Rising mountains, cooling oceans prompted spread of invasive species 450 million years ago

This slab of rock contains fossils of invasive species that populated the continent of Laurentia 450 million years ago after a major ecological shift occurred. Ohio University geologists found that rising mountains and cooling oceans prompted the spread of these invasive species. -  Alycia Stigall
This slab of rock contains fossils of invasive species that populated the continent of Laurentia 450 million years ago after a major ecological shift occurred. Ohio University geologists found that rising mountains and cooling oceans prompted the spread of these invasive species. – Alycia Stigall

New Ohio University research suggests that the rise of an early phase of the Appalachian Mountains and cooling oceans allowed invasive species to upset the North American ecosystem 450 million years ago.

The study, published recently in the journal PLOS ONE, took a closer look at a dramatic ecological shift captured in the fossil record during the Ordovician period. Ohio University scientists argue that major geological developments triggered evolutionary changes in the ancient seas, which were dominated by organisms such as brachiopods, corals, trilobites and crinoids.

During this period, North America was part of an ancient continent called Laurentia that sat near the equator and had a tropical climate. Shifting of the Earth’s tectonic plates gave rise to the Taconic Mountains, which were forerunners of the Appalachian Mountains. The geological shift left a depression behind the mountain range, flooding the area with cool water from the surrounding deep ocean.

Scientists knew that there was a massive influx of invasive species into this ocean basin during this time period, but didn’t know where the invaders came from or how they got a foothold in the ecosystem, said Alycia Stigall, an Ohio University associate professor of geological sciences who co-authored the paper with former Ohio University graduate student David Wright, now a doctoral student at Ohio State University.

“The rocks of this time record a major oceanographic shift, pulse of mountain building and a change in evolutionary dynamics coincident with each other,” Stigall said. “We are interested in examining the interactions between these factors.”

Using the fossils of 53 species of brachiopods that dominated the Laurentian ecosystem, Stigall and Wright created several phylogenies, or trees of reconstructed evolutionary relationships, to examine how individual speciation events occurred.

The invaders that proliferated during this time period were species within the groups of animals that inhabited Laurentia, Stigall explained. Within the brachiopods, corals and cephalopods, for example, some species are invasive and some are not.

As the geological changes slowly played out over the course of a million years, two patterns of survival emerged, the scientists report.

During the early stage of mountain building and ocean cooling, the native organisms became geographically divided, slowly evolving into different species suited for these niche habitats. This process, called vicariance, is the typical method by which new species originate on Earth, Stigall said.

As the geological changes progressed, however, species from other regions of the continent began to directly invade habitats, a process called dispersal. Although biodiversity may initially increase, this process decreases biodiversity in the long term, Stigall explained, because it allows a few aggressive species to populate many sites quickly, dominating those ecosystems.

This is the second time that Stigall and her team have found this pattern of speciation in the geological record. A study published in 2010 on the invasive species that prompted a mass extinction during the Devonian period about 375 million years ago also discovered a shift from vicariance to dispersal that contributed to a decline in biodiversity, Stigall noted.

It’s a pattern that’s happening during our modern biodiversity crisis as well, she said.

“Only one out of 10 invaders truly become invasive species. Understanding the process can help determine where to put conservation resources,” she said.

Scientists reveal secrets of ancient ocean in new book

The Rheic Ocean separated two major land masses 430 million years ago. - art by: Christina Ullman, Ullman Design
The Rheic Ocean separated two major land masses 430 million years ago. – art by: Christina Ullman, Ullman Design

Call it the ocean that time forgot. About 400 million years ago, the Rheic Ocean played a big role in Earth’s history. When this massive body of water closed, the Appalachians were lifted to Himalayan heights and the planet’s continents slammed together to form the supercontinent of Pangaea. Dinosaurs and early mammals evolved to traverse the large swath of land, spreading life to every corner of the globe.

But the Rheic Ocean doesn’t get much attention in the field of geology today. In fact, American texts give usually credit to an older ancient sea, the Iapetus, for creating the Appalachians.

Ohio University geologist Damian Nance and colleagues now hope to set the record straight with a new book published this fall by the Geological Society of America. It pulls together recent data from a team of UNESCO-funded scientists in the United States, Germany, Britain, Portugal, Turkey and several former Eastern Block countries who have spent years combing for better geological evidence of this ancient ocean and its legacy.

The Rheic Ocean opened 480 million years ago and, by 430 million years ago, separated two major land masses. To the north was Laurentia, which comprised North America, Europe, Greenland and part of Asia. To the south lay Gondwana, which comprised Africa, South America, Antarctica, Australia and India.

The sea closed some 340 million years ago, which pushed the continents together and created two mountain ranges: the Appalachian mountains of North America and the Variscan Belt of Europe, which runs across southern Europe and North Africa from Ireland to the Czech Republic and from Morocco to the Black Sea. Both mountain belts have eroded greatly over time, “shadows of their former selves,” Nance noted.

When the Atlantic Ocean opened and pushed the pieces of Pangaea apart again, geological evidence of the Rheic Ocean’s line of closure became buried – or was carried half a world away to Europe. That’s why scientists in areas ranging from Texas to Turkey have had to puzzle together remaining scraps of information to reconstruct this sea’s important history.

Nance, for example, has found evidence of the Rheic Ocean in rock formations in Mexico. That also suggests that the range is a bit longer than previously thought, said the scientist, who recently served as an expert on the topic for a National Geographic television program that will air in early 2008.

The creation and break up of supercontinents is one of the hot topics in geological sciences, Nance said, as these major plate movements impact climate change and lead to extinctions. “All hell breaks loose when they get together. When they break up, they are responsible for massive sea level changes,” he said.

The Earth’s land masses have merged into supercontinents at least twice – 300 million years ago to create Pangaea and 1 billion years ago to form Rodinia – and may have done so repeatedly through Earth history. Scientists project that in 250 million years, the closure of the Atlantic Ocean will merge North and South America with Africa and Eurasia, forging what some call “Pangaea Ultima.”

Sound like science fiction? Researchers already see evidence of this future world, Nance said. The Atlantic is about as old and as wide as an ocean can get. As it ages, the floor of an ocean becomes heavier and colder, eventually sinking into the Earth’s interior like a plank of waterlogged wood. Geologists see signs of seabed collapse off Gibraltar and anticipate that the Atlantic floor off the American East Coast and African west coast will be the first to sink.

Learning more about the ancient oceans and continental shifts not only helps scientists predict future geological changes, Nance added, but can suggest where on Earth certain natural resources may lie.

Ancient maps of the world also explain certain synergies in what are now disparate locations – the similar feel of Newfoundland and Scotland, for example, despite their separation by the Atlantic Ocean – and also some surprises. For much of Earth’s history, our Sunshine State was part of Africa. “If it wasn’t for the closure of the Rheic Ocean,” Nance says, “we wouldn’t have Florida. It actually belongs to Mauritania.”