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GLY3603
Paleontology
Module 4 – Lecture 4
Life of the Ordovician.
Changes in Life Through Time:
The Paleozoic Era
A. Invertebrate Evolutionary Trends
1. Mobility to Stability.
• Cambrian assemblages were mostly mobile forms of metazoans, trilobites the most
abundant of them.
• The Ordovician Radiation included organisms that were sessile and had hard parts.
• An Early Ordovician change in seawater chemistry facilitated precipitation of calcite
over aragonite (a less stable mineral) from seawater and the formation of
widespread “hardgrounds” for invertebrate attachment.
A. Invertebrate Evolutionary Trends
Continued
2. Increased diversity and the rise of the “Paleozoic Fauna”
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A great adaptive radiation of invertebrate fauna.
Number of families of invertebrates increased from about 150 at the end of the Cambrian
to 400 during the Ordovician.
The Cambrian fauna was replaced by a fauna that persisted throughout the rest of the
Paleozoic, a period of more than 230 million years.
Epifaunal suspension feeders, especially bryozoans, brachiopods, corals and stalked
echinoderms, dominated this fauna, the “Paleozoic Fauna”.
Infauna included suspension feeders in shallow burrows and deposit feeders
Epifaunal deposits feeders, scavangers, grazers, parasites and predators also composed
the community
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A. Invertebrate Evolutionary Trends
Continued
3. Feeding Adaptations
• Suspension feeding of fixed organisms above the substrate, i.e. epifaunal
suspension feeding, was the predominant type of feeding.
• Various filtering devices evolved for filter-feeding: lophophores of bryozoans
and brachiopods, tentacles of corals, gills of bivalves, brachioles and arms of
crinoids and blastoids, and others.
A. Invertebrate Evolutionary Trends
Continued
3. Feeding Adaptations continued.
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Abundant suspension feeders maximized their ability to obtain food in crowded competition by:
• Feeding at a variety of heights above the sea floor, called tiering. Most food moves
horizontally in bottom currents so tiered animals do not feed from the same water moving
past them.
• Invertebrates fed on a variety of food sizes. Bryozoans fed on particles <100 microns in size
while cnidarians can eat small fish. Size selection of food was just another way of
differentiating suspension feeder food resources.
When the Ordovician began epifaunal suspension feeders were low in height with minimal tiering.
With Ordovician invertebrate radiation came increased tiering with crinoids reaching 50 to 100 cm
of height above the seafloor.
First burrowing (shallow) infaunal suspension feeders accompanied increased predation.
First significant predators (eurypterids and cephalopods).
B. Rise and diversification.
• First appearances occurred of corals, clams, bryozoans, starfish, echinoids or
crinoids.
• The articulate brachiopods were rare during the Cambrian but began a period of
major diversification in the shallow-water marine environment.
• Brachiopods dominated the Ordovician shallow water non-reef environments.
• Types were very much facies related with inarticulates very restricted to finer
sediments.
• Most Ordovician brachiopods had a wide hinge and coarse ribs (e.g. Orthids).
• Most Cambrian ones bore chitinous shells and were small. Ordovician
brachiopods were larger and more diverse.
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B. Rise and diversification continued.
• Foraminifera protozoans made their first appearance and increased in
abundance and diversity later in the Paleozoic.
• Ostracods first appeared and became “large” and abundant in some limestone.
• The first corals and tabulate (always colonial) corals appeared in the Lower
Ordovician but were small and subordinate to tabulate corals and
stromatoporoids.
• Crinoids became abundant for the first time.
• Stalked echinoderms including crinoids and blastoids diversified. Although rare,
the first crinoidal limestone (limestone with an abundance of crinoids) was
deposited.
B. Rise and diversification continued.
• “Cystoids”, a heterogeneous group of blastozoans, reached their maximum
abundance in the Ordovician and Silurian but were never very abundant.
• Very early in the period primitive starfish and brittle stars appeared and in the
Middle Ordovician the first sea urchin (echinoid) joinrd in.
• Bryozoans first appeared and diversified greatly becoming a major contributor to
Ordovician limestone.
