Phylogenomics, origin and diversification of anthozoans (Phylum Cnidaria)

Anthozoan cnidarians (corals and sea anemones) include some of the
world's most important foundation species, capable of building
massive reef complexes that support entire ecosystems. Although previous
molecular phylogenetic analyses have revealed widespread homoplasy of the
morphological characters traditionally used to define orders and families
of anthozoans, analyses using mitochondrial genes or rDNA have failed to
resolve many key nodes in the phylogeny. With a fully resolved,
time-calibrated phylogeny for 234 species constructed from hundreds of
ultraconserved elements and exon loci, we explore the evolutionary origins
of the major clades of Anthozoa and some of their salient morphological
features. The phylogeny supports reciprocally monophyletic Hexacorallia
and Octocorallia, with Ceriantharia as the earliest diverging hexacorals;
two reciprocally monophyletic clades of Octocorallia; and monophyly of all
hexacoral orders with the exception of the enigmatic sea anemone
Relicanthus daphneae. Divergence dating analyses place Anthozoa in the
Cryogenian to Tonian periods (648–894 Ma), older than has been suggested
by previous studies. Ancestral state reconstructions indicate that the
ancestral anthozoan was a solitary polyp that had bilateral symmetry and
lacked a skeleton. Colonial growth forms and the ability to precipitate
calcium carbonate evolved in the Ediacaran (578 Ma) and Cambrian (503 Ma)
respectively; these hallmarks of reef-building species have subsequently
arisen multiple times independently in different orders. Anthozoans formed
associations with photosymbionts by the Devonian (383 Ma), and
photosymbioses have been gained and lost repeatedly in all orders.
Together, these results have profound implications for interpretation of
the Precambrian environment and the early evolution of metazoans.