Page 252 - Introduction to Paleobiology and The Fossil Record

ORIGIN OF THE METAZOANS 239

Neoproterozoic Paleozoic Mesozoic Cenozoic
Vendian C O S D C P Tr Jr K
Cnidaria
Porifera
Mollusca
Brachiopoda
Ctenophora
Onychophora
Arthropoda
Priapulida
Echinodermata
Annelida
Chordata
Hemichordata
Tardigrada
Bryozoa
Nematoda
Nemertea
Echiura
Entoprocta
Rotifera
Nematomorpha
Placozoa
“Mesozoa”
Platyhelminthes
Gnathostomulida
Gastrotricha
Acanthocephala
Loricifera
Kinorhyncha
Pogonophora
Sipuncula
Phoronida
Urochordata

Figure 10.4 Appearance of the main animal phyla and some other high-level taxonomic groups.
Geological period abbreviations are standard, ranging from Cambrian (C) to Cretaceous (K). (Based on
Valentine 2004.)

kingdom. The first metazoans were multicel-
Invertebrate body and skeletal plans
lular with one main cell type and peripheral
Life on our planet has been evolving for nearly collar cells or choanocytes, equipped with a
4 billion years. Molecular data suggest meta- whip or fl agellum (Nielsen 2008). There are
zoans have probably been around for at least three main body plans (Table 10.1).
at 550 myr, during which time, according to The parazoan body plan, seen in sponges, is
some biologists, as many as 35 separate phyla characterized by groups of cells usually orga-
have evolved. In recent years, new molecular nized in two layers separated by jelly-like mate-
phylogenies have completely changed our rial, punctuated by so-called wandering cells or
views of animal relationships and thus the amoebocytes; the cell aggregates are not differ-
importance of invertebrate body and skeletal entiated into tissue types or organs. In fact
plans. They are important from a functional molecular phylogenetic studies have suggested
point of view, but are potentially highly mis- that sponges are paraphyletic (see p. 262) so
leading if simply read as telling an evolution- this is only a grade of organization.
ary story. Despite the infi nite theoretical The diploblastic grade or body plan, typical
possibilities for invertebrate body plans, rela- of cnidarians and the ctenophorans, has two
tively few basic types have actually become layers – an outer ectoderm and an inner endo-
established and many had evolved by the derm and epithelia. These two layers are sepa-
Cambrian (Fig. 10.4). These body plans are rated by the acellular, gelatinous mesogloea.

usually defined by the number and type of The triploblastic body plan, seen in most
enveloping walls of tissue together with the other animals, has three layers of tissues from
presence or absence of a celom (Fig. 10.5). the outside in: the ectoderm, mesoderm and
The basic unicellular grade is typical of protist endoderm. Superimposed on this body plan is
organisms and is ancestral to the entire animal the bilateral symmetry that defi nes the bilate-