Marine life of the early Paleozoic
Based on statistical work by Jack Sepkoski, marine invertebrate communities are often broken down into three separate “evolutionary faunas”:

• The Cambrian fauna (or Trilobite fauna): trilobites, archaeocyathids, hyoliths, monoplacophorans, inarticulate brachiopods, primitive echinoderms
• The Paleozoic fauna (or Brachiopod fauna): articulate brachiopods, stony and lacy bryozoans, stromatoporoids, cephalopods, crinoids and blastoids, starfish, graptolites
• 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 anomalocaridids);
• Only shallow burrowers and short encrusters: most life concentrated near sediment- water interface;
• Reef builders: archaeocyathid sponges (in earlier Cambrian only: almost no middle or later Cambrian reefs)

Some important groups:

• Trilobites:
  • Group of arthropod found only in Paleozoic
  • EXTREMELY common in Cambrian and Ordovician; still common but lower diversity in rest of Paleozoic
  • Benthic epifaunal detritovores (backwards facing mouth).
  • Known from many growth stages, thousands of species, trace fossils, etc.
  • The main index fossils of the Cambrian.

  Other Cambrian life includes first appearances of:

  • Brachiopods:
    • Two-shelled filter feeders
    • Dominant groups in Cambrian are "inarticulates": linguates (infaunal forms with calcium phosphate shells) and craniids epifaunal forms with calcite shells)
    • Articulate brachiopods (epifaunal with calcite shells) are present but rare
  • Echinoderms:
    • 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) tooth-like elements
    • Soft tissue preservation allows us to see that they had flattened elongate "eel-like" bodies
    • 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.

Life moves away from substrate-water interface: Appearance of deep burrows (worms, clams, etc.) and of tall attached epifauna (bryozoans, crinoids, blastoids, etc.).

Some important groups:

• Crinoids and blastoids:
  • Stalked echinoderms
  • Filter-feeders
• Receptaculitids: calcareous green algae forming small reef mounds
• Graptolites:
  • Planktonic colonial animals related to vertebrates and echinoderms
  • Main index fossils of the Ordovician
  • Mostly preserved by carbonization
• Nautiloids:
  • Shelled cephalopods
  • 1rst common large predators
• Bryozoans:
  • Sessile colonial filter-feeders
  • Often forming calcareous skeletons, either massive (stony bryozoans) or delicate (lacy bryozoans)
• Rugose corals (horn corals or tetracorals):
  • Sessile, mostly solitary, with very large polyps
• Tabulate corals:
  • Sessile, colonial corals.
• Stromatoporoids:
  • 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, echinoids, etc.).

1rst tabulate-stromatoporoid reefs (more important in middle Paleozoic). Fish diversity increases, but still jawless. 1rst 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
• More advanced jawless fish: development of paired fins as stabilizers
• Towards the end of the Silurian, the earliest jawed fish
• Radiation of bivalves (clams)

New taxa:
Eurypterids:

• "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: 2 January 2008