Page 268 - Introduction to Paleobiology and The Fossil Record

ORIGIN OF THE METAZOANS 255

Box 10.7 Larvae and the Ordovician radiation

Many factors, mainly ecological and environmental, have been invoked to explain the great Ordovi-

cian biodiversification or Ordovician radiation. Did the diversification have its origins in the plank-
ton? Most early bilaterians probably had benthic lecithotrophic larvae (see p. 241). But the Cambrian
oceans, relatively free of pelagic predators, offered great possibilities. Exploitation of the water
column by larvae occurred a number of times independently, turning the clear waters of the Early
Cambrian into a soup of planktonic organisms in the Ordovician. The fossil record and molecular
clock data suggest that at least six different feeding larvae developed from non-feeding types between
the Late Cambrian and Late Silurian (Peterson 2005). In addition to planktotrophic larvae, the
oceans were rapidly colonized by diverse biotas of other microorganisms such as the acritarchs (see

p. 216). The dramatic diversification of the suspension-feeding benthos coincides with the evolution
of planktotrophy in a number of different lineages (Fig. 10.18). These factors had an undoubted

effect on the diversification of Early Paleozoic life, which reached a plateau of diversity during the
Ordovician.

suspension feeding taxa
trace fossil genera
Planula
Cnidaria
1800
Tadpole
Ascidia
1600 Crinozoa
Feeding Eleutherozoa
1400 Dipleuruala
Dipleurula
Hemichordata
Tornaria
1200 Veliger Pteriomorphia
Number of taxa/genera 1000 Veliger Nuculoida

Vetigastropoda
800

Phyllodocida
600 Sorbeochoncha
Trochophores Feeding
trochophores Spionidae+
Serpulidae
400
Hoplonemertea
Pilidium
200 Heteronemertea
Ecdysozoa
0
Early M L E M L E ML E M Late Early Late Early Late
Ediacaran
Cambrian Ordovician S Devonian Carboniferous Permian
600 500 400 300 Ma
Figure 10.18 Origin of larval types and the Ordovician radiation as deduced from the fossil
record and molecular clock data. The numbers of genera of key suspension-feeding taxa are
indicated on the histogram in light tint, and, in dark tint, the numbers of genera of trace fossils.
(Based on Peterson 2005.)