Reefs through time

Question 1

Which reef structures can be recognised in the geological record?

Answer 1

Talus (slope off the fore reef)
Fore reef
Back reef
Lagoon

Question 2

What is the prominent type of reef thought to exist in the Precambrian? Where are these likely to exist today?

Answer 2

Microbial reefs - covered in microbial film, which glues the top levels of the sediment and builds structures.

Likely found in hyper saline environments today where grazers aren’t found in large numbers.

Question 3

What is the prominent reef type thought to be exist in the Cambrian? Describe the structure of this reef.

Answer 3

Archaeocyathids (funny sponges).

These sponges live on microbial thrombolite reefs. “Crinoids” were also thought to live on these reefs.

Question 4

Which two main groups of corals form a Silurian reef?

Answer 4

• Rugose/tetra corals
• Tabulate corals

Question 5

What is the basic structure of a rugose coral?

Answer 5

Septa insertions inserted in 4’s radiating from a central structure known as a columella.
Typically cup or horn shaped.
Often solitary but where colonial, they exhibited interconnectivity.

Question 6

What is the basic structure of a tabulate coral?

Answer 6

Lack conspicuous septa and have horizontal walls known as tabula (resemble floors or steps).
They were all colonial and often formed “honeycomb-like” structures

Question 7

When was the first point in geological history where corals were involved in reef structure?

Answer 7

The Silurian (third period of the Paleozoic)

Question 8

Why are silurian reefs so well understood/studied?

Answer 8

Silurian reef limestone preserves reef details; limestone is also often used in developments such as cars and has therefore been well studied in the US and Canada.

Question 9

What are the three main eras in geological history?

Answer 9

1. Paleozoic
2. Mesozoic
3. Cenozoic

Question 10

What are the three main geological periods associated with the early Paleozoic?

Answer 10

1. Cambrian
2. Ordovician
3. Silurian

Question 11

What are the three main geological periods associated with the late Paleozoic?

Answer 11

1. Devonian
2. Carboniferous
3. Permian

Question 12

What are the three main geological periods associated with the Mesozoic?

Answer 12

1. Triassic
2. Jurassic
3. Cretaceous

Question 13

Summarise the Devonian reefs. Is this well studied?

Answer 13

Alike to the Silurian reefs -> tabulate and rugose corals, with crinoids and trilobites etc.

This is also reasonably well studied as it is a limestone reef too, so gets quarried.

Question 14

What is the skeletal composition of rugose and tabulate corals?

Answer 14

Calcite (the other CaCO3 option other than aragonite).

Question 15

What are the three main stages in the growth of Devonian reefs? What is each stage associated with?

Answer 15

1. Pioneer structures -> stick coral forms
2. Intermediate structures -> platy-hemispherical forms
3. Mature (full reef structure off the seabed).

Question 16

What are the largest reefs known as from the Devonian? Are they bigger than the Great Barrier reef?

Answer 16

Mega reefs -> 8-9 reefs were larger than the Great Barrier Reef

Question 17

Is there a coral latitudinal trend in the Paleozoic and if so what is it?

Answer 17

There is!

Massive colonial corals are associated with lower latitudes, with decreasing size, coloniality, evolutionary complexity and rank with increasing latitude. Microbial mud mound reefs (remember the precambrian reefs) are found at high latitudes.

Question 18

What is the mass extinction after the Devonian?

Answer 18

The F-F mass extinction

Question 19

What did the F-F mass extinction cause for corals and reefs?

Answer 19

Corals died. This caused a switch back towards microbial reefs from corla-stromatoporoid reefs

Question 20

Other than corals, which major reef builders were crucial to reef systems in Silurian and Devonian periods.

Answer 20

Stromatoporoids -> likely sponges that also formed calcite CaCO3 skeletons.

Question 21

Quickly summarise the Paleozoic reef structures and extinctions up to the end of the Devonian.

Answer 21

Archaeocyath reefs (in the Cambrian)

First coral mass extinction event

Recovery, leading to stromatoporoid/coral reef expansion in the Ordovician, Silurian and Devonian.

F-F mass extinction (switch back to microbial reefs with coral death)

Question 22

What is an example of a plot that can be used to show the number of genera of a type of coral over time?

Answer 22

Spindle plot

Question 23

What is the main reef structure in the Carboniferous period?

Answer 23

Thrombolite reefs (microbial structures).
These reefs had lots of crypts (holes/pores), giving the reefs great diversity.
Rugose corals are present but are “add ons” to the reef rather than the dominant structure.

Question 24

Describe the reefs in the Permian.

Answer 24

• Based on byrozoans and sponges
• High diversity within crypts

Question 25

What happened after the Permian mass extinction?

Answer 25

> 90% of all species went extinct. This caused a shift from very high diversity to limited diversity and reef environment.

Question 26

What does LIPs stand for? What are they?

