Mass extinctions are rapid global decreases in Earth's biodiversity, with five key events identified over the planet's history, arguably the most famous of which occurred ~66 million years ago during the Cretaceous, which brought the rein of dinosaurs to an end. However, the largest mass extinction is attributed to the Permian, during which it is estimated that >95% of all life on Earth was eradicated.

The cause of this devasting event is still debated, with advocates for both a large asteroid impact that caused dust to plume into the atmosphere, blocking sunlight and generating acid rain, or significant volcanism that released copious amounts of CO2 into the atmosphere and made the oceans toxic to marine life.

New research published in Chemical Geology lends further support to the latter theory.

Diverse and full of sea life, the Earth's Devonian era—taking place more than 370 million years ago—saw the emergence of the first seed-bearing plants, which spread as large forests across the continents of Gondwana and Laurussia.

However, a mass extinction event near the end of this era has long been the subject of debate. Some scientists argue the Late Devonian mass extinction was caused by large-scale volcanic eruptions, causing global cooling. Others argue a mass deoxygenation event caused by the expansion of land plants was to blame.

A recently published study in the journal Communications Earth & Environment led by researchers at IUPUI now posits that both factors played a role—and draws attention to the environmental tipping points the planet faces today.

An international team of researchers led by botanists at the University of Vienna, Austria, has analyzed the morphological diversity of fossilized flowers and compared it with the diversity of living species. They found that flowering plants had already produced a large number of different flower types shortly after their emergence in the Cretaceous period, and this earliest floral diversity was greater than that today.

The study has been published in the journal New Phytologist.

Movement of rivers, mountains, oceans and sediment nutrients at the geological timescale are the central drivers of Earth's biodiversity, research published in Nature has revealed.

The research also shows that biodiversity evolves at similar rates to the pace of plate tectonics, the slow geological processes that drive the shape of continents, mountains and oceans.

"That is a rate incomparably slower than the current rates of extinction caused by human activity," said lead author Dr. Tristan Salles from the School of Geosciences.

The research looks back over 500 million years of Earth's history to the period just after the Cambrian explosion of life, which established the main species types of modern life.

Once a favored food of grazing dinosaurs, an ancient lineage of plants called cycads helped sustain these and other prehistoric animals during the Mesozoic Era, starting 252 million years ago, by being plentiful in the forest understory. Today, just a few species of the palm-like plants survive in tropical and subtropical habitats.

Like their lumbering grazers, most cycads have gone extinct. Their disappearance from their prior habitats began during the late Mesozoic and continued into the early Cenozoic Era, punctuated by the cataclysmic asteroid impact and volcanic activity that mark the K-Pg boundary 66 million years ago. However, unlike the dinosaurs, somehow a few groups of cycads survived to the present.

A study, "Nitrogen Isotopes Reveal Independent Origins of N2-Fixing Symbiosis in Extant Cycad Lineages," appearing Nov. 16 in the journal Nature Ecology & Evolution has concluded that the cycad species that survived relied on symbiotic bacteria in their roots, which provide them with nitrogen to grow. Just like modern legumes and other plants that use nitrogen fixation, these cycads trade their sugars with bacteria in their roots in exchange for nitrogen plucked from the atmosphere.