Spiralians 1: lophophorates

Key points

• Three spiralian invertebrate groups have lophophores, a filamentous feeding organ: brachiopods, bryozoans and phoronids.

• Brachiopods are twin-valved shellfish, with a lophophore and usually a pedicle, adapted to a wide range of life strategies on the seafloor.

• The phylum Brachiopoda is currently divided into the linguliformeans, with organo-phosphatic shells, and the craniiformeans and rhynchonelliformeans, both with calcareous shells.

• Paleozoic communities were dominated by orthides and strophomenides, together with a variety of spire-bearing forms; rhynchonellides and terebratulides are typical of the lower-diversity post-Paleozoic brachiopod assemblages.

• Brachiopods dominated the filter-feeding benthos of the Paleozoic but never fully recovered in abundance or diversity from losses during the end-Permian mass extinction.

• Living brachiopods are relatively rare, occupying mostly cryptic and deep-water habitats.

• Bryozoans are colonial invertebrates with lophophores, commonly displaying marked non-genetic variation across a wide range of environments.

• The Stenolaemata dominated Paleozoic bryozoan faunas, with only the cyclostomes surviving the combined effects of the end-Permian and end-Triassic mass extinctions; as the cyclostomes continued to decline after the end-Cretaceous extinction event, the cheilostomes radiated to dominate Cenozoic assemblages.

We may consider here under the name Molluscoidea, the two groups of animals which are known respectively as the Polyzoa [Bryozoa] and the Brachiopoda. These two groups, in many respects closely allied to one another, present affinities on the one hand to the Worms and on the other hand to the Mollusca . . .

R.A. Nicholson and R. Lydekker (1890) Manual of Palaeontology, 3rd edn

What do lampshells, moss animals and the rare tube-dwelling phoronids, or horseshoe worms, have in common? They may look very different, but these three phyla, the Brachiop-oda, Bryozoa and Phoronida, all possess a complex feeding organ, the lophophore, and have similar body cavities or celoms. Nevertheless the relationships among the three are not yet fully resolved, although the phoronids probably lie close to or may even be part of the group, the bryozoans are more distantly related. Our understanding has not changed much since 1890, but new molecular studies may help resolve these uncertainties in the next 10 years.

The phoronids are tube-dwelling, wormlike lophophorates, with the 10 or so described species divided between two genera, Phoronis and Phoronopsis. These animals lack a mineralized skeleton and pursue burrowing or boring life strategies with near-cosmopolitan distributions. The phylum has a long though questionable geological history, as some authors suggest that Precambrian and Lower Paleozoic records of the vertical burrow Skolithos (see p. 523) may possibly be the work of phoronids. The ichnogenus Talpina, present as borings in both Cretaceous belem-nite rostra and Tertiary mollusk shells, may also have been constructed by phoronids.

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