Earth History, Ch. 13 1
Ch. 13—Early Paleozoic Cambrian and Ordovician systems
• Phanerozoic Eon spans geologic time from end of Proterozoic Eon up to the present
– Cenozoic Era (65 Ma—present)
– Mesozoic Era (250 Ma—65 Ma)
– Paleozoic Era (543 Ma—250 Ma)
• Cambrian and Ordovician systems comprise the Early Paleozoic
Cambrian
System
Ordovician
System
Early
Early
Middle
Middle
Late
Late
543
495
443
Earth History, Ch. 13 2
Early Cambrian life
Cambrian
Period
• Conodonts (primitive chordates) appeared in earliest Cambrian time
Earth History, Ch. 13 3
Conodonts
Earth History, Ch. 13 4
• Middle part of Early Cambrian is called “Tommotian Stage”
• Tommotian fauna is the earliest diverse “shelly” fauna
– Did shells evolve in response to the earlier evolution of teeth?
– Variety of animals that cannot be related to anything in post-Tommotian time
– First sponges, brachiopods
– First reefs: built by archaeocyathids
Cambrian
Period
Earth History, Ch. 13 5
Tommotian fauna
Primitive mollusk
Fossils of
unknown biologic
affinity
Earth History, Ch. 13 6
Tommotian fauna
Archaeocyathid—A calcareous animal
that first appeared in the Tommotian, then
built the earliest true reefs somewhat later
in Early Cambrian time
Earth History, Ch. 13 7
Cambrian and Ordovician Life
• “Cambrian explosion”: Early and Middle Cambrian appearance of most phyla of invertebrates
• Ordovician was a major time of evolutionary radiation; also appearance of graptolites, rugose corals, tabulate corals, stromatoporoids, land plants (?)
• By late Ordovician, complex marine communities had become established
Earth History, Ch. 13 8
spore plant cells
Primitive land plants are known from Silurian rocks, but so far only spores and sheets of cells have been
recovered from the Late Ordovician
Earth History, Ch. 13 9
Ordovician
radiation
Cambrian
explosion
Earth History, Ch. 13 10
The Cambrian Explosion
The rapid diversification of animals
Earth History, Ch. 13 11
Geologic time • Age of Earth is 4.6 billion years
• If time were distance and 4.6 billion years
equalled 46 feet, then
– 100 million years = 1 ft
– 1 million years = 0.01 ft
4.6 Ga
today
Earth History, Ch. 13 12
• 540 Ma (40.6 feet up the ladder)
– earliest evidence of predators
• 570 Ma (40.3 feet up the ladder)
– earliest evidence of metazoans
• 2.1 Ga (25 feet up the ladder)
– fossil eukaryotes
• 3.5 Ga (12 feet up the ladder)
– fossil prokaryotes and stromatolites
• 3.8 Ga (8 feet up the ladder)
– carbon isotope evidence of photosynthesis
Precambrian life
Earth History, Ch. 13 13
The “Cambrian Explosion”
Earth History, Ch. 13 14
“Cambrian Explosion”
• Chordates and most phyla of invertebrates originated between 520 Ma and 510 Ma
– 10 My = ~1” on the ladder of time
• Best faunas are Chengjiang (China) and Burgess Shale (British Columbia)
Burgess
ShaleChengjiang
bacteria X X
algae X X
sponges X X
cnidarians X X
ctenophores X X
brachiopods X X
mollusks X absent
hyoliths X X
priapulids X X
annelids X X
lobopods X X
arthropods X X
echinoderms X absent
hemichordates X X
chordates X X
Earth History, Ch. 13 15
The Burgess Shale
Walcott Quarry:
Protected as a
UNESCO “World
Heritage Site”
Earth History, Ch. 13 16
Cambrian paleogeography
Earth History, Ch. 13 17
Middle Cambrian of Laurentia
• Laurentian craton was
ringed by a concentric
pattern of sedimentary
environments
– Nearshore detrital
belt
– Shallow marine
carbonates
– Deep-water deposits
Earth History, Ch. 13 18
The Burgess Shale
• Location of the Burgess Shale quarry is in the
deep-water belt, but immediately adjacent to the
shallow marine carbonate belt
• Exquisite preservation of Burgess Shale fossils is
attributed to rapid burial in oxygen-free
environment
– Animals probably lived in shallow water carbonate
setting, then were swept into deeper water by turbidity
currents
Earth History, Ch. 13 19
The Burgess Shale
Mt Stephen,
near Field BC
Earth History, Ch. 13 20
The Burgess Shale
Charles Doolittle Walcott,
discoverer of the Burgess
Shale (1909)
Earth History, Ch. 13 21
Walcott’s Bio
• Secretary of Smithsonian Institution
• President, National Academy of Sciences
• Member, National Research Council
• Co-founder, Carnegie Institution
• Co-founder, National Park Service
• Co-founder, Advisory Committee for Aeronautics (now NASA)
• Discovered Burgess Shale in 1909, led quarrying parties in 1910-13, 17, 19, 24
• Died in 1927 at age 77
Earth History, Ch. 13 22
Walcott’s Woes
• 1st wife died 1911; two sons died 1913,
1917
• Re-married 1914 (at age 64), but father-in-
law would not attend wedding because of
Walcott’s “questionable character”
Earth History, Ch. 13 23
Earth History, Ch. 13 24
Earth History, Ch. 13 25
Earth History, Ch. 13 26
Earth History, Ch. 13 27
Trilobite Olenoides,
with delicate appendages
preserved
Bizzare beasts of
the Burgess Shale
Earth History, Ch. 13 28
Modern onychophoran
Aysheaia
Bizzare beasts of the Burgess Shale
Earth History, Ch. 13 29
Pikaia
Aysheaia
Bizzare beasts of
the Burgess Shale
Earth History, Ch. 13 30
Hallucigenia
Opabinia
Canadaspis
Bizzare beasts of
the Burgess Shale
Earth History, Ch. 13 31
Marella
Sanctacaris
Nectocaris Bizzare beasts of
the Burgess Shale
Earth History, Ch. 13 32
Burgess seafloor scene
Earth History, Ch. 13 33
Burgess
seafloor scene
Earth History, Ch. 13 34
Significance of the Burgess Shale
• Was the Cambrian explosion truly a burst of
evolutionary diversification, or….
• Was there a long record of Precambrian
diversification for which there is no
preserved fossil record?
• Why have there been few new phyla since
Cambrian time?
