Cross references: Ediacaran Period Neoproterozoic MetazoaAbstract:
Parazoa Porifera Placozoa Pre-Bilateria Locomotion Coelenterata
Cambrian Explosion Bilateria Protostome Deuterostomes
Early origin of the bilaterian developmental toolkit (Goog)
"Whole-genome sequences from the choanoflagellate Monosiga brevicollis, the placozoan Trichoplax adhaerens and the cnidarian Nematostella vectensis have confirmed results from comparative evolutionary developmental studies that much of the developmental toolkit once thought to be characteristic of bilaterians appeared much earlier in the evolution of animals.
The diversity of transcription factors and signalling pathway genes in animals with a limited number of cell types and a restricted developmental repertoire is puzzling, particularly in light of claims that such highly conserved elements among bilaterians provide evidence of a morphologically complex protostome-deuterostome ancestor. Here, I explore the early origination of elements of what became the bilaterian toolkit, and suggest that placozoans and cnidarians represent a depauperate residue of a once more diverse assemblage of early animals, some of which may be represented in the Ediacaran fauna (c. 585-542 Myr ago)."
Full length PDF available online for free.
"The presence of many ‘bilaterian’ developmental tools in morphologically
simpler organisms reinforces the view that the original role of these genes and regulatory networks was in the formation of specialized cell types in specific regions of the body, not necessarily in producing complex multicellular structures"