Palaeo - Cambrian Ecosystems

Question 1

The Cambrian Fauna and JS

Answer 1

• The Cambrian period is characterized by the sudden appearance of complex animal-dominated ecosystems.
• Jack Sepkoski showed that a ‘Cambrian fauna’ with a distinctive taxonomic composition arose in this period, before waning almost immediately through the Ordovician radiation.
• Sepkoski collated a database of the stratigraphic occurrence of marine families, allowing the first detailed analysis of the diversity of major groups (classes – the Linnean level below a phylum) through time – see the spindle plots
• Elegant statistical analysis showed, remarkably, that the diversity of each class could be described as a blending of three ‘fundamental trends’ (factors).

Question 2

3 faunal groups

Answer 2

3 groups (trajectories) and their occurrence through geologic time

1. Emerges in the Cambrian, is dominant and then rapidly wanes in the Ordovician, remaining almost non-existent for the rest of time - Trilobita
2. Rise up after Ordovician and remain dominant until PT extinction - Crinoidia
3. Dominant after the PT extinction - Gastropoda

Question 3

Species trends of the three faunas

Answer 3

• Some groups’ trends (e.g. Trilobites, articulate brachiopods, gastropod molluscs) map almost exactly onto one of the three factors – these groups are considered to represent a ‘Cambrian’, ‘Palaeozoic’ and ‘Recent’ fauna.
• Other groups (e.g. ostracod arthropods, corals) represent a blending of two or three factors.
• Some classes follow this well e.g. gastropods fit into 3, whilst other are more complex
• Blending these three factors together accounts for the vast majority (>90%) of the pattern of diversity through time; the residuals largely correspond to times of mass extinction or rapid diversification.
• Essentially are ‘3 faunas in which we can subdivide species’
• Sepkoski suggested that members the three different faunas might be characterised by different evolutionary parameters.

Question 4

Cambrian versus modern fauna

Answer 4

Could the Precambrian oceans, essentially devoid of macroscopic life, be analogous to a patch of ground recently cleared and about to undergo ecological succession? 
•	Cambrian
     o	Generalists (r-species) 
     o	High productive rate
     o	High speciation rate
•	Recent
     o	Specialised (K-species)
     o	Low productive rate
     o	Low speciation rate

Cambrian Fauna characteristics:
• Less competitive environment
• Less predation
• Metabolic inefficiency

Question 5

The Burgess Shale

Answer 5

• The Burgess Shale is an exceptional fossil deposit that offers an unrivalled glimpse into Cambrian ecology.
o Sedimentary setting
 Multiple horizons over 10m
 Corresponds to 10-100ka (short geological timescale)
• Two attributes make it particularly suitable for this sort of study:
o - Soft tissue preservation
 Preserves ‘complete’ communities
o - Discrete burial horizons
 Transport entire communities?
Fossiliferous beds of the Burgess Shale are 2 mm to 20 cm thick; each corresponds to a single obrution event, and they are separated by fossil-poor background sedimentation.
• Obrution events – slumps of sediment
o Everything churned up in med - muddled

Question 6

Is the site representative of broader Cambrian communities?

Answer 6

• Palaeolatitude: 15° N
o Close to equator – maximum diversity in this region
o Shows a diverse community because of a diverse environment?
• Connection to open ocean present?
• Pelagic trilobites - few endemics
o Shelly taxa match other Cambrian assemblages
o Most shelly taxa widespread across N. America and beyond
o Shelly assemblages resemble any other
o Matches Cambrian – representative
• Water depth
o Green algae present – must be in photic zones
 Shallow
 Have had some transportation however

Question 7

Problems with the Burgess Shale?

Answer 7

Problems and biases:
•	How does death assemblage relate to life assemblage?
•	Biases
o	Not everything preserved
	Underrepresents soft body organisms
o	Transport
o	Organism-substrate habitat
o	Motile organisms less likely to be buried

Accounting for decay
• Certain organisms (e.g. the annelid Burgessochaeta) occur in almost every fossiliferous bed, but at different states of decay.
• The level of decay in a particular bed seems to have no impact on species richness, suggesting that decay introduces little bias into the data.

Collection bias
• Early collectors treated the Burgess Shale as a homogeneous unit, quarrying slabs and taking them by packhorse to a pleasant meadow for splitting.
o Explosives, transported to camp – no bed labels – no bed by bed resolution available
o “One fossil per slab” – break up by fossils
o Discarded ‘boring fossils’ – diversity measurements skewed
o Parts and counterparts separated (counted twice)
• No records of collection practice were kept. The first palaeoecological study, based on these collections, was therefore limited in its precision.
• Later collectors labelled each slab with its stratigraphic horizon and sought to collect (or at least record) every specimen.