Page 402 - Introduction to Paleobiology and The Fossil Record

Chapter 15

Deuterostomes: echinoderms

and hemichordates

Key points

• 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 fi xed 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 (starfi sh) 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.