Name______________________________________________________________

Reef Builders through Time

In this activity, you will use the Paleobiology Database (PBDB) to explore the history of reef-building animals through time. You will use the PBDB to determine which animal groups were important components of reefs at different times in Earth history and document their evolution and extinction patterns. Next, you will investigate the relationship between reef builders and global climate and atmospheric carbon dioxide (CO2). Finally, you will use what you've learned to make predictions about how reefs may change in the future due to anthropogenic global warming.

Introduction

What do you think of when you hear the word "reef"? For most of us, we immediately envision a warm, shallow sea with abundant coral and colorful tropical fish, like the image below. Reefs have been formally defined in a variety of ways by both biologists and geologists. For our purposes, we will use a relatively inclusive definition provided by paleontologist Rachel Wood: a reef is a biogenic carbonate structure that develops topographic relief upon the sea floor.1

Using this definition, carbonate buildups constructed by various organisms at various scales and water depths can be considered reefs. The key feature uniting them all is the abundant production of biomineralized carbonate skeletons. Reef-building organisms, whether they are algae, sponges, corals, or other animal groups, invest a lot of energy into producing calcium carbonate (CaCO3) to support their soft tissues. This carbonate is typically made of either calcite or aragonite, two mineral polymorphs of CaCO3. In order to make a reef, the reef builder must be able to biomineralize efficiently.

Reef ecosystems today represent less than 1% of the ocean floor, but host over 25% of marine biodiversity.2 Paleontologists have shown that ancient reefs were also centers of biodiversity, though the groups making and living on the reef have changed over time. The fossil record shows that groups may have built reefs at some times during their stratigraphic range but not at others.

Modern and ancient reefs are sensitive indicators of environmental change. For example, anthropogenic global warming is implicated in the bleaching of the Great Barrier Reef, damaging over one-third of the corals.3 Can we paleontologists use our knowledge of ancient reefs to predict the future of reefs on Earth?

1 Wood, R. 2011. General evolution of carbonate reefs. Pp. 452-469 in: Hopley, D. (ed.) Encyclopedia of Modern Coral Reefs: Structure, Form and Process. Encyclopedia of Earth Sciences Series. Springer, Dordrecht.2 The Coral Reef Alliance. 2017. Coral reef biodiversity. https://coral.org/coral-reefs-101/coral-reef-ecology/coral-reef-biodiversity/3 Hughes, T.P., et al. 2018. Global warming transforms coral reef assemblages. Nature 556: 492-496.

Photograph taken on the Great Barrier Reef of Australia.Photo by Wise Hok Wai Lumhttps://commons.wikimedia.org/wiki/File:Great_barrier_reef.JPG

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Part 1. Key Reef Builders

Let's explore the geologic history of some animal groups that are known to be reef builders during different intervals of the Phanerozoic. Locate the Data Table at the back of this activity packet. You will find listed on the table seven taxonomic groups, including sponges, corals, brachiopods, and bivalve mollusks. Note that these reef-building groups are not all at the same taxonomic level.

For starters, we'll look up each group in the Paleobiology Database (PBDB) to learn more about its diversity through time.

1. Go to the Paleobiology Database website: https://paleobiodb.org.

2. Click on the blue "Explore" link.

3. Use the search box in the upper right of the screen to type in the name of the first animal group listed on the Data Table, "Archaeocyatha". Select this name from the list of taxa that appears. Double-check that you are selecting the right group — in this case, the class Archaeocyatha — and not a similar looking name, like the order Archaeocyathida.

4. Click on the Cambrian time period on the time scale at the bottom of the webpage.

5. Look for the little box in the lower right corner of the screen, which lists the total number of collections and occurrences in the PBDB for this group in this time period. Record the total number of Cambrian collections containing Archaeocyatha in the correct cell in the Data Table.

6. Now click on the Ordovician time period on the time scale. Note that the number of collections and occurrences has changed. Record the number of Ordovician collections containing Archaeocyatha on the Data Table.

7. Repeat this process for the other time periods listed on the Data Table.

8. Now repeat these steps for each reef-building group in the Data Table, recording the total number of collections for each time period. Be sure to close out the previous taxonomic filter before starting a new search, by clicking the x in the "Filters" box on the lower left of the screen.

Each collection represents a fossil locality from which that group was sampled. When a group is common and widespread, we expect it to show up in a lot of fossil collections.4 We can therefore use the number of collections in the PBDB as a rough proxy for the relative abundance of each group.

4 Note: Collections are specific fossil localities (geographically and stratigraphically constrained) while occurrences are the taxa found in a collection.

Scleractinian corals in the PBDB

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Looking over all your data, think about when in geologic time each group was most abundant. Since these are reef-building animals, those times should be times when reefs were dominated by that group.