• Trepostomes (“stony”) were the dominant bryozoans. Many of these were
branching stick-like or compact encrusting colonies.
• Three other orders appeared.
• In limestone and limy shale, bryozoans and brachiopods are the most
common invertebrate components.
B. Rise and diversification continued.
• Bivalves first appeared near the base of the Middle Ordovician, radiated and
became successful burrowing filter feeders but were not yet particularly
abundant.
• Gastropods diversified greatly.
• An increased diversity and abundance of acritarchs (organic-walled
phytoplankton of unknown affinity), the major phytoplankton group of the
Paleozoic Era, occurred. Acritarchs were the primary food source of the
suspension feeders that were becoming more common (more in Module 8).
• Trilobites reached their greatest abundance and diversity early in the period.
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C. Declines in invertebrate groups
and diversity.
• Invertebrate grazers (gastropods) decimated the algal
stromatolites that were so widespread before diversification of
metazoans.
• Late in the period trilobites began a long decline in abundance
and diversity until their extinction shortly before the end of the
Paleozoic.
D. Dominant invertebrates.
• Ordovician invertebrate communities dominated by
articulate brachiopods, bryozoa, and corals.
• Corals and bryozoa became primary reef builders. Other
reef builders included stromatoporoids and other
sponges.
• Most of the benthic community was composed of
bryozoans, brachiopods, corals, stalked echinoderms,
sponges, bilves, gastropods, graptolites, trilobits, starfish,
cephalopods, and conodonts.
E. Lesser contributors to Ordovician
assemblages.
• Cystoids, echinoids, bivalves, gastropods, non-stromatoporoid sponges,
ostracods, graptolites.
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F. Predators
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Common large predators, including shelled cephalopods.
• Rare and small in the Late Cambrian.
• The cephalopods were the dominant and fastest predators of the Ordovician and
were very common.
• Only 1 order in Cambrian but 10 in Ordovician.
• Most were open coiled (cyrtocone) or straight (orthocone).
• Some got between 15 and 30 feet and were the largest animals yet to live.
Eurypterids were significant predaceous arthropods.
• Cuticle skeleton so most fossils are carbon films.
• Most 10-20 cm but some up to 1.8 meters.
G. Reefs.
• Stromatoporoids
• Became a major reef forming group in the upper Middle Ordovician.
• Sponges, multilayered structure with small mounds on each layer and
star shaped canals centered on mounds.
• Interior usually recrystallized.
• Major reef formers to end of Devonian.
• Colonial rugose and tabulate corals began contributing to reefs in the
Middle Ordovician
G. Reefs.
• Development of an Ordovician reef:
• stabilization stage- stalked crinoids
• colonization stage- stromatoporoids and bryozoans
• diversification stage- corals, bryozoans, sponges, and stromatoporoids
• domination stage- stromatoporoids
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H. Guide fossils
• The three fossil groups most useful for Ordovician biostratigraphic correlation are the articulate
brachiopods, graptolites, and conodonts.
• Graptolites
• Pelagic and benthic colonial related to modern hemichordates.
• Along with the pelagic trilobites among the only fossils found in deep water shale.
• 13 zones in European Ordovician.
• Because some were pelagic they are very important in intercontinental correlation.
• Most frequently found in organic shale.
• Conodonts (see Module 8)
• Very abundant.
• Conodonts (phosphatic) are teeth of an eel-like creature.
• Chitinozoa (see Module 8)
• Extinct palynomorphs of unknown systematic position.
• Brachiopods
I. Vertebrates
• Vertebrates arose from Hemichordates (partial notochords), which
include modern acorn worms and extinct graptolites.
• Oldest from Cambrian Burgess Shale (Pikaia)
• Jawless vertebrates evolved in Late Cambrian, the armored Agnaths.
• Filter feeders
J. Plants.
• First very rare evidence of land plants in the form of cuticle and spores.
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K. Extinction.
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End of the Ordovician marked by a mass extinction.
57% generic extinction.
More than 100 families of marine invertebrates became extinct.
Large-scale loss of diversity in many groups, particularly stalked crinoids,
brachiopods, and trilobites.
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