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