Exercise 17.1

The Great Ordovician Biodiversification Event: The Lesser-Known but Larger Sibling of the Cambrian Explosion

(This exercise is based on Schmitz, B., D. A. T. Harper, B. Peucker-Ehrenbrink, S. Stouge, C. Alwmark, A. Cronholm, S. M. Bergström, M. Tassinari, and X. F. Wang. 2008. Asteroid breakup linked to the Great Ordovician Biodiversification Event. Nature Geoscience 1: 49–53.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)

INTRODUCTION

In Chapter 17 you were introduced to the famous period of evolutionary diversification commonly known as the Cambrian explosion. This name was inspired by the appearance in rock layers exposed in Wales (historically called Cambria) of fossil life forms that were more diverse, large, and complex than any that could be observed in older layers. It seemed that diverse and complex marine life had suddenly “exploded” onto the scene over a relatively short span of geological time. This well-known period of diversification is dwarfed, however, by a later ramp-up of diversity known as the Great Ordovician Biodiversification Event (GOBE), where biodiversity of certain groups appears to have approximately doubled in a period as short as a few million years, and family-level diversity tripled over the course of the geological age. The paper cited here proposes a possible explanation for this observation by correlating this biological diversification with an astronomical event that resulted in an increase in asteroid impacts on the Earth’s surface. The authors show that the presence of extraterrestrial chromite grains, which are evidence of meteorite impacts, are present in geological strata of the same age where the rapid increase in diversity occurred (see the green band in Figure 3, below).

In Figure 1 below, the authors show the presence of extraterrestrial chromite grains per kilogram of rock from 3 sites—two in Europe and one in China. In this figure, time advances from bottom to top as if we are looking at the actual stratigraphy. Chromite grains are shown with red dots for each site, plotted against the scales at the bottom. The horizontal dotted lines correct for different rates of sedimentation at the different sites, and can be though of as isoclines of geological time.

As you can see, the Arenig–Llanvirn boundary is characterized by a marked increase in the presence of meteoritic evidence, and in one site actual meteorites.

Figure 2 shows what was going on biologically during the same time-frame. This figure shows the number of species of brachiopods (the black trace), and also the origination and extinction of species (red and blue traces respectively) during the time bracketing the Arenig–Llanvirn boundary. You can see that there is a marked increase in diversity associated with the boundary, and also a series of spikes in both origination and extinction of species.

QUESTIONS

 

Question 1. The authors talk about both “intrinsic” and “extrinsic” factors in relation to biodiversification. What do these terms mean in this context?

 

Question 2. It may seem counterintuitive to suggest that a disruptive event like a meteorite impact would increase rather than decrease biodiversity. How might this be explained?

 

Question 3. The authors use the term “fossil meteorites.” Do you agree or disagree with their use of this term, and why?

Figure 1 Distribution of extraterrestrial (chondritic) chromite and osmium isotopes through Middle Ordovician sections in Sweden and China. Results are shown for sections at Kinnekulle (Höllekis and Thorsberg quarries) and southern Scania (Killeröd and Fagelsang sections), 350 km apart in southern Sweden, and the Puxi River and Fenxiang sections, 4 km apart in south-central China. The distribution of Os isotopes across the Höllekis section is also shown. The stratigraphic interval yielding abundant fossil meteorites in the Thorsberg quarry is indicated. The conodont biostratigraphy shown has been produced specifically for this study, using consistent taxonomic concepts for the different sections. M. ozarkodella = Microzarkodina ozarkodella.

 

Question 4. Notice in Figure 1 that so-called fossil meteorites were only found in the Höllekis and Thorsberg quarries, but not at the other sites. Why is this?

Figure 2  Total diversity of brachiopod species (number of species) through part of the Lower and Middle Ordovician in Baltoscandia. The results are based on bed-by-bed collections at eight localities. Note the dramatic increase in biodiversity (black line) and high extinction (blue line) and origination (red line) levels following the regional Volkhov–Kunda boundary, that is, the same level where extraterrestrial chromite appears and Os isotopes change in Fig. 2. B. triang.-navis = Baltoniodus triangulatus-navis. The dashed lines show the boundaries between the regional states.

 

Question 5. In Figure 2, if the blue trace is greater than the red trace, will the slope of the black trace be positive or negative?

 

Question 6. In Figure 2, when the blue and red traces are both zero, the black trace is level and horizontal (a slope of zero). What will the black trace look like if the blue trace and the red trace are both high (say at 10)?

 

Question 7. Do the data presented in this paper provide proof that meteorite impacts cause biodiversity to increase?

Figure 3  Global biodiversity change at family level through the early Palaeozoic era. Although this diagram from Sepkoski (1995; ref. 15) gives a good representation of the overall biodiversity trend, the resolution is too crude for correlation with field data. Trem. = Tremadocian (Global) and Tremadoc (British), Ash. = Ashgill, Lland. = Llandovery, We. = Wenlock, Lud. = Ludlow, F. = Floian, Dap. = Dapingian, Sand. = Sandbian, H. = Hirnantian. (Figure slightly modified from the current paper [green band added to highlight the time span considered in the paper]).

 

Question 8. In Figure 3 we can see a dramatic extinction event at the end of the Ordovician followed by a steep increase in biodiversity. What could explain this steep increase, and why is that case different from the case you considered in Question 2?