DESCRIPTION OF THE PROTOSTOMATA (MACALISTER 1879)
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PROTOSTOMATA LINKS
| Protostomata (pro-to-sto-MAH-tuh) is formed from two Greek roots that mean primary or first [proto (πρωτο)] and mouth [stoma (στόμα)]. The reference is to the group of metazoans in which the first infolding during gastrulation becomes the mouth. The taxon has several synonyms, but Protostomia (Grobben 1908) is the most widely used. We have elected to use the older term, Protostomata (MacAlister 1879). |
INTRODUCTION TO THE PROTOSTOMES
The protostomes form the largest group of bilaterians and include such successful and diverse groups as the arthropods and the mollusks. As such, they represent the largest group of living things on earth. Although they do have a few common features (see Table 1), the protostomes are quite variable in structure, developmental histories, and ecology.
| TABLE 1. Characters that define or are common among the taxa of protostomes. | |
| The gut, if present, develops from the first invagination of the blastula, the blastopore, in the formation of the gastrula. This feature provides the foundation of the name protostome, which means first mouth. | all protostomes |
| Most protostomes develop from the zygote by spiral clevage. | most protostomes (all protostomes in the system of Nielsen, 2001) |
| Those that develop a true body cavity form a schizocoelic coelom. | most protostomes |
| Many have a planktonic trochophore larva. | many protostomes |
| The main nerve cord is paired and ventral. | almost all protostomes |
This great abundance and diversity in form has led to a plethora of taxonomic systems, many of which are incompatible and inconsistent with each other. The Major Clades of the Animal Kingdom illustrates some of the incompatibilities in that Nielsen (2001), Raff (2001), Adouette et al. (2000), Brusca and Brusca (2003), and Tudge (2000) all divide the protostome phyla differently and even include different phyla within the protostomes. In general, taxonomic systems based on structural characters (e.g. Nielsen 2001; and Brusca and Brusca 2003) separate the protostomes into groups that include the Cycloneuralia and Articulata plus a number of unassociated clades. The phylogenetic systems based on molecular characters (e.g. Tudge 2000; Raff 2001), however, separate them into the Ecdysozoa and the Spiralia (also called Lophotrochozoa). (Go to ALTERNATE VIEWS OF PROTOSTOME TAXONOMIC SYSTEMS for further explanations and comparisons.)
We have followed the system of Edgecombe et al. (2011; see Figure 1) for which they draw on molecular, structural, developmental, and fossil evidence to produce two monophyletic protostome clades: Ecdysozoa (clade 12), and Spiralia (=Lophotrochozoa; clade 13). Chaetognatha (clade 10) is basal and a sister to Ecdysozoa + Spiralia.
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| FIGURE 1. Our interpretation of Edgecombe et al. (2011) highlighting the Protostome taxa (defined by clade 9). |
| ALTERNATE VIEWS OF PROTOSTOME TAXONOMIC SYSTEMS Phylogenetic taxonomic systems that serve to organize the protostomes seem to point to fundamentally 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 2 and 3. They are similar to each other in that the protostome clades are defined by spiral cleavage. Also, Figures 2 and 3 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 4 and 5 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 (=Spiralia). 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. Valentine (2004) attempted to resolve the systematic chaos by using molecular as well as structural and fossil evidence with four clades of bilaterians: Ecdysozoa, Lophotrochozoa, Paracoelomata, and Deuterostomata. Even in his system, there were several problematic groups, namely the Platyhelminthes, which were problematic in almost all systems. He removed the Platyhelminthes from the group that Giribet et al. (2000) called the Platyzoa (thus making the name problematic). He also changed the name of the clade to the Paracoelomata, a reference to their pseudocoelomate organization. The more recent work by Giribet et al. (2007; Figure 6) and Dunn et al. (2008; Figure 7) confirms the Ecdysozoa-Lophotrochozoa (Spiralia?) structure of the protostomes. Within the Lophotrochozoa, however, there is no monophyletic lophophorate clade. The Bryozoa (Ectoprocta) are basal in the Platyzoan clade and the Entoprocta are basal in the Trochozoan clade, positions that suggest the primitive nature of the lophophore. Another curious outcome is that the Tardigrada does not emerge with the Pan-Arthropods but with the other Ecdysozoa. Edgecombe et al. (2011) is a more formalized topology that summarizes molecular work of the previous 15 years and offers a more standardized set of terms for the higher groups (see Figure 1). We have color-coded the taxa in Figures 2-5 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. The more recent analyses of Giribet et al. (2007, Figure 6) and Dunn et al. (2008, Figure 7) show the Platyhelminthes emerging in a derived clade within the Platyzoa. |
FIGURE 2. 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 3. 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 5. 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
FIGURE 6. A figure from Giribet et al. (2007). Green squares indicate taxa for which expressed sequence tags were available.
FIGURE 7. A figure from Dunn et al. (2008)
| LITERATURE CITED Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass. Dunn, C.W., A. Hejnol, D.Q. Matus, K. Pang, W.E. Browne, S.A. Smith, E. Seaver, G.W. Rouse, M. Obst, G.D. Edgecombe, M.V. Sørensen, S.H.D. Haddock, A. Schmidt-Rhaesa, A. Okusu, R.M. Kristensen, W.C. Wheeler, M.Q. Martindale, and G. Giribet. 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature. 452: 745-749. Edgecombe, G. D., G. Giribet, C. W. Dunn, A. Hejnol,R. M. Kristensen, R. C. Neves, G. W. Rouse, K. Worsaae, and M. V. Sorensen. 2011. Higher-level metazoan relationships: recent progress and remaining questions. Organisms Diversity and Evolution. DOI 10.1007/s13127-011-0044-4. Giribet, G., C.W. Dunn, G.D. Edgecombe, and G.W. Rouse. 2007. A modern look at the Animal Tree of Life. Zootaxa. 1668: 61-79. MacAlister, A. 1879. Zoology of the Invertebrate Animals. Henry Holt and Co. New York. Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. 2nd Edition. Oxford University Press. Oxford. Raff, R. A. 2001. Metazoan phylogeny. In: Anderson, D.T., ed. Invertebrate Zoology. Oxford University Press. Oxford, UK. pp. 424-446. Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York. Valentine, J. W. 2004. On the Origin of Phyla. University of Chicago Press. Chicago. pp. 614. |
| By Jack R. Holt. Last revised: 04/07/2013 |
