Palćos:		Invertebrates
METAZOA		Protostomia

 

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Protostomia

 

Metazoa |--Porifera (paraphyletic?) `--+--Radiata `--Bilateria |--Deuterostomia `--Protostomia |--Ecdysozoa | |--Tardigrada | `--Arthropoda | |--Trilobita | `--Pancrustacea | |--Crustacea | `--Insecta `--Lophotrochozoa |--Mollusca `--Annelida

The Protostomes include the great majority of invertebrates, such as arthropods, molluscs, annelids, and many less well-known taxa

The protostome condition is defined by a spiral and determinant cleavage in the early stages of embryo development, schizocoelous coelom formation (as the archenteron (embryonic gut) forms the coelom begins as splits within the solid mesodermal mass, and the formation of the blastopore (the original opening) into the mouth

There is a tendency among researchers in the field of molecular phylogeny to divide the Protostomia into two further groups, the Ecdysozoa and Lophotrochozoa. However, not all molecular phylogeneticists accept this approach.

Ecdysozoa and Lophotrochozoa

Protostomia after Aguinaldo et. al. 1997:

<==o Protostomia
   |--o LOPHOTROCHOZOA
   `--o ECDYSOZOA

 Recently metazoan phylogeny has suggested that the Protostomia can be subdivided into two further clades. The molecular phylogenetic tree of Aguinaldo et al. divides the protostomes into the two clades, Ecdysozoa and Lophotrochozoa.

Giribet & Ribera (1998), using 18S rDNA sequences also divide the protostome coelomates into two clades, the Ecdysozoa and the Spiralia.

abstract of a paper.  Describes using DNA sequencing as an alternative to morphology in determining Protostome phylogeny.  From this abstract:

"Here we report a cladistic approach to metazoan evolution including 133 18S rDNA sequences of 31 animal phyla. Despite the difficulties associated with analyzing large data sets, our data suggest that the Bilateria and Protostomia are monophyletic. The internal phylogeny of the protostomes is divided into two main clades. One clade includes the classical protostome worms (annelids, sipunculans, echiurans, pogonophorans, and vestimentiferans), mollusks, nemerteans, "lophophorates," platyhelminths, rotiferans, and acanthocephalans, although the internal resolution of the clade is very low. The second clade includes arthropods and other molting animals: tardigrades, onychophorans, nematodes, nematomorphans, kinorhynchs, and priapulans. The arthropods and related phyla lack a ciliated larvae, lack a multiciliate (locomotory) epithelium, and share many features, notably, a reduced coelomic cavity and the presence of a cuticle which molts."

Also The Ancestry of the Hox Cluster - Guillaume Balavoine - from the abstract

"The 18S rDNA phylogeny of metazoans leads to the picture that the bulk of the bilaterian phyla is split into a double dichotomy giving three great superphyla, Deuterostomia, Ecdysozoa (including nematodes) and Lophotrochozoa (including brachiopods). The four-gene cluster in C. elegans must therefore be the remnant of a larger cluster. Comparison of the Hox [cluster of] genes sequences appears to confirm the position of some phyla in this three-branched tree of the Bilateria, notably brachiopods"

A different molecular phylogenetic study by Winnepennickyx et al. (1998) presents a cladogram that doesn't agree with Aguinaldo et al., although it, too, contains a clade made up of  priapulids and arthropods.

Alternative phylogeny after Winnepennickyx et al. (1998)

<==o Protostomia after Winnepennickyx et al., 1998
   |-- Mollusca 
   |-- Annelida [+ Vestimentifera & Pogonophora sensu Rouse, 2001]  
   |--+-- Phoronida 
   |  `-- Brachiopoda 
   |-- Entoprocta 
   |-- Bryozoa [Ectoprocta] 
   |-- Gastrotricha
   |-- Lobopoda [Onychophora sensu lato] (lobopods, velvet worms)
   |-- Nemata [Nematoda] 
   `--+-- Priapulida
      `--+-- Crustaceomorpha
         `-- Hexapoda (insects)

Note: This tree is from molecular phylogenetic study made by Winnepennickyx, Backeljau & Kristensen (1998). This doesn't agree with a study by Aguinaldo et al. (1997), although it too contains a clade made by priapulids and arthropods. It also contains clade made by phoronids and brachiopods. 

