20 Evolution and the History of Life: The End-Ordovician Mass Extinction

The Geologic Timeline and the Context of Early Life

The history of life on Earth is documented across a vast geologic timescale spanning approximately $4.5 imes 10^{9}$ years. The earliest organic structures are recorded around $3.8 imes 10^{9}$ years ago during the Precambrian. The timeline is divided into eons, eras, periods, and epochs. The Phanerozoic Eon, which represents the time of visible life, begins with the Paleozoic Era, starting with the Cambrian Period approximately $542 imes 10^{6}$ years ago. The Paleozoic continues through the Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian (together forming the Carboniferous), and Permian periods. Following the Paleozoic is the Mesozoic Era, consisting of the Triassic, Jurassic, and Cretaceous periods. The Cenozoic Era follows the extinction of the non-avian dinosaurs and includes the Tertiary and Quaternary periods. Within these are various epochs, such as the Paleocene, Miocene, Pliocene, Pleistocene, and the current Holocene. Mass extinctions serve as major markers within this timeline, often defining the boundaries between these geologic divisions.

Defining Mass Extinctions and the Big Five Great Crises

Mass extinctions are significant biological events that often conjure images of catastrophic asteroid impacts; however, they are defined by the widespread and rapid decrease in the biodiversity on Earth. In the history of life, there are five major events referred to as the "Big Five" great crises. These include: 1. The End-Ordovician extinction; 2. The Late Devonian extinction; 3. The End-Permian extinction (the most severe in Earth's history); 4. The Late Triassic extinction; and 5. The End-Cretaceous (K/T) extinction, which famously ended the reign of the dinosaurs. These events are characterized by three major impacts on the biosphere: the extinction of successful groups, a subsequent biological rebound, and a fundamental shift in the ecology of the planet.

The Three Major Impacts of Mass Extinctions

The first impact is Extinction, where many successful biological groups are simply obliterated or severely crippled. For example, the Trilobita were significantly impacted throughout several events. Specific trilobite orders and suborders recorded in the fossil record include Agnostina ($ext{Lower Cambrian}$-$ext{Upper Ordovician}$), Olenina ($ext{Cambrian}$-$ext{Upper Ordovician}$), Redlichiina ($ext{Lower Cambrian}$-$ext{Middle Cambrian}$), and the Phacopina ($ext{Lower Ordovician}$-$ext{Upper Devonian}$). The Asaphida, which once comprised nearly one in five of all described fossil trilobites, were decimated, with Raphiophorus being the lone surviving genus.

The second impact is the Rebound. Following a massive drop in the number of genera—which can fall from approximately $4000$ to below $1000$ during major events—there is a recovery phase where diversity begins to climb again. This is visible in the fossil records of the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, and Neogene.

The third impact is Ecological Shift. Mass extinctions often change the dominant fauna. In the Paleozoic, the oceans were dominated by Rugose corals, Tabulate corals, Brachiopods (such as Thurmanella), Bryozoans, and Crinoids. Post-extinction ecological landscapes often see a rise in different groups like Bivalves (such as Chlamys, Lopha, Gervillella, Gryphaea, Pleuromya, and Myophorella), Snails, Starfish, and Sea Urchins (such as Nucleolites).

Generalized Causes of Mass Extinctions and the Primary Driver

Several mechanisms are proposed for mass extinctions. Sea level change is a primary factor, involving rapid regressions (shallowing) or rapid transgressions (deepening). During the End-Ordovician, the global sea level dropped by approximately $160160 ext{m}$. Oceanographic changes also play a role, with chemical evidence suggesting collapsed upwelling and anoxia (oxygen depletion), as seen in the first pulse of the Late Permian extinctions. Volcanic activity is another major driver; the Late Permian saw massive eruptions in the Siberian Traps, while the End-Triassic is associated with volcanics, sills, and dike swarms in the High Atlas and North America. Bolide impacts, such as the one that created the Chicxulub crater (evidenced by magnetic gravity anomaly maps), are also significant. However, all these possibilities are linked to a single central driver: large-scale climate change.

