Cambrian

What is the timescale of the Cambrian?

541 to 485 mya

Cambrian Explosion

Cambrian Explosion
-Mostly sea-life
-Limited evidence for freshwater habitats, little life on land
-No insects or vertebrates on land until Middle Paleozoic

Cambrian animal life divided into three intervals, each with a new, characteristic fauna, which are:

Cambrian animal life divided into three intervals, each with a new, characteristic fauna
-Lowermost Cambrian fauna
-Tommotian fauna
-Later Cambrian fauna.

Base of Cambrian System marked by:

-burrows - requires oxygen; trace fossil
-branched burrows different from Neoproterozoic burrows, increasing behavioral complexity

Lowermost Cambrian Fauns:
-Simple skeletal fossils – why?

Current hypothesis: to avoid predators in the newly expanding food web; some with many plates
-Many small calcium carbonate skeletons,
tube or vase-shaped
-Teeth – why are these significant? Sign of predators and made of calcium phosphate.

Tommotian fauna

• Richer fauna made of calcite, small skeletal fossils, unlike any extant phyla or younger fossils; 3-4 million yr period.

-Skeleton formation influenced by seawater chemistry; Mg/Ca ratio determines whether calcite or aragonite is precipitated

-Later Cambrian and Ordovician, Mg/Ca <2, favored calcite – trilobites, brachiopods,
corals, echinoderms
• Some organisms probably also precipitated shells from calcium phosphate (apatite, like our teeth)
• Probably reflected evolutionary battle between skeletonized animals and their predators

Late-lower Cambrian Explosion, Large animals with skeletons

• Large animals with skeletons
– Trilobites; lived throughout the Paleozoic (541 to 252 mya)

-Arthropods with calcified
segmented skeletons; similar to extant phyla.

Other abundant Early Cambrian animal groups

Other abundant Early Cambrian animal groups
– Monoplacophoran mollusks
– Inarticulate brachiopods
– Echinoderms

• Chengjiang fauna

– Soft-bodied creatures including:
• Cnidarians
• Predatory
worms
• Anomalocarids
– Huge carnivores (2 m)
– Swimmers
– Impaled prey

Early Cambrian Modes of Life

Early Cambrian Modes of Life
• Limited modes of life
• Seafloor, free-living deposit feeders
– Extract organic matter from sediments
– Trilobites, arthropods
• Suspension feeders
– Collect organic matter from the water
– Eocrinoids
• Attach by stalk; abundant Monoplacophorans (molluscs) grazed microbial mats (stromatolites) growing on seafloor

Upper water column:
• Acritarchs (below), eukaryotic algae
• Trilobites, jellyfish

-Stromatolites

-Stromatolites

– Less abundant; more restricted, sub-tidal region
– Weak grazing pressure in inter-tidal zone

-Microbialites

-Microbialites
– Thin organic layer in sedimentary rock, trapped sediment in intertidal zone, could tolerate changes in temp, salinity

Thrombolites

-Thrombolites
– Thin organic layer in sedimentary rock, trapped sediment, lots of holes from predators, little layering

Cambrian Reefs

Reefs
– Early Cambrian
– Archeocyathids
– Suspension feeders
– Probably sponges with
mats, calcite
– Went extinct at the end of early

Cambrian Evolutionary experimentation

Cambrian Evolutionary experimentation
– Bizarre echinoderm classes
• Few species and genera
– Tried out many, short-lived body plans.

Middle and Late Cambrian

Middle and Late Cambrian
– 15 Million year duration
– Expansion of many groups
• Trilobites
• Echinoderns
• Conodonts
– Early fish
• Isolated bony external plates

Cambrian Extinctions

Cambrian Extinctions
-Six Cambrian mass extinctions affected shallow-water trilobites
-One of these extinctions also killed archaeocyathid reef builders
-Each extinction was followed by an evolutionary radiation of trilobites

Cambrian Paleogeography

Cambrian Paleogeography
-Cratons formed supercontinent early during Cambrian
-Progressive flooding of continents.

-Regression in Middle Cambrian and again in Late Cambrian