Base of Cambrian (and thus of the Paleozoic, and Phanerozoic):
Historically, the first appearance of macroscopic shelly fossils (the
Cambrian Explosion)
At present, first appearance of burrowing trace fossil Tretpichnus pedum (formerly Phycodes pedum or Tricophychus pedum), very likely the burrows of priapulid worms
This event is dated radiometrically at about 541 Ma, and is thus older (by about 15 million years or more) than the "Cambrian Explosion" proper
The "Long Fuse" of the Cambrian Explosion:
Fortunian Age (once "Manykaian Age", first part of Terrenuevian Epoch): first good infaunal trace fossils, some rare calcareous fossils
Cambrian Age 2 (formerly "Tommotian Age", second part of Terreneuvian Epoch):
Small Shelly Fauna or (SSFs):
c. 1-3 mm long
Various calcareous (also some silica, some calcium phosphate) fossils
Represent a variety of organisms: sponges, mollusks, annelids, possible brachiopods
Archaeocyathids:
Main Early Cambrian reef formers, probably a type of sponge
Cone-in-cone structure, calcareous skeleton
Small (few cms tall)
1rst appear in "Tommotian"; die off at end of Cambrian Epoch 2
Cambrian Age 3 (formerly "Atdbanian Age", first part of Cambrian Epoch 2): The Cambrian Explosion proper:
Appearance of macroscopic calcareous (and silica and calcium phosphate) hard parts across the Tree of Life, especially within Lophotrochozoa (in brachiopods and mollusks), Ecdysozoa (in arthropods), and in Deuterostomia (in echinoderms and vertebrates)
Because these groups are either known or inferred to have been present earlier in the Cambrian (or even Ediacaran), the Cambrian Explosion is now known to have NOT been the sudden origin of these groups. Instead, it appears to be a biogeochemical event: a time when conditions were such that biomineralization became easier for animals, and many different already-existing lineages could take advantage of it. (That doesn't mean that there isn't a big diversification because of it: after all, being able to make skeletons was extremely useful and there are adaptive radiations because of it. But it does mean that the groups themselves had a long history already.)
More on Cambrian life later.
The Cambrian Substrate Revolution
Evidence for microbial matgrounds in shallow marine sediments (siliciclastic & carbonate) during Neoproterozoic:
Layers well laminated, almost no vertical bioturbation
Water-Sediment interface probably very sharp
Postulated ecohabits include: mat scratchers, mat miners, mat stickers, mat encrusters
During Cambrian, rise of some bioturbators, with limited vertical mining abilities.
Presence of surviving mat scratchers (diverse ancestral mollusks, esp. diverse monoplacophorans and polyplacophorans), and some probable mat-sticking echinoderms (helioplacoids). As rise of deeper burrowers and grazing increases in late Cambrian, increasingly Phanerozoic-style substrates:
Poorly laminated sediments in shallow water
Water-Sediment interfaced blurred and "soupy"
Extinction of taxa like helioplacoids that attached directly to hard sediment-water interface;
migration of mat scratchers and mat miners upwards (to rocky shores) or downwards (to deep water)
Paleogeography and Tectonics of
the Cambrian
Pannotia (supercontinent) breaks apart at or near the base of the Cambrian. Breaks up into:
Gondwana (modern South America, Africa, Madagascar, India, Antarctica, Australia,and assorted fragments): mostly emergent in Cambrian
Laurentia (modern North America (except for far west) and Greenland); mostly submerged by end of Cambrian
Separated from Baltica and Gondwana by Iapetus Ocean
Baltica (much of modern western and northern Europe): mostly submerged
Separated from Gondwana by Tornquist Sea, from Siberia by Paleo-Asian Ocean
Siberia (much of northwestern Asia)
various Asian blocks: all mostly submerged
Global (eustatic) transgressions in Middle & Late Cambrian:
In western Laurentia, huge onlap sequences (the Tonto Group exposed from California to the Grand Canyon)
Across much of world, very large carbonate banks form
Paleogeography and Geology of the Ordovician:
Gondwana moves poleward
During Late Ordovician, major (but brief) ice age.
In eastern Laurentia, Early & Middle Ordovician huge stable carbonate platform (continuation of Cambrian).
During Middle-Late Ordovician, the Taconic Orogeny (first of the Appalachian orogenies):
Small microcontinent (or "exotic terrane") Avalonia breaks off from Gondwana, travels through Iapetus (proto-Atlantic) Ocean
Volcanic arc produced as Avalonia comes close to eastern Laurentia, producing giant ash marker bed (the Millbrig Bed of North America, the Big Bentonite of western Europe)
In eastern Laurentia, major down drop of foreland basin generates thick flysch and thicker molasse ("clastic wedge") deposits extend far into inland seas of Laurentia.
Collision and suturing between Avalonia and Laurentia possible in northern part of Appalachian region; however, still seem to have been separate further south.
Glaciers at end of Ordovician produce pulses of extinction.
Evolution of Life in the Cambrian and Ordovician
Based on statistical work by Jack Sepkoski, marine invertebrate communities are often broken down into three separate "evolutionary faunas":
The Paleozoic fauna (or Brachiopod fauna): rhynchonelliform 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, 1-2 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)
Dominant groups in Cambrian are "inarticulates": linguates (infaunal forms with calcium phosphate shells) and craniforms epifaunal forms with calcite shells)
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.
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. 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
![Paleogeography](https://www.geologypage.com/wp-content/uploads/2014/02/Cambrianperiod.jpg"){kind=link}
