Deuterostomes: echinoderms and hemichordates
• Echinoderms today include sea urchins, starfish and sea cucumbers. They are all equipped with a water vascular system, a mesodermal skeleton of calcitic plates with a stereom structure, pentameral symmetry and tube feet.
• During the Cambrian radiation many bizarre forms evolved. The spindle-shaped Helicoplacus may be part of the stem group for the entire phylum but did not survive the Cambrian substrate revolution.
• Pelmatozoans were mainly fixed echinoderms and include the blastoids, crinoids and cystoids; the crinoids include four classes: the Inadunata, Flexibilia, Camerata and Articulata.
• The echinoids were part of the mobile benthos. During the Mesozoic irregular groups, adapted for burrowing, evolved from the more regular forms that characterized the Paleozoic.
• Asteroids (starfish) were more important in post-Paleozoic rocks; their Triassic radiation may have inhibited the re-radiation of some key groups of brachiopod.
• Carpoids are traditionally classed with the echinoderms, although some have argued they were ancestral to chordates; they were probably stem-group echinoderms.
• Graptolites are hemichordates closely related to the living rhabdopleurids with similarly constructed rhabdosomes and ultrastructure.
• Dendroids, with autothecae and bithecae together with many stipes, and graptoloids, with generally fewer stipes and only one type of theca, are the two most common grap-tolite orders.
• Graptolites probably pursued benthic (dendroids), planktic (dendroids and graptoloids) and automobile (graptoloids) lifestyles.
• Graptoloids evolved rapidly and were widespread, the ideal zone fossils in rocks of Ordovician-Silurian and Early Devonian age.
The echinoderms and hemichordates appear to be two very different groups of animals, one characterized by five-fold symmetry and a water vascular system, the other a group of odd stick-like colonial organisms. Surprisingly, both are closely related to each other and, moreover, are not so distant from ourselves, the chordates. Both groups are deu-terostomes; the first opening to develop in the embryo is the anus and a second forms the mouth. The group has a dipleurula larva and a body cavity that developed from an extensions of the embryonic gut (see p. 240). Modern morphological and molecular analyses indicate that the echinoderms and hemi-chordates are in fact sister groups (Smith 2005). A small, extinct group - the Vetulicolia - so far known only from the Cambrian, has also been related to the deuterostomes because of similar gill structures and the absence of limbs. But although recent finds from Utah have suggested that this group has more in common with the arthropods and probably belongs to the ecdysozoans (see p. 361), the group remains an enigma (see Box 15.10).
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