Showing posts with label Cretaceous extinction. Show all posts
Showing posts with label Cretaceous extinction. Show all posts

Friday, October 28, 2011

Land Animals & Ecosystems Decimated During the Permian Extinction

To the left. Lystrosaurs escaped the destruction of the Permian catastrophe as did the meter high spore-tree Pleuromeia it is feeding on. Illustration credit: Victor Leshky.

PROVIDENCE, R.I. [Brown University] — The cataclysmic events that marked the end of the Permian Period some 252 million years ago were a watershed moment in the history of life on Earth. As much as 90 percent of ocean organisms were extinguished, ushering in a new order of marine species, some of which we still see today. But while land dwellers certainly sustained major losses, the extent of extinction and the reshuffling afterward were less clear.

In a paper published in the journal Proceedings of the Royal Society B, researchers at Brown University and the University of Utah undertook an exhaustive specimen-by-specimen analysis to confirm that land-based vertebrates suffered catastrophic losses as the Permian drew to a close. From the ashes, the survivors, a handful of genera labeled "disaster taxa," were free to roam more or less unimpeded, with few competitors in their respective ecological niches. This lack of competition, the researchers write, caused vicious boom-and-bust cycles in the ecosystems, as external forces wreaked magnified havoc on the tenuous links in the food web. As a result, the scientists conclude from the fossil record that terrestrial ecosystems took up to 8 million years to rebound fully from the mass extinction through incremental evolution and speciation.

"It means the (terrestrial ecosystems) were more subject to greater risk of collapse because there were fewer links" in the food web, said Jessica Whiteside, assistant professor of geological sciences at Brown and co-author on the paper.

The boom-and-bust cycles that marked land-based ecosystems' erratic rebound were like "mini-extinction events and recoveries," said Randall Irmis, a co-author on the paper, who is a curator of paleontology at the Natural History Museum of Utah and an assistant professor of geology and geophysics at Utah.

The hypothesis, in essence, places ecosystems' recovery post-Permian squarely on the repopulation and diversification of species, rather than on an outside event, such as a smoothing out of climate. The analysis mirrors the conclusions reached by Whiteside in a paper published last year in Geology, in which she and a colleague argued that it took up to 10 million years after the end-Permian mass extinction for enough species to repopulate the ocean — restoring the food web — for the marine ecosystem to stabilize.

"It really is the same pattern" with land-based ecosystems as marine environments, Whiteside said. The same seems to hold true for plants, she added.

Some studies have argued that continued volcanism following the end-Permian extinction kept ecosystems' recovery at bay, but Whiteside and Irmis say there's no physical evidence of such activity.

The researchers examined nearly 8,600 specimens, from near the end of the Permian to the middle Triassic, roughly 260 million to 242 million years ago. The fossils came from sites in the southern Ural Mountains of Russia and from the Karoo Basin in South Africa. The specimen count and analysis indicated that approximately 78 percent of land-based vertebrate genera perished in the end-Permian mass extinction. Out of the rubble emerged just a few species, the disaster taxa. One of these was Lystrosaurus, a dicynodont synapsid (related to mammals) about the size of a German shepherd. This creature barely registered during the Permian but dominated the ecosystem following the end-Permian extinction, the fossil record showed. Why Lystrosaurus survived the cataclysm when most others did not is a mystery, perhaps a combination of luck and not being picky about what it ate or where it lived. Similarly, a reptilian taxon, procolophonids, were mostly absent leading to the end-Permian extinction, yet exploded onto the scene afterward.

"Comparison with previous food-web modeling studies suggests this low diversity and prevalence of just a few taxa meant that links in the food web were few, causing instability in the ecosystem and making it susceptible to boom-bust cycles and further extinction," Whiteside said.

The ecosystems that emerged from the extinction had such low animal diversity that it was especially vulnerable to crashes spawned by environmental and other changes, the authors write. Only after species richness and evenness had been re-established, restoring enough population numbers and redundancy to the food web, did the terrestrial ecosystem fully recover. At that point, the carbon cycle, a broad indicator of life and death as well as the effect of outside influences, stabilized, the researchers note, using data from previous studies of carbon isotopes spanning the Permian and Triassic periods.

"These results are consistent with the idea that the fluctuating carbon cycle reflects the unstable ecosystems in the aftermath of the extinction event," Whiteside said.

Friday, July 22, 2011

Baenid Turtles Survived the K-T Extinction

New fossil localities from North Dakota and Montana have produced the remains of a turtle that survived the 65 million-year-old meteorite impact that wiped out the dinosaurs. The resulting study, published in the latest issue of the Journal of Vertebrate Paleontology, suggests that Boremys, a turtle that belongs to a group known as the baenids (bay-een-ids) survived the extinction event unharmed. Baenids are a group of extinct river turtles native to North America that flourished from approximately 80 million to 42 million years ago.
Diagram of the many types of turtles that survived the K/T extinction event. The skulls of these turtles are illustrated (as viewed from above) as are their fossil record (thick blue bars) and evolutionary relationships (thin lines connecting bars). The red line marks the extinction event, and is matched with the photographic of the geologic section in the background. The rocks below the red line were deposited during the Age of Dinosaurs (Mesozoic Era), whereas those above the line were deposited at the beginning of the Age of Mammals (Cenozoic Era). Note that eight different lineages of turtles survived the event, while only two went extinct at the event. Image courtesy of Erik Freeman.

The lead author, Tyler Lyson from Yale University, has been collecting turtles from the western United States for years, and immediately realized the importance of these new specimens, "This find further confirms that turtles were not fazed by the meteorite that killed the dinosaurs 65 million years-ago."

The survival of turtles during this massive extinction event appears counterintuitive with what we know about other organisms. While other groups of animals show high rates of extinction at the Cretaceous/Paleocene (K/T) boundary 65-million-years ago, this new discovery provides more data indicating that turtles were largely unaffected by the meteorite impact. Lyson and colleagues found that if you just looked at turtles during this time, you would not even notice that one of the largest extinction events in Earth’s history had occurred.

The researchers recovered the new Boremys turtle remains from rocks in southwestern North Dakota and eastern Montana. These rock formations, known as the Hell Creek and Fort Union, respectively, have been actively collected for fossils by paleontologists for more than 100 years.  This new study proves that even in well-searched localities, new fossil discoveries can still be uncovered. Co-author Dr. Walter Joyce acknowledged his surprise with the new discovery, "At first I did not believe Tyler when he told me that he had found Boremys in Paleocene deposits."

Lyson and his co-authors believe that features of the shell of Boremys did not help its chances of preservation in the fossil record. The turtle had very thin shell bones relative to other baenids, and they remained unfused throughout the turtle’s life. These features likely led to its skeleton being scattered after death, making the chances of a whole shell being preserved very low. The authors also noted that the features of the shell were very similar to another turtle species living at the same time, which lead to initial misidentification.

With the discovery of this turtle, Lyson and his colleagues can now say that at least eight types of baenid turtles survived the meteorite impact that killed so many other animals.