Page 289 - Introduction to Paleobiology and The Fossil Record

276 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

rounded by tentacles with stinging cells or enteron, forming radial partitions that increase
nematocysts, functions both as a mouth and the area of absorption of nutrients. These
an anus. There is thus no head or tail, and mesenteries can, in the case of the corals,
nutrients and waste pass through the same secrete calcium carbonate to form solid, calci-

opening. The body itself, although diploblas- fied partitions or septa. Most species are
tic, is, in fact, composed of three layers; the found in marine environments although
inner endoderm and the outer ectoderm both hydrozoans can be very abundant in fresh-
consist of living cells while the intervening water habitats.
mesoglea is a gelatinous, acellular substance
containing rare cells. The outer layer of the
body wall contains cnidoblast cells that Classifi cation: design and relationships of
contain the primed stings or nematocysts that the main groups
are usually confi ned to the tentacles. A primi- The phylum Cnidaria is usually split into
tive nerve net is embedded in the mesoglea. three classes: hydrozoans, scyphozoans and
Fingers of endoderm commonly poke into the anthozoans (Box 11.4). The hydrozoans

Box 11.4 Classifi cation of Cnidaria

The phylum is characterized by radial symmetry, with the ectoderm and endoderm separated by the
mesoglea; the enteron has a mouth surrounded by tentacles with stinging cells. The phylum ranges
from Upper Precambrian to Recent. The putative medusoid Brooksella, which predates the Ediacara
fauna may, in fact, be a trace fossil. The group has a wide range of body plans (Fig. 11.18).
Class HYDROZOA

• This includes six main orders of small, usually polymorphic forms. Each has an undivided enteron
and solid tentacles, and may form colonies. There are six main orders; the Chondrophora con-
tains some of the oldest cnidarians
• Ediacaran to Recent
Class SCYPHOZOA

• Mainly jellyfish, contained in the Scyphomedusae, which are only preserved in Lagerstätten. The

extinct Conulata is often included here since the group has a tetrameral symmetry and apparently
has tentacles. Their long conical shells, for example Conularia, are composed of chitinophos-
phate; conulates appeared in the Cambrian and were extinct by the Mid Triassic
• Ediacaran to Recent
Class ANTHOZOA
• These are exclusively marine, and most are sessile, colonial forms (though they have mobile
planula larvae). The three subclasses, Ceriantipatharia, Octocorallia and Zoantharia (including
the orders Rugosa, Tabulata and Scleractinia), all lack medusoid stages, possess hollow tentacles
and have the enteron divided, longitudinally, by vertical septa. Both solitary and colonial forms
occur. The class includes corals, sea anemones and sea pens. Octocorals often produce spicules
that occur as microfossils
• Ediacaran to Recent
Class CUBOZOA

• The sea wasps and box jellyfish have both medusae and polyps and are mainly restricted to

tropical and subtropical latitudes
• Carboniferous to Recent