Context: Recently, Stanford researchers have found strong evidence to support the theory that a devastating extinction event was caused by a lack of oxygen in Earth’s oceans approximately 444 million years ago. 
Background 

• Today, human-caused global warming is the primary cause of marine oxygen loss, and a variety of species are at risk. 

Key Points

• The investigation was focused on the Late Ordovician Mass Extinction, which is one of the “Big Five” mass extinction events in Earth’s history. 
• The most famous of the great die-offs is the Cretaceous-Paleogene event that wiped out three-quarters of all plant and animal species, including the dinosaurs, about 65 million years ago. 
• During the Late Ordovician event nearly 450 million years ago, most life was confined to the oceans and only a few plants had moved onto land. The continents were still mostly connected as a single giant landmass called Gondwana. 
• The initial wave of extinction was triggered by a global cooling that impacted much of Gondwana. Approximately 444 million years ago, a second wave of extinction set in that is theoretically tied to ocean anoxia. 
• It found that by the end of the Late Ordovician event, around 85 percent of marine species vanished from the fossil record. 
• Previous studies have derived ocean oxygen concentrations through the analysis of ancient sediments containing isotopes of metals such as uranium and molybdenum. 
• These elements have different chemical reactions in anoxic conditions compared to well-oxygenated conditions. 
• Now,researchers developed a new model which combined the existing metal isotope data with new data from samples of black shale, which was recovered from the Murzuq Basin in Libya. The shale was deposited in the geological record during the mass extinction event.  
• Based on the model, the researchers concluded that severe and prolonged anoxia must have occurred across large volumes of Earth’s deep seabeds. 
• The research indicates that the deoxygenation across modern oceans will push many species toward extinction. And by expanding our thinking of how oceans have behaved in the past, we could gain some insights into the oceans today.

Other Four Mass Extinctions Event

• The Late Devonian: This period is now regarded as a number of 'pulses' of extinction spread over 20m years, beginning 380m years ago. This extinction has been linked to major climate change, possibly caused by an eruption of the volcanic Viluy Traps area in modern-day Siberia. A major eruption might have caused rapid fluctations in sea levels and reduced oxygen levels in the oceans.
• The Middle Permian:  Scientists have recently discovered another event 262m years ago that rivals the 'Big Five' in size. This event coincided with the Emeishan eruption in what's now China, and is known to have caused simultaneous extinctions in the tropics and higher latitudes.
• The Late Permian: The Late Permian mass extinction around 252m years ago dwarfs all the other events, with about 96% of species becoming extinct. The extinction was triggered by a vast eruption of the Siberian Traps, a gigantic and prolonged volcanic event that covered much of modern day Siberia, which led to a cascade of environmental effects.
• The Late Triassic: The Late Triassic event, 201m years ago, shares a number of similarities with the Late Permian event. It was caused by another large-scale eruption, this time of the Central Atlantic Magmatic Province, which heralded the splitting of the supercontinent Pangaea and the initial opening of what would later become the Atlantic Ocean.