Systematics or Phylogenetics

Sponges (Porifera) are traditionally divided into three major classes according to the composition of their organic skeleton (Degnan et al 2010). These classes are placed within two groups symplasm that is comprised of glass and cellularia that is comprised of cellular tissue.  Symplasma contains Hexactinellida (glass sponges), and cellularia contanins calcarea (calcareous sponges) and demospongiae (silicious sponges) (Rupper, Fox & Barnes 2004).

Sponges are generally recognised as the oldest surviving metazoan pelagic lineage (Srivastava et. al 2010), even described by some as living fossils (Muller 1998). This is supported by a recent analysis of sponge sequences, suggesting the maximum likelihood to be that sponges to be paraphyletc (consist of of the decendants of the LCA) (Sperling et al. 2009). This leads to the implication that the LCA was sponge like. There is still contravery, and a debate if sponges are indeed paraphyletic or monophyletc, which would have a direct impact on the reconstruction of the LCA (Degnan et al 2010).

LCA Phylogenic Assumption (composed from information, Sterling et al. 2009)