Geology Reference
In-Depth Information
impact of predation during the Cambrian was great, lead-
ing some scientists to hypothesize that the rapid evolution
of a shelly invertebrate fauna was a response to the rise of
predators. With predators playing an important role in the
Cambrian marine ecosystem, any mechanism or feature that
protected an animal would certainly be advantageous and
confer an adaptive advantage to the organism.
Scientists currently have no clear answer as to why
marine organisms evolved mineralized skeletons during the
Cambrian explosion and shortly thereafter. They undoubt-
edly evolved because of a variety of biologic and environ-
mental factors. Whatever the reason, the acquisition of a
mineralized skeleton was a major evolutionary innovation
that allowed invertebrates to successfully occupy a wide
variety of marine habitats.
divided into two main groups: the fl oaters, or
plankton
, and
the swimmers, or
nekton
.
Plankton are mostly passive and go where currents
carry them. Plant plankton, such as diatoms, dinofl agellates,
and various algae, are called
phytoplankton
and are mostly
microscopic. Animal plankton are called
zooplankton
and are
also mostly microscopic. Examples of zooplankton include
foraminifera, radiolarians, and jellyfish. The nekton are
swimmers and are mainly vertebrates such as fi sh; the inver-
tebrate nekton include cephalopods.
Organisms that live on or in the seafloor make up the
benthos
. They are characterized as
epifauna
(animals) or
epiflora
(plants)—those that live on the seafloor—or as
infauna
—animals that live in and move through the sediments.
The benthos are further divided into those organisms that stay
in one place, called
sessile
, and those that move around on or
in the seafl oor, called
mobile
.
The feeding strategies of organisms are also important
in terms of their relationships with other organisms in the
marine ecosystem. There are basically four feeding groups:
suspension-feeding
animals remove or consume micro-
scopic plants and animals, as well as dissolved nutrients from
the water;
herbivores
are plant eaters;
carnivore-scavengers
are meat eaters; and
sediment-deposit feeders
ingest sedi-
ment and extract the nutrients from it.
We can defi ne an organism's place in the marine ecosys-
tem by where it lives and how it eats. For example, a brachio-
pod is a benthonic, epifaunal suspension feeder, whereas a
cephalopod is a nektonic carnivore.
An ecosystem includes several
trophic levels
, which are
tiers of food production and consumption within a feeding
hierarchy. The feeding hierarchy and hence energy fl ow in
an ecosystem comprise a food web of complex interrelation-
ships among the producers, consumers, and decomposers
(
MARINE LIFE
Having considered the origin, differentiation, and evolution of
the Proterozoic-Cambrian marine biota, we now examine the
changes that occurred in the marine invertebrate community
during the Paleozoic Era. Rather than focusing on the history
of each invertebrate phylum (Table. 21.1), we will survey the
evolution of the Paleozoic marine invertebrate communi-
ties through time, concentrating on the major features and
changes that took place. To do that, we need to briefl y examine
the nature and structure of living marine communities so that
we can make a reasonable interpretation of the fossil record.
In analyzing the present-day marine ecosystem, we must
look at where organisms live, how they get around, and
how they feed (
Figure 21.2). Organisms that live in the
water column above the seafl oor are called
pelagic
. They are
◗
◗
Figure 21.3). The
primary producers
,or
autotrophs
, are
TABLE 21.1
The Major Invertebrate Groups and Their Stratigraphic Ranges
Phylum Protozoa
Class Sarcodina
Order Foraminifera
Order Radiolaria
Phylum Porifera
Class Demospongea
Order Stromatoporoida
Phylum Archaeocyatha
Phylum Cnidaria
Class Anthozoa
Order Tabulata
Order Rugosa
Order Scleractinia
Phylum Bryozoa
Phylum Brachiopoda
Class Inarticulata
Class Articulata
Cambrian-Recent
Cambrian-Recent
Cambrian-Recent
Cambrian-Recent
Cambrian-Recent
Cambrian-Recent
Cambrian-Oligocene
Cambrian
Cambrian-Recent
Ordovician-Recent
Ordovician-Permian
Ordovician-Permian
Triassic-Recent
Ordovician-Recent
Cambrian-Recent
Cambrian-Recent
Cambrian-Recent
Phylum Mollusca
Class Monoplacophora
Class Gastropoda
Class Bivalvia
Class Cephalopoda
Phylum Annelida
Phylum Arthropoda
Class Trilobita
Class Crustacea
Class Insecta
Phylum Enchinodermata
Class Blastoidea
Class Crinoidea
Class Echinoidea
Class Asteroidea
Phylum Hemichordata
Class Graptolithina
Cambrian-Recent
Cambrian-Recent
Cambrian-Recent
Cambrian-Recent
Cambrian-Recent
Precambrian-Recent
Cambrian-Recent
Cambrian-Permian
Cambrian-Recent
Silurian-Recent
Cambrian-Recent
Ordovician-Permian
Cambrian-Recent
Ordovician-Recent
Ordovician-Recent
Cambrian-Recent
Cambrian-Mississippian
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