Answer 26

Large Igneous Provinces Large fissure eruptions on a planetary scale.

Question 27

What can be the consequences of LIPs on the planetary system?

Answer 27

- Block light (causing nuclear winters) - Short term ozone destruction - High CO2 -> can cause ocean acidification - High sea temperatures (symbiont loss?)

Question 28

How does current climate change compare to historic LIPs?

Answer 28

Very similar, except climate change is occurring over ~300 years, whilst LIPs occurred over periods of up to 2000 years. Demonstrates the rapid change we are causing.

Question 29

Is there a correlation in geological history between LIPs and mass extinction events?

Answer 29

Yes -> although not all LIPs caused extinctions. This is thought to vary with the rate of change (faster = more likely for mass extinctions) although this is not fully understood.

Question 30

When did Scleractinians first appear in geological time (as prominent reef components)? What was their distribution?

Answer 30

In the early Triassic. Locally distributed around the equator in the early Triassic, but a global spread by the late Triassic.

Question 31

What are scleractinian corals thought to have evolved from?

Answer 31

Originally thought they evolved from carboniferous corals (heterocorals). Actually think they evolved from soft anemones, developing the ability to lay down CaCO3 skeletons.

Question 32

When is the first scleractinian fossil thought to originate from?

Answer 32

The Ordovician, over 450 mya.

Question 33

Which polymorph of calcium carbonate is more geologically unstable?

Answer 33

Aragonite -> typically dissolves and precipitates as calcite over thousands/millions of years. Therefore, fossils are typically replaced or recrystallised.

Question 34

What is the typical pattern of CaCO3 polymorphs over time?

Answer 34

Generally either calcite precipitating or aragonite precipitating organisms (usually following the dominant chemistry).

Question 35

What has molecular analysis revealed about the origins of Scleractinian corals? Which clocks were used?

Answer 35

Mitochondrial 16s rDNA and 28S rDNA were used as molecular clocks, which could be compared against the fossil record. This showed the origin of Scleractinian corals to be 425 mya, suggesting they were present during the Ordovician etc, but only became dominant after the rugose and tabulate extinctions.

Question 36

What happened during the Triassic-Jurassic extinction?

Answer 36

Caused widespread reef collapse. Restructuring occurred in the Jurassic, with different Scleractinian corals and more dominant stromatoporoids again, as well as less obvious symbiosis.

Question 37

What were the conditions over the course of the Triassic?

Answer 37

Initially anoxic after the Permian mass extinction. Large structures form in the late Triassic. Extinction occurs during the Triassic-Jurassic mass extinction.

Question 38

How can symbiotic relationships through geological time be assessed?

Answer 38

Using δ¹³C and δ¹⁸O isotopes. Photosynthetic activity of zooxanthellae alters carbon and oxygen isotope ratios in coral skeletons. These isotopic signatures can indicate the presence or absence of symbionts.

Question 39

How can you tell if a symbiotic relationship was present in geological aragonitic coral skeletons?

Answer 39

The skeletal δ¹³C and δ¹⁸O proportions overlap with modern coral zooxanthellates (organisms in symbiosis), suggesting they likely hosted photosynthetic symbionts.

Question 40

On an isotopic plot for symbiosis across geological time, what do the CAL and WAL lines represent?

Answer 40

CAL = trend for modern, shallow water symbiotic corals WAL = trend of deep-water non-symbiotic corals

Question 41

What can be understood in terms of which geological time periods had symbiotic corals from an isotope plot?

Answer 41

There is evidence of symbiotic corals in the Triassic. However, this seems to break down in the Upper Jurassic, where isotope proportions seemed to align more closely with modern non-symbiotic corals. However, the presence of modern zooxanthellates suggests reacquisition of symbionts occurred.

Question 42

Why is it easier to study Paleozoic corals using isotopic plots than Metazoic corals? Where symbiotic relationships present in this period?

Answer 42

Paleozoic corals were typically associated with calcite, which is more geologically stable. There is evidence that they did, fractioning near modern and triassic symbiotic corals.

Question 43

What other method other than carbon and oxygen isotopes can be used to understand symbiotic relationships over geological time (e.g., used for Devonian corals)?

Answer 43

d15N in organic matter can be extracted from the skeleton and compared to living corals (symbiotic and non-symbiotic). If the nitrogen offset is similar, then the symbiotic relationship can be estimated.

Question 44

What were cretaceous reefs typically composed of? What kind of polymorphic reefs formed?

Answer 44

Bivalves (rather than corals), such as rudist bivalves. Typically calcite reefs.

Question 44

What morphological feature of coral skeletons makes them useful to calculate time in different geological periods? What was identified because of this?

Answer 44

Growth banding -> a layer is added daily, with the density of this band indicating seasonality. The world was spinning faster back in the Devonian, with 400 day years -> tidal drag caused by the moon has slightly slowed Earth's spin.