Phylogeny and Adaptive Radiation
Phylogeny and
Adaptive Radiation
The phylogenetic position of the lopho- phorates has generated much controversy and debate. Sometimes they have been considered protostomes with some deuterostome characters, and at other times deuterostomes with some protostome characters. Brusca and Brusca* contend that there is overwhelming evidence that they are a monophyletic group and are deuterostomes. On the other hand, sequence analysis of the gene coding for small-subunit ribosomal RNA provides evidence that they are protostomes.† They appear clearly allied with Annelida and Mollusca and very close to Entoprocta within superphylum Lophotrochozoa. Their common possession of a lophophore is a unique synapomorphy. Other features, such as a U-shaped digestive tract, metanephridia, and tendency to secrete outer casings may be homologous within the clade, but they are convergent with many other taxa.
Division of the coelom into three parts (trimerous, or tripartite) is a feature shared with deuterostomes, but the character must be convergent if lophophorates are protostomes. Furthermore, some authors question the trimerous nature and homologies of the coelom in some lophophorates (for example, whether the space in the epistome of inarticulate brachiopods is a protocoel, whether the mesocoel and metacoel in brachiopods are homologous to those spaces in other lophophorates, and whether the body coelom of ectoprocts is homologous to that of brachiopods and phoronids). The blastopore origin of the mouth in phoronids and mosaic development in ectoprocts are typical protostome characters. Their larvae have been called trochophore-type in the past, although the resemblance to trochophores of annelids and molluscs is not close.
All lophophorates are filter feeders, and most of their evolutionary diversification has been constrained by this function. The tubes of phoronids vary according to their habitats. Many adaptations of ectoprocts seem related to miniaturization of individual zooids and colony formation. Various ectoprocts build their protective exoskeletons of chitin or gelatin, which may or may not be impregnated with calcium and sand. Brachiopod variations occur largely in their shells and lophophores.
The phylogenetic position of the lopho- phorates has generated much controversy and debate. Sometimes they have been considered protostomes with some deuterostome characters, and at other times deuterostomes with some protostome characters. Brusca and Brusca* contend that there is overwhelming evidence that they are a monophyletic group and are deuterostomes. On the other hand, sequence analysis of the gene coding for small-subunit ribosomal RNA provides evidence that they are protostomes.† They appear clearly allied with Annelida and Mollusca and very close to Entoprocta within superphylum Lophotrochozoa. Their common possession of a lophophore is a unique synapomorphy. Other features, such as a U-shaped digestive tract, metanephridia, and tendency to secrete outer casings may be homologous within the clade, but they are convergent with many other taxa.
Division of the coelom into three parts (trimerous, or tripartite) is a feature shared with deuterostomes, but the character must be convergent if lophophorates are protostomes. Furthermore, some authors question the trimerous nature and homologies of the coelom in some lophophorates (for example, whether the space in the epistome of inarticulate brachiopods is a protocoel, whether the mesocoel and metacoel in brachiopods are homologous to those spaces in other lophophorates, and whether the body coelom of ectoprocts is homologous to that of brachiopods and phoronids). The blastopore origin of the mouth in phoronids and mosaic development in ectoprocts are typical protostome characters. Their larvae have been called trochophore-type in the past, although the resemblance to trochophores of annelids and molluscs is not close.
All lophophorates are filter feeders, and most of their evolutionary diversification has been constrained by this function. The tubes of phoronids vary according to their habitats. Many adaptations of ectoprocts seem related to miniaturization of individual zooids and colony formation. Various ectoprocts build their protective exoskeletons of chitin or gelatin, which may or may not be impregnated with calcium and sand. Brachiopod variations occur largely in their shells and lophophores.
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