SYSTEMATIC BIOLOGY RETURN TO THE PROTOSTOMES
HOME SYLLABUS WEEKLY ASSIGNMENTS J. SYSTEMATIC BIOLOGY TAXA OF LIFE
COMPARISONS OF PROTOSTOME PHYLOGENETIC TAXONOMIC SYSTEMS

Phylogenetic taxonomic systems that serve to organize the protostomes seem to point to funadmentally different apomorphies within this huge collection of taxa.  Structural systems like those of Brusca and Brusca (2003) and Nielsen (2001) are shown below in Figures 1 and 2.  They are similar to each other in that the protostome clades are defined by spiral cleavage.  Also, Figures 1&2 both have the clade, Cycloneuralia, a group defined by the anterior ganglia (brain) wrapping around the anterior end of the food tube.  They also group the segmented animals together (the Articulata).  Nielsen (2001) further groups the Sipunculans and Molluscans with the articulates as a clade defined by the development of a schizocoelom, a true coelom that develops from slits in the mesoderm.  The typical "bryozoan" phyla (Entoprocta and Ectoprocta) are included in their protostome clades as well and thus separate the "bryozoa" from the other lophophorates that they place in the Deuterostomata.

The molecular systems (see Figures 3 and 4 from Tudge, 2000 and Raff, 2001, respectively) have focused on different defining synapomorphies and thus separate the protostomes into two large groups: the Ecdysozoa and the Lophotrochozoa.  The Ecdysozoa are defined by taxa that have or evolved from organisms with cast their outer coverings as they grow.  The Lophotrochozoa similarly are defined by taxa that have or evolved from organisms with a trochophore larva and ciliary feeding structures (lophophores), so all of the lophophorates are included in the protostomes.  Also, the phyla of the "cycloneuralia" and "articulata" of the structural systems are spread between the Lophotrochozoa and Ecdysozoa.  Thus, the two systems are not at all compatible.

The chaos of protostome systematics seems to have been resolved quite nicely by Valentine (2004), who uses molecular as well as structural and fossil evidence with four clades of bilaterians: Ecdysozoa, Lophotrochozoa, Paracoelomata, and Deuterostomata. Even in his system, there are several problematic groups, namely the Platyhelminthes, which is problematic in all current systems.  Likely the flatworms are paraphyletic.  Indeed, the acoel flatworm may be the sister group to all the bilateria.  He removed the Platyhelminthes from the group that Giribet et al. (2000) called the Platyzoa (thus making the name problematic).  He changed the name of the clade to the Paracoelomata, a reference to their pseudocoelomate organization.

I have color-coded the taxa in Figures 1-3 to indicate their grades of structural development.  The eucoelomates are blue, pseudocoelomates are red, and the acoelomates are green.  Note that only in Figures 1 and 4 do the Platyhelminthes emerge as primitive groups.

Figure 1.  The protostomes as separated by Brusca and Brusca (2003).  This includes two large clusters: the Cycloneuralia and the Articulata along with several independent clades.

BLUE = EUCOELOMATES; RED = PSEUDOCOELOMATES; GREEN = ACOELOMATES

Figure 2.  The protostomes as separated by Nielsen (2001).  This also sets apart the Cycloneuarlia as a clade.  The cladogram further sets apart a larger clade that includes the Articulata of Brusca and Brusca (2003) and the Mollusca-Sipuncula clade as a coherent group called the Schizocoela. 

BLUE = EUCOELOMATES; RED = PSEUDOCOELOMATES; GREEN = ACOELOMATES

Figure 3.  This is a cladogram that summarizes current molecular phylogenetics in the protostomes.  It is a generalized figure from Tudge (2000).  In this system, the protostomes are divided into two large groups: the Ecdysozoa and the Lophotrochozoa.

BLUE = EUCOELOMATES; RED = PSEUDOCOELOMATES; GREEN = ACOELOMATES

Figure 4. This is a cladogram of some protostome phyla modified from Raff (2001).  Note that all of the pseudocoelomates are missing and that the Platyhelminthes are considered primitive.  Still, of the separation of the of the two major protostome clades falls according to the Ecdysozoa-Lophotrochozoa hypothesis.

BLUE = EUCOELOMATES; RED = PSEUDOCOELOMATES; GREEN = ACOELOMATES


This page is maintained by Jack R. Holt.  Last modified: 01/04/07