Phylogeny
Phylogeny
The early embryological development of sipunculans, echiurans, and annelids is almost identical, showing a very close relationship among the three. It is also similar to molluscan development. Some authors group the four phyla into a supraphyletic assemblage called “Trochozoa” because of the common possession of a trochophore larva. Other similarities, too, point to close relationship of the sipunculans to the echiurans and annelids, such as the nature of the nervous system and body wall. Sipunculans and echiurans are not metameric and thus are more primitive in that characteristic than annelids. They probably represent collateral evolutionary lines that branched from protoannelid stock before the origin of metamerism.
Several characters suggest relationship of Pogonophora to Annelida, as we noted previously. Molecular sequence analysis supports close relationships of Echiura, Sipuncula, and Pogonophora to Annelida and Mollusca in Lophotrochozoa.
Phylogenetic affinities of Pentastomida are uncertain. They have some similarities to Annelida. Their larval appendages and molting cuticle, however, are arthropod characteristics. Their larvae resemble tardigrade larvae. Most modern taxonomists align them with arthropods, however, and evidence is accumulating that they are most closely related to the crustacean subclass Branchiura. This evidence includes similarities in morphology of their sperm and in base sequences of ribosomal RNA. If pentastomids really are close to the branchiurans, then their status as a phylum should be revoked, and they should be classified as crustacean arthropods.
Onychophorans share a number of characteristics with annelids: metamerically arranged nephridia, muscular body wall, pigment cup ocelli, and ciliated reproductive ducts. Characteristics shared with arthropods include the cuticle, tubular heart and hemocoel with open circulatory system, presence of tracheae (probably not homologous), and large size of the brain. Unique characteristics include oral papillae, slime glands, body tubercles, and suppression of external segmentation.
Some authors believe the onychophorans should be included with the arthropods, but that would involve redefining the phylum Arthropoda. Manton* recommended placing the Onychophora with the myriapods and insects in the phylum Uniramia. However, most authors believe that the differences seem to warrant keeping them in a separate phylum (Figure 21-17). Sequence analysis supports placement of Onychophora in Ecdysozoa.
Affinities of tardigrades are among the most puzzling of all animal groups. They have some similarities to rotifers, particularly in their reproduction and their cryptobiotic tendencies, and some authors have called them pseudocoelomates. Their embryogenesis, however, would seem to put them among the coelomates. The enterocoelic origin of the mesoderm is a deuterostome characteristic. Other authors identify several important synapomorphies that suggest grouping them with arthropods (Figure 21-17). Sequence analysis supports alignment with arthropods in Ecdysozoa.
Recent discoveries of Cambrian fossil pentastomids and tardigrades and additional fossil onychophorans strongly suggest that these small phyla arose during the Cambrian explosion, just as did the major phyla. Because this period was long before terrestrial vertebrates evolved, the identity of the hosts for Cambrian pentastomids remains enigmatic; some authors have suggested that they might have been conodonts.
The early embryological development of sipunculans, echiurans, and annelids is almost identical, showing a very close relationship among the three. It is also similar to molluscan development. Some authors group the four phyla into a supraphyletic assemblage called “Trochozoa” because of the common possession of a trochophore larva. Other similarities, too, point to close relationship of the sipunculans to the echiurans and annelids, such as the nature of the nervous system and body wall. Sipunculans and echiurans are not metameric and thus are more primitive in that characteristic than annelids. They probably represent collateral evolutionary lines that branched from protoannelid stock before the origin of metamerism.
Several characters suggest relationship of Pogonophora to Annelida, as we noted previously. Molecular sequence analysis supports close relationships of Echiura, Sipuncula, and Pogonophora to Annelida and Mollusca in Lophotrochozoa.
Phylogenetic affinities of Pentastomida are uncertain. They have some similarities to Annelida. Their larval appendages and molting cuticle, however, are arthropod characteristics. Their larvae resemble tardigrade larvae. Most modern taxonomists align them with arthropods, however, and evidence is accumulating that they are most closely related to the crustacean subclass Branchiura. This evidence includes similarities in morphology of their sperm and in base sequences of ribosomal RNA. If pentastomids really are close to the branchiurans, then their status as a phylum should be revoked, and they should be classified as crustacean arthropods.
Onychophorans share a number of characteristics with annelids: metamerically arranged nephridia, muscular body wall, pigment cup ocelli, and ciliated reproductive ducts. Characteristics shared with arthropods include the cuticle, tubular heart and hemocoel with open circulatory system, presence of tracheae (probably not homologous), and large size of the brain. Unique characteristics include oral papillae, slime glands, body tubercles, and suppression of external segmentation.
Some authors believe the onychophorans should be included with the arthropods, but that would involve redefining the phylum Arthropoda. Manton* recommended placing the Onychophora with the myriapods and insects in the phylum Uniramia. However, most authors believe that the differences seem to warrant keeping them in a separate phylum (Figure 21-17). Sequence analysis supports placement of Onychophora in Ecdysozoa.
Affinities of tardigrades are among the most puzzling of all animal groups. They have some similarities to rotifers, particularly in their reproduction and their cryptobiotic tendencies, and some authors have called them pseudocoelomates. Their embryogenesis, however, would seem to put them among the coelomates. The enterocoelic origin of the mesoderm is a deuterostome characteristic. Other authors identify several important synapomorphies that suggest grouping them with arthropods (Figure 21-17). Sequence analysis supports alignment with arthropods in Ecdysozoa.
Recent discoveries of Cambrian fossil pentastomids and tardigrades and additional fossil onychophorans strongly suggest that these small phyla arose during the Cambrian explosion, just as did the major phyla. Because this period was long before terrestrial vertebrates evolved, the identity of the hosts for Cambrian pentastomids remains enigmatic; some authors have suggested that they might have been conodonts.
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| Figure 21-17 Cladogram depicting hypothetical relationships of Onychophora and Tardigrada to arthropods. Onychophorans diverged from the arthropod line after development of such synapomorphies as hemocoel and growth by ecdysis. They share several primitive characters with annelids, such as metameric arrangement of nephridia, but molecular evidence indicates inclusion in Ecdysozoa, not Lophotrochoza. Note that the tracheal system of onychophorans is not homologous to that of arthropods but represents a convergence. |
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