GEOL 102 Historical Geology
Spring Semester 2017
The Early Paleozoic: The Cambrian and Ordovician
So now we are at the Phanerozoic:
Global and Regional Geology of the Cambrian and Ordovician
Starting with the Paleozoic:
Phanerozoic Eon: 541 - 0 Ma
Paleozoic Era: 541.0 - 252.902 Ma
Base of Cambrian (and thus of the Paleozoic, and Phanerozoic):
- Historically, the first appearance of macroscopic shelly fossils (the
- 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
- 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
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) glaciations.
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 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, 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)
Some important groups:
Other Cambrian life includes first appearances of:
- 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 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
- Receptaculitids: 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(lacy bryozoans)
- 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, 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
- More 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
To Lecture Notes.
Last modified: 19 January 2017