27 Evolution and the History of Life: The End-Permian Mass Extinction

Geological Context and the History of Life

The history of life on Earth is categorized into broad eras and specific periods that trace the evolution and extinction of organic structures. The Precambrian era began approximately $4.5\text{ billion years ago}$, with the earliest organic structures appearing around $3.8\text{ billion years ago}$. The Paleozoic era, spanning from approximately $542\text{ million years ago}$ to $251\text{ million years ago}$, includes the Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian periods. Following the Paleozoic is the Mesozoic era ($251\text{ million}$ to $65\text{ million years ago}$), comprising the Triassic, Jurassic, and Cretaceous periods. The Cenozoic era encompasses the Tertiary and Quaternary periods, the latter of which includes the Pleistocene and Holocene epochs. Within the Tertiary period, subdivisions include the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs.

The End-Permian Mass Extinction: "The Great Dying"

The End-Permian mass extinction is recognized as the largest biological crisis in Earth's history, earning the moniker "The Great Dying." According to research by Steven Stanley published in the Proceedings of the National Academy of Sciences, the cumulative losses during this event were catastrophic across all domains of life. The extinction claimed approximately $57\%$ of all biological families, $62\%$ of all genera, $81\%$ of marine species, and $70\%$ of terrestrial vertebrate species. Data indicates that the number of genera plummeted from a peak of nearly $4000$ to approximately $1000$ during the transition from the Permian to the Triassic period.

The Two-Phase Extinction Model

The crisis at the end of the Permian is often characterized by two distinct extinction pulses. The first was the Terminal Guadalupian extinction, which primarily struck marine organisms, resulting in the loss of about $35\%$ of all marine genera. The second and more severe pulse was the Terminal Permian extinction (the Permo-Triassic Boundary). This event was global, impacting both marine and terrestrial ecosystems, and stands as the first mass extinction to profoundly devastate terrestrial organisms.

Comprehensive List of Extinct Taxa

The End-Permian event resulted in the total extinction of several major groups that had been prominent throughout the Paleozoic. These include:

• Corals: Both Tabulate Corals and Rugose Corals went entirely extinct.

• Arthropods: The iconic Trilobites and the Eurypterids (sea scorpions) were completely lost.

• Echinoderms: Blastoid echinoderms, such as Pentremites tulipiformis, and three major groups of Crinoids vanished.

• Mollusks: Goniatitic Ammonoids went extinct.

• Brachiopods: Three entire orders of Brachiopods were wiped out.

• Fish: The Acanthodian fish (e.g., Acanthodes) disappeared from the fossil record.

• Synapsids: The Pelycosaur synapsids and twenty distinct families of therapsid synapsids were lost.

While some groups suffered near-total extinction, only a few lineages of ammonoids, bryozoans, and crinoids managed to survive into the Triassic.

The Terrestrial Record in the Karoo Basin

The Karoo Basin in South Africa provides the only complete and richly fossiliferous terrestrial sedimentary record spanning the End-Permian extinction events. In the Late Permian, the basin supported a highly diverse therapsid fauna, including various Dinocephalia, Biarmosuchia, and Gorgonopsia. However, following the extinction event, this diversity was replaced in the earliest Triassic by a nearly monospecific assemblage dominated by Lystrosaurus. This genus is often cited as a "peak evolutionary form" or a disaster taxon because of its ability to thrive while almost all other terrestrial vertebrate competitors perished.

Paleogeography and Environmental Collapse

By approximately $255\text{ million years ago}$, the Earth's landmasses had coalesced into the supercontinent Pangea, surrounded by the vast Panthalassic Ocean and the Paleo-Tethys and Tethys Oceans. This geographic configuration contributed to extreme environmental shifts. Evidence shows that forests virtually disappeared during the transition; woody conifers were replaced by non-woody lycopods. Furthermore, the dominant fluvial systems changed from meandering streams to braided streams, a transition interpreted as a result of the collapse of plant cover on riverbanks, which previously stabilized soil.

Marine Anoxia and Geochemical Evidence

There is abundant evidence for deep-ocean anoxia during the End-Permian. Geological records from uplifted deep-sea sediments in Japan show a transition where highly oxidized hematite was replaced by gray chert. Extreme anoxia at the end of the Permian is marked by the presence of black shales, and ocean waters remained dysoxic (containing very low oxygen concentrations) until well into the Middle Triassic. Scientists suggest that the ocean was stagnant, with oxygen-poor, carbon dioxide-rich, and hydrogen sulfide-rich deep waters potentially upwelling onto shallow shelves, though the exact mechanism for such a sudden mixing remains a subject of debate.

Isotopic Excursions and Climate Extremes

The Permo-Triassic boundary is marked by significant isotopic excursions. A sharp negative carbon isotope excursion ($\delta^{13}C$) indicates massive releases of light carbon into the atmosphere, consistent with extreme global warming. Oxygen isotope ($\delta^{18}O$) values derived from conodonts allow for the estimation of oceanic temperatures. These estimates suggest upper ocean temperatures reached as high as $104^\circ F$ or $40^\circ C$. Because photosynthesis becomes extremely difficult above $35^\circ C$ and water temperature is inversely related to oxygen solubility, these hot waters were likely hypoxic or anoxic. This shift is also visible in the sedimentology; the cool, moist conditions that supported the Glossopteris flora in the southern hemisphere were replaced by Triassic "red beds." These red beds, colored by ferric oxide, indicate hot, dry conditions and the dehydration of brown iron hydroxides.

The Siberian Traps as the Primary Kill Mechanism

The leading theory for the cause of this warming and subsequent mass extinction involves Large Igneous Provinces (LIPs). Specifically, the Siberian Traps represent a massive accumulation of igneous rock and lava flows covering an area roughly the size of the United States. The eruptions began approximately $252\text{ million years ago}$, coinciding precisely with the extinction event, and lasted for up to one million years. The primary kill mechanism is believed to be the massive release of carbon dioxide ($CO_2$), largely triggered by the volcanic heat burning through continent-sized deposits of fossil coal buried during the Carboniferous period. This resulted in a simple but catastrophic temperature rise that destabilized global ecosystems.