Chapter 15
The Cambrian
Explosion and Beyond
Multicelluarity
First multicellular organisms began
approximately 600 Mya
First animals about 565 Mya
In about 40 My nearly every major
animal phylum appeared in fossil record
The Cambrian Explosion
The Phanerozoic Eon
Visible life
Nature of the Fossil Record
Fossil = any trace left by an organism
that lived in the past
Fossils originally defined boundaries of
eons, eras, periods, and epochs
Which part of the organism is preserved
and available for study?
What kinds of habitats produce fossils?
Nature of the Fossil Record
Four categories of fossils
Compression and Impression
• Organic matter is buried in water- or windborne sediment before decomposition
• Weight of sediment causes impression in
material below
Permineralized fossils
• Structures buried in sediment and dissolved
minerals precipitate in cells
Nature of the Fossil Record
Four categories of fossils
Casts and Molds
• Remains decay or shells dissolve after
being buried in sediment
• Molds are unfilled spaces
• Casts form when new material infiltrates
space, fills it, and hardens to rock
Nature of the Fossil Record
Four categories of fossils
Unaltered remains
• Preserved in environments that discourage
loss from weathering, scavenging, and
decomposition
• Humans in peat bogs, mammoths in ice,
dried dung from giant ground sloths,
animals trapped in amber
• Rarest type of fossil
Nature of the Fossil Record
Key factors for fossilization
Durability
Burial
Lack of oxygen
Fossils are primarily hard structures
deposited in river deltas, beaches,
floodplains, marshes, and sea floors
Nature of the Fossil Record
Biases in fossil record
Geographic
• Lowland and marine habitats
Taxonomic
• Marine organisms dominate fossil record but
make up only 10% of extant species
• Soft parts such as flowers almost never
preserved
Temporal
• Most recent species dominate
– Not destroyed by tectonic activity yet
Nature of the Fossil Record
Geologic time scale divided into eons,
eras, periods, epochs, and stages
Determined by diagnostic fossils
Absolute dated after discovery of
radioactive isotopes
• Before relative dating used
Lengths of time units not equal
Three eras of Phanerozoic
• Paleozoic, Mesozoic, Cenozoic
The Cambrian Explosion
Nearly all animal phyla that ever existed
first appear in fossil record during
Cambrian
Very short geologic time
Animal body plans (bauplans)
diversified incredibly during this time
Three key divisions of body plans
determined by embryology
The Cambrian Explosion
Diploblasts and Triploblasts
Two or three embryonic layers
Ectoderm, Endoderm, and Mesoderm
• Endoderm forms gut
• Ectoderm forms skin
• Mesoderm forms muscles, and most organs
Radially symmetrical animals are
diploblastic
Bilateria are triploblastic
The Cambrian Explosion
Coelomates, Pseudocoelomates, and
Acoelomates
Coelom = fluid filled cavity derived from
mesoderm
• Increases mobility
Acoelomates include flatworms
Pseudocoelomates include roundworms
• Cavity not completely lined by peritoneum
The Cambrian Explosion
Protostomes and Deuterostomes
All have true coeloms
Difference in gastrulation
Protostomes form mouth first
• Flatworms, roundworms, arthropods,
annelids, mollusks
Deuterostomes form anus first
• Echinoderms and Chordates
The Cambrian Explosion
All of these bauplans arose during
Cambrian Explosion
Other major morphological innovations
too
Segmented bodies
Shells
Appendages
Notochords
The Cambrian Explosion
Two major fossil faunas give evidence
of emergence of new bauplans
Ediacaran faunas
565—544 Mya
Burgess Shale
520—515 Mya
The Cambrian Explosion
Ediacaran faunas
First discovered in Australia in 1940s
• Now found at 20 sites around world
Late Precambrian
Compression and impression soft-bodied
fossils
Sponges and jellyfish
• Maybe annelids, arthropods, and mollusks
The Cambrian Explosion
Burgess Shale
Found in British Columbia
• Chengjian Biota found in Yunnan, China
– 525—520 Mya
Impression and compression fossils
Arthropods, annelids, mollusks, chordates
• Agnathans
Little overlap with Ediacaran faunas
The Cambrian Explosion
Earliest members of most animal
lineages appeared suddenly in fossil
record at same time around globe
No obvious precursor older than
Ediacaran fauna
Was the Cambrian Explosion really
explosive?