Q1. For each group, indicate the time period (or periods!) when you predict it would have been an important component of reef ecosystems in the table below.

Archaeocyatha

Stromatoporoidea

Guadalupiidae

Tabulata

Scleractinia

Richthofenioidea

Radiolitidina

Q2. Which groups do you predict may have lived together in a reef ecosystem and at which times?

Now let's see how well the number of collections serves as a proxy for the genus-level diversity of each group through time.

9. In the PBDB Navigator, search for each group in turn again. Remember to clear the filters and zoom all the way out before each search so all global occurrences are included.

10. Click on the graph icon in the toolbar in the upper left of the screen.

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11. A plot of sampled-in-bin diversity will appear. Ensure that the taxonomic level is set to "Genus" and the temporal resolution is set to "Age". Use these diversity plots to answer the following questions.

Q3. Fill in the time period(s) of peak diversity for each group in the table below. Be sure to include all times with high diversity, not just a single tallest peak.

Archaeocyatha

Stromatoporoidea

Guadalupiidae

Tabulata

Scleractinia

Richthofenioidea

Radiolitidina

Q4. How well do the times of peak diversity match your predicted times based on the number of collections containing each group (from Q1)? Identify any discrepancies.

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With this understanding of diversity through time, let's now explore the record of extinction in each of these groups. Reef ecosystems are sensitive to environmental perturbations and we therefore expect elevated extinction rates in reef-building groups during times of global climate change.

Q5. Use the Data Table of collection numbers to predict when you expect to see elevated extinction rates for each group. Fill in those predicted times in the appropriate column of the table below.

Predicted Times of Elevated Extinction Rate

Observed Times of Elevated Extinction Rate

Archaeocyatha

Stromatoporoidea

Guadalupiidae

Tabulata

Scleractinia

Richthofenioidea

Radiolitidina

Now let's use the PBDB to calculate actual extinction rates for each group.

12. In the PBDB Navigator, search for each group in turn again. Don't forget to clear the filters and zoom all the way out before each search!

13. Click on the graph icon in the left-hand toolbar.

14. Ensure that the taxonomic level is set to "Genus" and the temporal resolution is set to "Age".

15. Click on the "Use advanced diversity curve generator button".

16. Under "Choose data to display", select "sampled in-bin diversity" and "extinction rate" and shut off the other options. This is also a great time to read about how these values are calculated!

17. Important: Note that the extinction rate graph plots downward, with higher rates plotting lower on the screen.

Q6. Use the extinction rate plots to fill in the right-hand column in the table above.

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Q7. How well do your predictions match the observed times of heightened extinction for each group? Identify any discrepancies.

Q8. While large databases like the PBDB are tremendously useful for conducting powerful analyses, "big data" does come with some tradeoffs. Errors in data entry and programming can creep in. Looking back at your observations so far, identify at least one error or bug you suspect might not be accurate. (Hint: You might consider the pattern of stratigraphic occurrences for the groups after you look up their first and last appearances.) Can you explain what caused the error?

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Of course, the data collection and interpretation you've done so far have been at a fairly coarse temporal scale. Let's take a closer look at what's going on in the mid-Paleozoic, a particularly interesting time for reef ecosystems.

Q9. Which two groups were likely important reef-builders in the Devonian? Fill in their names as "Group A" and "Group B" in the table below.

Q10. For each of these two groups, repeat the process of stepping through time and recording total collection numbers, but at the finest temporal scale available for the Silurian, Devonian, and Mississippian (i.e., stratigraphic ages). To see collections for each age, click on the smallest boxes in the time scale. Fill in the table below with the total number of collections for each group and time period.

Group A = Group B =

RhuddanianAeronianTelychianSheinwoodianHomerianGorstianLudfordianPridoliLochkovianPragianEmsianEifelianGivetianFrasnianFamennianTournaisianViseanSerpukhovian

Q11. Based on these data, when do you predict reefs dominated by these two groups together suffered the single most significant extinction?

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Part 2. Carbon, Climate, and Reefs

The concentration of greenhouse gases such as carbon dioxide (CO2) in Earth's atmosphere is positively correlated with global temperatures. High atmospheric CO2 is also associated with ocean acidification and lower levels of oxygen in the oceans (called dysoxia). All three of these parameters (warming, acidification, and dysoxia) can have a negative impact on marine life, including reef ecosystems. On the other hand, low atmospheric CO2 is associated with colder temperatures and the establishment of continental ice sheets, which decrease sea levels and produce cooler oceans, which might also impact reefs.