The End-Ordovician Mass Extinction: Climate and CO2 Thresholds

Throughout the Ordovician, the global climate cooled gradually. During the Middle Ordovician, atmospheric $CO_2$ is estimated to have been between $14 ext{--}22$ times present-day levels. For glaciation to occur, models suggest a reduction of $CO_2$ levels down to approximately $8$ times present-day levels. Various factors contributed to the drawdown of $CO_2$. The Taconic Orogeny increased silicate weathering, a process that removes $CO_2$ from the atmosphere. Additionally, seawater chemistry (Strontium isotopes) indicates a high eruption rate of basalt. The positioning of large landmasses over the intertropical convergence zone further increased weathering due to high rainfall. These factors combined to lower $CO_2$ from $14 ext{--}22$ times present-day levels to about $12$ times present-day levels. However, since $12$ is higher than the threshold of $8$, additional mechanisms were required to trigger the glaciation.

The Carbon Cycle and Isotopic Excursions

To understand perturbations in the carbon cycle, scientists measure the ratio of $^{12}C$ and $^{13}C$. Photosynthesizers prefer the lighter isotope, $^{12}C$, meaning organic carbon is enriched in $^{12}C$. Mass extinctions are frequently associated with isotopic excursions. At the end of the Ordovician, during the Hirnantian substage, there was a massive positive carbon and oxygen excursion in proxies for ancient seawater. A positive excursion indicates that more organic carbon ($^{12}C$-rich) was being buried. This left the remaining system enriched in $^{13}C$. The increased burial of organic carbon indicates a reduction in the greenhouse gas $CO_2$ reservoir, leading to global cooling.

Biphasic Extinction: The Two Pulses of the End-Ordovician

The End-Ordovician event consisted of two distinct pulses of extinction centered around the glaciation of the southern supercontinent, Gondwanaland, which was situated over the South Pole.

1. The first pulse occurred at the onset of glaciation and the start of sea level fall. This primarily affected deep-shelf benthic faunas, with trilobites being among the most strongly affected groups.
2. The second pulse occurred at the sudden end of the ice age when ice caps melted and the climate warmed rapidly. This led to a sudden sea level rise, causing the extinction of shallow-shelf faunas, significantly impacting brachiopods and corals. During the height of the ice age, a cosmopolitan fauna known as the "Hirnantia" fauna temporarily flourished.

The Role of Early Land Plants in Weathering

Additional evidence for $CO_2$ drawdown comes from the fossil record of early land plants. The first spores from land plants appear in the Middle Ordovician around $462 imes 10^{6}$ years ago ($462 ext{Ma}$). These spores possess a thick covering to prevent desiccation, a feature common to all land plants that reproduce via spores. Although these early land plants (mosses, liverworts, and hornworts) were non-vascular and lacked roots, they significantly increased chemical weathering. By weathering silicate rocks and phosphate deposits, these non-vascular plants were the final factor that pushed atmospheric $CO_2$ below the critical threshold required for the glaciation that triggered the mass extinction.

Biological Scale and Selectivity of the End-Ordovician Event

The End-Ordovician mass extinction is the second largest of the "Big Five" events, surpassed only by the End-Permian. Approximately $22\%$ of all extant families and $60\%$ of all extant genera became extinct. This event was statistically more severe than the asteroid impact at the end of the Cretaceous. However, it is considered a "cull" rather than a selective extinction because almost no higher taxa (such as orders or classes) were completely lost. This makes it a bizarre outlier compared to other mass extinctions that were taxonomically selective (such as the loss of non-avian dinosaurs). While groups were whittled down—Brachiopods lost one-third of their families and three-quarters of their genera—the higher taxonomic framework survived, and the fauna of the subsequent Silurian period remained ecologically almost identical to that of the Late Ordovician.