The Cambrian Explosion
Estimated phylogeny of all animals
using DNA sequence data
Did the branching events take place 565—
525 Mya?
Molecular clock estimated earliest
branches to be 900 Mya
• Way before Cambrian Period
• Chordates and echinoderms 1 Bya
• Protostomes and deuterostomes 1.2 Bya
The Cambrian Explosion
Results indicate much earlier radiation
and no Cambrian “explosion”
If true, older fossils should be found
Earliest chordate is from Chengjiang
fauna 530 Mya
Therefore, chordates must be older than
this time
The Cambrian Explosion
If molecular clock is correct, animal
lineages are much older
But morphological explosion did occur in
Cambrian
Maybe ecological explosion
New benthic and pelagic predators filling
open niches
Rising oxygen in sea water made larger
size possible
New ways of feeding and locomoting
Macroevolutionary Patterns
Adaptive radiation = single species
quickly diversifies into many species
Hawaiian Drosophila
Galápagos finches
New ecological niches available to species
Can examine a phylogeny and see long
later branches and short internodes
Macroevolutionary Patterns
Stasis
Long periods of little morphological change
followed by rapid periods of speciation
Darwin proposed speciation was gradual
Eldredge and Gould (1972) proposed
punctuated equilibrium
• Stasis and rapid diversification
• “Fits and starts”
• Part of argument is problem of scale
– Slow for biologists is rapid for paleontologists
Macroevolutionary Patterns
Punctuated equilibrium
Includes anagenesis = change in a species
without diverging into other species
• Phyletic transformation
Opposite is cladogenesis = one species
splitting into two
Some species appear to demonstrate stasis
• Living fossils
• Horseshoe crabs, ginkos
Extinction
Rate of global extinctions has not been
constant
Raup broke last 543 My into 1 My
increments to examine constancy of
extinction rate
Found several mass extinctions
Over 60% of living species went extinct
Extinction
Benton quantified how many extinctions
occurred over last 510 My
Found the Big Five
Terminal Ordovician 440 Mya
Late Devonian 365 Mya
End Permian 250 Mya
End Triassic 215 Mya
Cretaceous-Tertiary (K-T) 65 Mya
Extinction
Over 96% of all extinctions did not occur
during mass extinction events
Background extinctions
Likelihood of any clade becoming extinct is
constant and independent of how long taxa
have been in existence
• Survivorship curves indicate this
Geographic range of species correlates
negatively with extinction probability
Extinction
K-T High Impact Extinction
It is now well-known that an asteroid at
least 10 km in diameter struck Chicxulub,
Mexico causing the K-T extinction
Evidence from irridium deposits, shocked
quartz, and microtekites
• Glass spheres created by high impact and
temperature
Extinction
K-T impact caused several events that
may have killed the species
Organisms near impact site were probably
directly killed by heat and flying particles
Sulfur dioxide created by sea water
vaporization caused acid rain, scattering of
solar radiation, and global cooling
Other particles thrown into air added to
shading and cooling
Extinction
K-T impact
Massive earthquakes triggered
• 13 on Richter scale
Giant tsunamis 4 km high
Tsunami and global cooling killed oceanic
phytoplankton
Decline of most groups lingered 500,000 y
60—80% of all species died
Extinction
K-T impact
Species affected differently
• Mammals, amphibians, crocodilians, turtles,
insects, and ferns not affected much
• Dinosaurs, birds, ammonites, and land plants
greatly affected
• Reasons not understood
• Species with narrow geographic ranges affected
most
North America affected most
• Near impact site
Recent Extinctions: Humans
Some people fear human-caused
extinctions will rival Big Five
20% of all birds lost by human
colonization
Most loss is on islands
Birds eaten by humans, killed by
introduced species, habitat loss
Recent Extinctions: Humans
Because of rising human population and
current rate of extinction, potential
losses are great
Probably rate of loss will slow because
most vulnerable species have already been
wiped out
Deforestation still increasing and high
diversity areas are being newly exploited
Time will tell if humans cause it to be the
Big Six