Below is a figure summarizing how the concentration of atmospheric CO2 has changed over the Phanerozoic. The plot includes curves derived from geochemical modeling (labeled Models) and a curve based on geological and paleontological datasets (labeled Measurements). While the different curves vary in their precise values, they generally agree on overall trends. Note that the timescale in this figure runs in the opposite direction to those in the PBDB.

History of atmospheric CO2 concentration. Yellow, green, and purple curves (with error envelopes) represent three different geochemical models while the blue curve and dots represent proxy data. CO2 concentration given in parts per million by volume (ppmv).Figure by Robert A. Rohde from published data for Global Warming Art project.https://commons.wikimedia.org/wiki/File:Phanerozoic_Carbon_Dioxide.png

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Q12. Consider the reefs of the mid-Paleozoic. Based on the plot of atmospheric CO2, what environmental changes do you predict might explain the loss of the Silurian-Devonian reef ecosystems?

Q13. The table below lists time periods when reefs were widespread. For each time period, list the key reef building groups for that time interval (based on your answer to Q1), and then indicate whether atmospheric CO2 was relatively high or low based on the figure above.

Key reef builders CO2 high or low?

Calcite or aragonite?

Cambrian

Silurian-Devonian

Pennsylvanian-Permian

Jurassic-Cretaceous

Neogene

Q14. Some workers5 have suggested that times of high atmospheric CO2 were associated with higher rates of seafloor spreading, which decreases the ratio of magnesium to calcium (Mg/Ca) in seawater. Low Mg/Ca tends to favor animals that use calcite to make their skeletons, while higher Mg/Ca favors aragonite producers. Given this relationship between climate, ocean chemistry, and biomineralization, indicate which time periods in the table above would favor calcite- or aragonite-using reef builders.

5 For a review of this concept and evidence supporting it, see: Ries, J.B. 2010. Review: geological and experimental evidence for secular variation in seawater Mg/Ca (calcite-aragonite seas) and its effects on marine biological calcification. Biogeosciences 7: 2795-2849.

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Q15. To test your predictions about calcite vs. aragonite in reef builders, use Web and textbook resources to determine the common mineralogy of each reef building group. Use this information to fill in the table below.

Calcitic or aragonitic?

Archaeocyatha

Stromatoporoidea

Guadalupiidae

Tabulata

Scleractinia

Richthofenioidea

Radiolitidina

Q16. Do your observations in Q15 match your predictions in Q14? Explain.

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Part 3. The Future of Reefs

While the plot of atmospheric CO2 through the Phanerozoic shows a decline in CO2 levels since the mid-Paleogene, this trend is reversing. Human activities, such as burning fossil fuels and deforestation, are rapidly putting more CO2 into the atmosphere. Indeed, Earth recently crossed the 410 ppm mark, a CO2

concentration Earth hasn't experienced since the Pliocene (3 Ma), prior to the start of the Quaternary Ice Age. Various models suggest Earth may reach 720-1,000 ppm as early as the year 2100.6

Q17. Based on the Phanerozoic atmospheric CO2 plot, when was the last time Earth experienced a CO2 concentration of 1,000 ppm?

Q18. As atmospheric CO2 increases, which mineral — calcite or aragonite — do you predict will be favored among future reef builders?

6 Intergovernmental Panel on Climate Change (IPCC). 2014. Summary for Policymakers. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp. http://www.ipcc.ch/report/ar5/syr/

Atmospheric CO2 concentration in parts per million (ppm) for the last 800,000 years. As of 2018, concentrations have reached 411 ppm.Figure from Scripps Institute of Oceanography, The Keeling Curve.https://scripps.ucsd.edu/programs/keelingcurve/

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Q19. Given everything you have learned over the course of this activity, what predictions can you make about how reefs will change due to anthropogenic global warming? Explain your reasoning.

Cluster of coral species on the Great Barrier Reef of AustraliaPhoto by Toby Hudson https://en.wikipedia.org/wiki/Coral_reef_protection#/media/File:Coral_Outcrop_Flynn_Reef.jpg

Name______________________________________________________________

DATA TABLE

Look up each reef-building group listed below in the PBDB Navigator. Click on each time period on the time scale at the bottom of the screen. Record the number of collections in the database for each group in each time period in the table.

Phylum or Class:

Porifera Porifera Porifera Anthozoa Anthozoa Brachiopoda Bivalvia

Group: ClassArchaeocyatha

ClassStromatoporoidea

FamilyGuadalupiidae

SubclassTabulata

OrderScleractinia

SuperfamilyRichthofenioidea

SuborderRadiolitidina

Cambrian

Ordovician

Silurian

Devonian

Mississippian

Pennsylvanian

Permian

Triassic

Jurassic

Cretaceous

Paleogene

Neogene