Results of Cambrian Explosion

Origin of major body plans

First diverse ecosystem in oceans

Animalia, bilateria, protostomia, deuterostomia.....

Diverse fauna full of different sizes & ecologies

e.g. surface dwellers, burrowers & sessile animals

First large
animal predators

Modern-type food webs

anomalocaris = largest - up to 1m !

many aspects ancient Cambrian food webs surprisingly similar to those of today

i.e. % omnivores/herbivores

distribution predator-prey links etc.

Evidence Cambrian Substrate Revolution

i.e. anomalocaris preyed on trilobites

Effects Substrate Revolution

Substrate

Ottoia
505 Myr

burrower proper

hunted w/ proboscis sticking up out of sediment

T. Pedum fossil Burgess Shale

fan shaped, 3D burrow in sediment connecting to surface

oldest trace fossil? appears start Cambrian

priapulid worm

Evidence
Type?

Animals leave burrows in sediment

Burrows can be preserved as trace fossils

Trace fossils

records of behaviour

Why
burrow?

protection

anchorage

feeding

Skolithos

vertical burrows made by worm-like animals

some @ "Pipe Rock" in NW Scotland

v. common at beginning Cambrian

a surface on/in which organisms live

New environment for colonisation

Oxygenated sediment (previously anoxic)

Made sediment wetter & softer

Microbial mats could no longer thrive

sediment churned up too much

were common in Ediacaran

Diversity Trends

John Phillips 1800-1874

Created diversity curve of fossils/time

Diversity=number of species

Controlled by:
originations - extinctions

Increasing diversity from Cambrian onwards

John Sepkoski 1948-1999

Spent >20 years collecting information on fossil occurrences

--> quantified trends over time (in oceans)

compiling enormous computer database of fossil occurrence data

Sepkoski Curve

Cambrian

great diversity increase

little set back

extinction at end

Ordivician

great diversity increase (exponential)

recovered diversity from Cambrian decline ~100 Myr

Evolutionary Faunas

Distinct set of organisms that evolve & go extinct in concert

product of Sepkoski's compilation & quantification

Cambrian Fauna

trilobites, inarticulates, polychaetes

Dominant in Cambrian

Paleozoic Fauna

cephalopods, echinoderms, corals etc

Declined afterwards slowly

before trickling out 100s Myr later

Mass Extinctions

Five main mass extinctions & many smaller ones

help separate evolutionary faunaas

beset/punctuate history of life

End-Cambrian Extinction

NOT a big mass extinction, just smaller

fewer absolute total Cambrian species ∴ fewer lost

--> tho quite severe wrt % species lost

accompanied by several others:

  • Botomian Event ~517 Ma
  • Dresbachian Event ~502 Ma
  • End-Cambrian Extinction ~488 Ma

Causes End-Cambrian

poorly understood

(1) Beginning complex life ∴ vulnerable?

(2) Glaciation ?

(3) Depletion marine waters ?

Consequences

Cambrian fauna decimated

Lots of extinctions

Paleozoic fauna had chance to radiate in ensuing Ordivician