Phoronid - Evolutionary History - Family Tree

Family Tree

Phoronids, brachiopods and bryozoans (ectoprocts) are collectively called lophophorates, because all feed using lophophores. From about the 1940s to the 1990s, family trees based on embryological and morphological features placed lophophorates among or as a sister group to the deuterostomes, a super-phylum that includes chordates and echinoderms. In the early development of their embryos, deuterostomes form the anus before the mouth, while protostomes form the mouth first.

Nielsen (2002) views the phoronids and brachiopods as affiliated with the deuterostome pterobranchs, which also filter-feed by tentacles, because the current-driving cells of the lophophores of all three have one cilium per cell, while lophophores of bryozoans, which he regards as protosomes, have multiple cilia per cell. Helmkampf, Bruchhaus and Hausdorf (2008) summarise several authors' embryological and morphological analyses which doubt or disagree that phoronids and brachiopods are deuterostomes:

• While deutersomes have three coelomic cavities, lophophorates such as phoronids and brachiopods have only two.
• Pterobranchs may be a sub-group of enteropneusts ("acorn worms"). This suggests that the ancestral deuterostome looks more like a mobile worm-like enteropneust than a sessile colonial pterobranch. The fact that lophophorates and pterobranchs both use tentacles for feeding is probably not a synapomorphy of lophophorates and deuterostomes, but evolved independently as convergent adaptations to a sessile lifestyle.
• The mesoderm does not form by enterocoely in phoronids and bryozoans, but does in deuterostomes, while there are disagreements about whether brachiopods form the mesoderm by enterocoely.

Bilateria	Acoelomorpha (Acoela and Nemertodermatida) Deuterostomia (Echinoderms, chordates, etc.) Protostomia Ecdysozoa (Arthropods, nematodes, priapulids, etc.) Lophotrochozoa Bryozoa Annelida and phyla merged into them Mollusca Phoronida and Brachiopoda Nemertea Dicyemida Myzostomida Platyzoa Platyhelminthes Other Platyzoa

Relationships of Phoronida to other Bilateria:

From 1988 onwards analyses based on molecular phylogeny, which compares biochemical features such as similarities in DNA, have placed phoronids and brachiopods among the Lophotrochozoa, a protostome super-phylum that includes molluscs, annelids and flatworms but excludes the other main protostome super-phylum Ecdysozoa, whose members include arthropods. Cohen wrote, "This inference, if true, undermines virtually all morphology–based reconstructions of phylogeny made during the past century or more."

While analyses by molecular phylogeny are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear. The Lophotrochozoa are generally divided into: Lophophorata (animals that have lophophores), including Phoronida and Brachiopoda; Trochozoa (animals many of which have trochophore larvae), including molluscs, annelids, echiurans, sipunculans and nemerteans; and some other phyla (such as Platyhelminthes, Gastrotricha, Gnathostomulida, Micrognathozoa, and Rotifera).

Molecular phylogeny indicates that Phoronida are closely related to Brachiopoda, but Bryozoa (Ectoprocta) are not closely related to this group, despite using a similar lophophore for feeding and respiration. This implies that the traditional definition "Lophophorata" is not monophyletic. Recently the term "Lophophorata" has been applied only to the Phoronida and Brachiopoda, and Halanych thinks this change will cause confusion. Some analyses regard Phoronida and Brachiopoda as sister-groups, while others place Phoronida as a sub-group within Brachiopoda, implying that Brachiopoda is paraphyletic. Cohen and Weydman's analysis (2005) concludes that phoronids are a sub-group of inarticulate brachiopods (those in which the hinge between the two valves have no teeth and sockets) and sister-group of the other inarticulate sub-groups. The authors also suggest that the ancestors of molluscs and the brachiopod+phoronid clade diverged between 900 Ma and 560 Ma, most probably about 685 Ma.