Alternative phylogeny after Peterson and Eernisse (2001)

<==o Protostomia
   |--o  Deuterostomia
   `--+--o LOPHOTROCHOZOA
      `--o ECDYSOZOA

Peterson & Eernisse (2001) arrive at several different trees through a combination of methods. They indicate that the Lophotrochozoa are paraphyletic and hence not a true clade. Interestingly, all but the morphology only approach showed the sponges (Porifera) to be paraphyletic (i.e. that sponges were the direct ancestors of other forms of animal life) rather than a side-branch

Paleontologist Conway Morris presents a different tree again, and it is clear that there is a lot to be resolved before we can have a clear picture of the early branchings of the animal evolutionary tree.

Coelomate phyla can be divided into two groups variously called the Schizocoela and Enterocoela, or the Protostomia and Deuterostomia (this is shown at the bottom of the above cladogram). These are distinguished by embryonic cleavage patterns, the fate of the blastopore and coelom formation.

One important difference involves the cleavage patterns; the division of cells in the early embryo. Protostomes are characterized by a spiral (the planes of cell division are diagonal to the vertical axis of the embryo) and determinant (the goal of each embryonic cell is established very early) cleavage. Deuterostomes undergo radial (parallel or perpendicular to the vertical axis of the embryo) and indeterminate cleavage (each early embryonic cell retains the capacity to develop into a complete embryo if isolated from other cells)

There is a strange symmetry in the fate of the Blastopore (the first opening of the archenteron which forms during gastrulation.). In Protostomes ("mouth first") the blastopore forms the mouth; in deuterostomes ("mouth second") - blastopore forms the anus.

Another difference involves how the mesoderm surrounds the body cavity. It can either pouch off or split entirely. In Schizocoela or Protostomia, it splits. In the Enterocoela or Deuterostomia, the mesoderm (or archenteron, the original gut) pouches off.

Protostomia after Conway Morris, 1993

(with slight modification following and Zrzavý, et al., 2001)

<==o Protostomia sensu Zrzavý, et al., 2001 
   |--o ASCHELMINTHES [Pseudocoelomata]
   |  |-- Xenoturbellida
   |  |-- Platyhelmithes 
   |  `--o Acanthognatha Cavalier-Smith, 1998
   |     |--o Neotrichozoa Zrzavy, et al., 1998 [Monokonta Cavalier-Smit, 1998]
   |     |  |-- Gastrotricha
   |     |  `-- Gnathostomulida
   |     `--o Prosomastigozoa
   |        |-- Myzostomida sensu Zrzavý, et al., 2001 
   |        `--+-- Cycliophora Funch & Kristensen, 1995
   |           `--o Syndermata Ahrlichs, 1995 sensu Zrzavý, et al., 2001
   |              |-- Monogononta [Rotifera: Monogononta]
   |              `--o Lemniscea Carey, et al., 1996
   |                 |-- Bdelloida [Rotifera: Bdelloidea]
   |                 `--+-- Seisonida [Rotifera: Seisonidea]
   |                    `-- Acanthocephala 
   `--+--o ARTHROPODOMORPHA
      |  |-- Cephalorhyncha
      |  |--+--+-- †Protoconodonta
      |  |  |  `-- Chaetognatha
      |  |  `--+-- Nematomorpha
      |  |     `-- Nemata [Nematoda] 
      |  `--+--o †Sprigginida
      |     |  |?- †Bomakellia
      |     |  `-- †Spriggina floundersi
      |     |?- Tardigrada (water bears)
      |     `-- Arthropoda (arthropods) [polyphyletic???]
      `--+-- Entoprocta 
         |-- Bryozoa [Ectoprocta]
         |--+-- †Chancelloriida
         |  `-- †Sachitida
         `--+-- †Siphogonuchitida
            `--+--o †Halkieriida
               |  `-- †Halkeria evangelista
               `--+--o "MOLLUSCOMORPHA"
                  |  `--+?- †Tullimonsterida
                  |     `--+-- Mollusca 
                  |        `--+-- †Hyolitha
                  |           `-- Sipuncula [Sipunculoidea] 
                  `--+--o "ANNELIDAMORPHA"
                     |  |?- †Wiwaxia corrugata
                     |  `--+?- †Canadia spinosa
                     |     |--+-- †Amiskwiida
                     |     |  `-- Nemertea [Nemertinea] 
                     |     `--+-- Lobatocerebrida
                     |        `--+--+-- †Banffida
                     |           |  `-- Echiura [Echiuroidea] 
                     |           `-- Annelida [+ Vestimentifera & Pogonophora sensu Rouse, 2001]  
                     `--+--o LOPHOPHORATA
                        |  |-- Phoronida 
                        |  `--+-- †Tommotiida
                        |     `-- Brachiopoda 
                        `--+-- †Palaeoscolecida
                           `--+-- Kinorhyncha 
                              `--+-- Loricifera 
                                 `-- Priapulida [Priapuloidea]