Page 266 - Introduction to Paleobiology and The Fossil Record
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ORIGIN OF THE METAZOANS 253
Budd &
Traditional Gould 1989 Fortey et al. 1996 Jensen 2000
Recent
Relative
disparity
Time
Cambrian
?
Figure 10.16 Modes of the Cambrian explosion. (Based on Budd & Jensen 2000.)
Finally, during the Mid Ordovician, the Earth
Ordovician radiation
was bombarded with asteroids that appear in
During an interval of some 25 myr, during the some way also to be linked to the biodiver-
Mid to Late Ordovician, the biological com- sification (Schmitz et al. 2008). Taken together,
ponent of the planet’s seafl oors was irrevers- these conditions were ideal for all kinds of
ibly changed. A massive hike in biodiversity speciation processes and the evolution of eco-
was matched by an increase in the complexity logical niches. Most significant was the diver-
of marine life (Harper 2006). The event wit- sification of skeletal organisms, including the
nessed a three- to four-fold increase in, for brachiopods, bryozoans, cephalopods, con-
example, the number of families, leveling off odonts, corals, crinoids, graptolites, ostra-
at about 500; these clades would dominate codes, stromatoporoids and trilobites that we
marine life for the next 250 myr. Nevertheless will read about later.
the majority of “Paleozoic” taxa were Whereas the Cambrian explosion involved
derived from Cambrian stocks. With the the rapid evolution of skeletalization and a
exception of the bryozoans (see p. 313), no range of new body plans, together with the
new phyla emerged during the radiation, extinction of the soft-bodied Ediacara biota
although more crown groups emerged from and the appearance of the Bilateria, the Ordo-
the stem groups generated during the Cam- vician diversification generated few new
brian explosion. higher taxa, for example phyla, but witnessed
The great Ordovician radiation is one of a staggering increase in biodiversity at the
the two most signifi cant evolutionary events family, genus and species levels. This taxo-
in the history of Paleozoic life. In many ways nomic radiation, which included members of
the Ordovician Period was unique, enjoying the so-called “Cambrian”, “Paleozoic” and
unusually high sea levels, extensive, large epi- “Modern” evolutionary biotas (see p. 538),
continental seas, with virtually fl at seabeds, set the agenda for much of subsequent marine
and restricted land areas, many probably rep- life on the planet against a background of
resented only by archipelagos. Magmatic and sustained greenhouse climates. Although
tectonic activity was intense with rapid plate many outline analyses have been made, there
movements and widespread volcanic activity. are relatively few studies of the ecological and
Island arcs and mountain belts provided environmental aspects of the Ordovician
sources for clastic sediment in competition diversification (Bottjer et al. 2001). Moreover
with the carbonate belts associated with most the causes of the event, and its relationship to
of the continents. Biogeographic differentia- both biological and environmental factors,
tion was extreme, affecting plankton, nekton are far from clear. Evolution of the plankton,
and benthos, and climatic zonation existed, however, may have been a primary factor
particularly in the southern hemisphere. (Box 10.7).
