A New Perspective on the Late Ordovician Mass Extinction

Earth has experienced mass extinction events, which have been caused by volcanism, meteorite impacts, and climate change. Particularity, the Late Ordovician Mass Extinction (LOME) was the first of the big five extinction events that have affected metazoan life in Earth’s history (Sepkoski, 2002), identified by an abrupt decrease in the diversity trend after the Great Ordovician Biodiversification Event (GOBE), encapsulating the most significant increase. In recent decades, studies have aimed to decrypt the biological signal shown by the fossil record during the LOME, assess the timing and duration of the event, and clarify its link to environmental changes.

In February 2019, Rasmussen et al. presented a biodiversity curve with four times higher resolution than previous authors (e.g., Alroy et al., 2008 and Sepkoski, 2002) for the Early Paleozoic (540 to 420 million years) and compared this curve to potential controlling variables such as temperature, sea-level change, oxygen in the atmosphere, plate tectonics, and carbon cycle. The biodiversity curve for the earliest Paleozoic periods (Cambrian, Ordovician, and Silurian) showed gradual biodiversity increases within two radiation events, the Cambrian Explosion and GOBE, and a 10 to 12 million years long extinction interval, identified as the LOME.

In order to estimate biodiversity in time bins of 2.3 million years, Rasmussen and his colleagues (2019) used two different datasets of more than 185,000 fossil occurrences each, filtered from the raw data registered in the Paleobiology Database and binned in the time slices manually or automatically.  The two datasets were standardized by using methods such as classical rarefaction and shareholder quorum subsampling. Based on these two standardized datasets, the authors calculated richness trends using Alroy’s approach (DSQS), Hill number or Shannon’s entropy adapted by Chao et al. (DHill), and a capture-recapture model built from a subset of ca. 25,000 occurrences and fitted to a “superpopulation approach” by using Laake’s interface (DCR).

The trends obtained were similar (Figure 1). Especially, those generated by DSQS and DHill approaches showed more coincidence because both methods are designed to compare samples of equal completeness. At the same time, they presented higher volatility than DCR due to uneven sampling. In addition, the curve of probability-based diversities (DCR) differed radically in magnitude from DSQS and DHill confirming a rising richness tendency from Cambrian to Middle Ordovician. However, in contrast to previous curves, it showed stepwise changes during the Early Paleozoic instead of abrupt increases and decreases.
Traditionally, the LOME is an event that occurred around 450–440 million years ago and eradicated nearly 85% of the marine species in a two-phased episode. The first phase happened in the late Katian, affecting organisms that lived in the shallow and deep water column, and the most devastating second phase, in the late Hirnantian, impacting the fauna in different water depths. The biodiversity curve presented by Rasmussen et al. (2019) showed three major drops starting in the mid-Katian, suggesting not only a new phase of extinction but also an earlier onset of the LOME and implying that the global hyperthermal event in the late Katian, the Boda Event, as well as the faunal dispersal and migration event, known as Richmondian Invasion, might be included as part of the LOME.
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Previously, the LOME had been attributed to cooling and warming variations in a short period and potential glaciation in a large portion of the terrestrial surface. However, Rasmussen et al. (2019) suggest a new perspective on the causes of the LOME and link the event to intensive volcanism that increased temperature, elevated the CO2 levels in the atmosphere and ocean, and triggered the first extinction phase. In addition, the authors include the Boda Event and Richmondian Invasion in the interval as periods when environmental conditions favored migration of fauna towards open niches in colder and deeper waters and enhanced existing species to broaden their geographic ranges. These migration events potentially buffered the effects of the volcanic event on the fauna, which later was affected by the greenhouse to icehouse fluctuations that, combined with the paleogeographic configuration, had a devastating effect on the organisms at the end of the Ordovician.Although the authors (Rasmussen et al., 2019) showed estimates of biodiversity for the Early Paleozoic in high-resolution by implementing a sophisticated new method, which accounts for not only sample completeness but also for survival, preservation, and origination probabilities, the correlation-causation approach for the biodiversity trends and the environmental variables could be improved by integrating higher resolution information to the analysis. In particular, further research regarding the LOME should estimate the extinction rates during the three phases to determine when the major loss in taxa happened and incorporate model selection to identify the drivers of the mass extinction when evaluating multiple causes for biodiversity trends. In this sense, it is also essential to extend Rasmussen et al.’s approaches to re-assess biodiversity throughout all geological time and continue testing their capture-recapture model.

References

• Alroy, J. (2010) The shifting balance of diversity among major marine animal groups: SCIENCE, 329, 1191–4.
• Alroy, J. et al. (2008) Phanerozoic trends in the global diversity of marine invertebrates: SCIENCE, 321, 97–100.
• Chao, A. et al. (2014) Rarefaction and extrapolation with Hill numbers: A framework for sampling and estimation in species diversity studies: Ecol. Monogr., 84, 45–67.
• Laake, J. L. (2013) RMark: An R interface for analysis of capture-recapture data with MARK: AFSC Processed Rep, Alaska Fisheries Science Center, NOAA, doi:10.1017/CBO9781107415324.004
• Rasmussen, C. M. Ø., Kröger, B., Nielsen, M. L. and Colmenar, J. (2019) Cascading trend of Early Paleozoic marine radiations paused by Late Ordovician extinctions: Proc. Natl. Acad. Sci. U. S. A., 116, 7207–7213.
• Sepkoski, J. J. (2002) A compendium of Fossil Marine Animal Genera: Bull. Am. Paleontol., 1–566.