GEOL 102 Historical Geology
Spring Semester 2012
The Early Paleozoic Era II: When Trilobites Ruled the Earth
Marine life of the early Paleozoic
Based on statistical work by Jack Sepkoski, marine invertebrate communities are often
broken down into three separate
- The Cambrian fauna (or Trilobite fauna): trilobites, archaeocyathids,
hyoliths, monoplacophorans, inarticulate brachiopods, primitive echinoderms
- The Paleozoic fauna (or Brachiopod fauna): rhynchonelliform brachiopods,
stony and lacy bryozoans, stromatoporoids, cephalopods, crinoids and blastoids, starfish,
- The Modern fauna (or Bivalve-Gastropod fauna): bivalves, gastropods,
vertebrates, echinoids, crustaceans, gymnolaemate bryozoans
All three categories exist in the Cambrian, and persist until the present (even if some
component members have died off). However, these "packages" of distantly related groups
tend to be common at the same time, or rare at the same time.
The Cambrian fauna dominates during the Cambrian, remains common in the Ordovician, and
became progressively rarer in the Silurian and later. The Paleozoic fauna is rare in the
Cambrian, becomes more common in the Ordovician, and dominates the rest of the Paleozoic:
it remains an important part of the Mesozoic and Cenozoic seas. The Modern fauna is very
rare in the Cambro-Ordovician, but continues a stead rise throughout the Phanerozoic: in
the post-Paleozoic it is the most abundant fauna.
Life in the Cambrian:
Very different from present seas, or even post-Ordovician seas:
- Almost no large animals, and very few predators (largest animal is only large
predator, 1 m long
- Only shallow burrowers and short encrusters: most life concentrated near sediment-
- Reef builders:
archaeocyathid sponges (in earlier Cambrian only: almost no middle
or later Cambrian reefs)
Some important groups:
Other Cambrian life includes first appearances of:
- Two-shelled filter feeders
- Dominant groups in Cambrian are "inarticulates":
linguates (infaunal forms with calcium phosphate shells) and craniiforms
epifaunal forms with calcite shells)
Rhynchonelliform (formerly "articulate") brachiopods ([mostly sessile] epifaunal with calcite shells) are present but rare
- Deuterostomes with five-fold body symmetry, calcite test, and a
specialized water-vascular system
- Most Cambrian
echinoderms were stalked (and thus sessile), but some were motile
Conodonts (appear in Late Cambrian):
- A group of chordates, very likely craniates, and possibly even vertebrates
- Known almost exclusively from their hard (calcium phosphate)
- Soft tissue preservation allows us to see that they had flattened elongate
"eel-like" bodies and the elements formed complex mouth parts
- Were probably fast swimming micropredators
- Survived until end of the Triassic.
- Other vertebrates (jawless "fish"):
- Cambrian vertebrates known from bony plates and impressions of
lamprey-like forms from Chengjiang.
Most Cambrian organisms are only known from their hard parts, but the Early Cambrian
Chengjiang site in China and the Middle Cambrian
Burgess Shale in British Columbia preserve soft-tissue impressions.
Terminal Cambrian Extinctions:
- Mass extinction of trilobites, primitive echinoderms
- Glaciation and anoxia both implicated
- Actually was most likely several pulses of mass extinctions
Life of the Ordovician:
Cambrian fauna still common, but Paleozoic fauna on the rise.
The Ordovician Radiation (also called the Great Ordovician Biodiversification Event or GOBE): Life moves away from substrate-water interface.
Appearance of deep burrows (worms, clams, etc.), of tall attached epifauna (bryozoans, crinoids,
blastoids, etc.), and some diversification of nektonic forms.
Some important groups:
- Stalked echinoderms
calcareous green algae forming small reef mounds
- Planktonic colonial animals related to echinoderms (and more distantly to vertebrates)
- Main index fossils of the Ordovician
- Mostly preserved by carbonization
- Paraphyletic grade of shelled
- 1rst common large predators
- Sessile colonial filter-feeders
- Evidence now of rare Cambrian bryozoans, but only diversify in Ordovician
- Often forming calcareous skeletons, either massive (stony bryozoans) or delicate
- Rugose corals
(horn corals or tetracorals):
- Sessile, mostly solitary, with very large polyps
- Tabulate corals:
- Sessile, colonial corals.
- Some evidence for possible rare Cambrian tabulates.
- Reef-forming sponges with calcareous skeletons.
Radiations of articulate brachiopods, gastropods (snails), echinoderms (especially
stalked crinoids and blastoids).
Decline of stromatolites: Probably due to more specialized grazers (gastropods,
1rst tabulate-stromatoporoid reefs
(more important in middle Paleozoic).
Fish diversity increases, but still jawless. The bony-armored jawless fish are sometimes called "ostracoderms":
this is a paraphyletic grade rather than a clade. Also, oldest good evidence of terrestrial plants.
Terminal Ordovician Extinctions:
- Disappearance of one third of all brachiopod and bryozoan families, as well as
many groups of conodonts, trilobites, and graptolites
- Associated with massive Gondwanan ice age
Silurian marine life:
- Decline of the Cambrian fauna: trilobites survive the terminal Ordovician extinctions,
but at reduced diversity
- Increase in the abundance and distribution of tabulate-stromatoporoid reefs
advanced jawless fish: development of paired fins as stabilizers
- Towards the end of the Silurian, the earliest jawed fish
- Radiation of bivalves (clams)
- "Sea scorpions", although found in brackish (and possibly fresh water) deposits
- Large arthropod predators (up to 3 m long)
- Some capable of short duration travel out of water
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Last modified: 19 January 2012