselves, move.
"  "Some Ediacaran strata with the texture characteristic of microbial mats contain fossils, and Ediacaran fossils are almost never found in beds that do not contain these microbial mats. Although microbial mats were once widespread, the evolution of grazing organisms in the Cambrian vastly reduced their numbers,

Ediacaran biota exhibited a vast range of morphological characteristics. Size ranged from millimetres to metres; complexity from "blob-like" to intricate; rigidity from sturdy and resistant to jelly-soft. Almost all forms of symmetry were present.  These organisms differed from earlier fossils by displaying an organised, differentiated multicellular construction and centimetre-plus sizes."  

Early origin of the bilaterian developmental toolkit (Goog) 
Full length PDF available online for free. 
"Rocks from the Ediacaran Period (635–542 Ma) immediately preceding the explosion of animals diversity in the Early Cambrian, contain a diversity of centimetre to metre-long fronds, discs and more complex forms, some of them superficially similar to modern animal groups. Although some members of this assemblage of fossils, known as the Ediacaran biota  (575–542 Ma), display apparent bilateral symmetry, none has evidence of appendages, eyes, a mouth (with one exception) or other characteristics of the higher bilaterians."