The Early Paleozoic Fauna:

1
The Early Paleozoic Fauna Ordovician radiation
of the Cambrian survivors
EPSC233 Earth Life History (Fall 2002)
2
Recommended reading STANLEY Earth System
History Chapter 13, pp. 348-355.
Keywords phyla (arthropods, poriferans,
brachiopods, echinoderms, mollusks,
hemichordates), reef builders (archeocyathids),
deposit feeders (trilobites, mollusks), filter
feeders (eocrinoids, crinoids, brachiopods,
mollusks), predators (cephalopods).
3
Apart from the occasional footprint, there is no
evidence of terrestrial animal life in Cambrian
and Ordovician sedimentary rocks. Oxygen levels
had probably reached values close to the present
atmospheric level.
4
Throughout the Paleozoic era, the rocks
containing the most diverse fossils are
typically ancient shallow-water marine
limestones, and particularly fossilized reefs.
5
Reef CaCO3framework of biogenic origin that rose
over the sea floor. This framework is solid from
the start and is not compacted or crushed during
burial.
6
The earliest reef builders in shallow Paleozoic
seas were not corals... Other organisms have the
ability to build housings of CaCO3 and thrive in
shallow water. Stromatolitic reefs of the
Precambrian were replaced by algal reefs (like
those next to the Burgess Shale) and by new types
of animals... the archeocyathids.
7
Archeocyathids were simple, filter-feeding,
sponge-like animals (phylum Porifera). Their
calcified skeletons built the earliest metazoan
(i.e. animal) reefs. Calcified corals had not
evolved yet.
8
These simple animals diversified and spread
rapidly over 200 genera are known worldwide from
the middle Cambrian. But they went extinct before
the end of the Cambrian period.
9
Poriferans (sponges) and cnidarians (jellyfish,
sea pens and corals), the simplest type of
animals found today, were probably among the
earliest to appear. Several Ediacaran fossils are
interpreted as poriferans or cnidarians because
of their radial symmetry.
10
Sponges (poriferans) had been around since the
latest Precambrian. Most of them leave little
trace in the fossil record. In some cases, their
soft body is supported by a flimsy skeleton of
mm-size spicules which falls apart upon death.
Usually, scattered spicules are the only remains
found.
11
An early Cambrian reef was a busy place good for
shelter and to ambush prey.
12
During the Ordovician period new animals will
evolve to build reefs. Cnidarians (sea anemones,
jellyfish) give rise to calcified
corals. Poriferans give rise to
stromatoporoids. A type of filter-feeding worm
gives rise to a new phylum, the bryozoans.
13
Primitive corals first appear in the Ordovician.
Some were solitary, horn shape (rugose corals).
Their growth bands have been used to calculate
the number of days in Paleozoic years.
14
Other corals are colonial, like this tabulate
coral Favosites.
15
stromatoporoids sponge-related (not corals)
Shapes like stromatolites, but the framework
clearly includes mineralized pillars and layers
that used to support a filter-feeding organism.
16

• Life in sediments
• advantage offers shelter
• disadvantage
• access to oxygen?
• how not to choke in mud
• Life on sediment
• but how not to sink in...

17
Brachiopods (a phylum of shelly invertebrates)
will become the most abundant shelly fauna of the
Paleozoic era. Most shells on todays beaches
are bivalve mollusks (clams, oysters,
mussels). Until the end of the Paleozoic,
brachiopods were the most successful group in the
niche occupied today by mollusks.
18

• Mollusks and brachiopods belong to two different
  phyla.
• They show similar use of hard parts even though
  they are different types of invertebrate
  animals.
• Their shells are
• anchors for muscles
• offer protection from predators
• help keep out sediment from breathing and
  feeding organs.

19
Brachiopods are adapted to life on top of the
sediment. Many have a pedicle (brown) to anchor
themselves to a solid object (reef, or other
shells or rock).