Showing posts with label mammals. Show all posts
Showing posts with label mammals. Show all posts

Saturday, May 14, 2011

Reptiles, Mammals, and the Triassic Climate

The skull of the procolophonid Hypsognathus 
was found in Fundy basin, Nova Scotia, which 
was hotter and drier when it was part of 
angaea. Mammals, needing more water, chose 
to live elsewhere. Photo Credit: Jessica 
Whiteside, Brown University.
More than 200 million years ago, mammals and reptiles lived in their own separate worlds on the supercontinent Pangaea, despite little geographical incentive to do so. Mammals lived in areas of twice-yearly seasonal rainfall; reptiles stayed in areas where rains came just once a year. Mammals lose more water when they excrete, and thus need water-rich environments to survive. Results are published in the Proceedings of the National Academy of Sciences.

PROVIDENCE, R.I. [Brown University] — Aggregating nearly the entire landmass of Earth, Pangaea was a continent the likes our planet has not seen for the last 200 million years. Its size meant there was a lot of space for animals to roam, for there were few geographical barriers, such as mountains or ice caps, to contain them.

Yet, strangely, animals confined themselves. Studying a transect of Pangaea stretching from about three degrees south to 26 degrees north (a long swath in the center of the continent covering tropical and semiarid temperate zones), a team of scientists led by Jessica Whiteside at Brown University has determined that reptiles, represented by a species called procolophonids, lived in one area, while mammals, represented by a precursor species called traversodont cynodonts, lived in another. Though similar in many ways, their paths evidently did not cross.

“We’re answering a question that goes back to Darwin’s time,” said Whiteside, assistant professor of geological sciences at Brown, who studies ancient climates. “What controls where organisms live? The two main constraints are geography and climate.”

Turning to climate, the frequency of rainfall along lines of latitude directly influenced where animals lived, the scientists write in a paper published this week in the online early edition of the Proceedings of the National Academy of Sciences. In the tropical zone where the mammal-relative traversodont cynodonts lived, monsoon-like rains fell twice a year. But farther north on Pangaea, in the temperate regions where the procolophonids predominated, major rains occurred only once a year. It was the difference in the precipitation, the researchers conclude, that sorted the mammals’ range from that of the reptiles.

Reptile precursor

The skull of the procolophonid Hypsognathus was found in Fundy basin, Nova Scotia, which was hotter and drier when it was part of Pangaea. Mammals, needing more water, chose to live elsewhere.

The scientists focused on an important physiological difference between the two: how they excrete. Mammals lose water when they excrete and need to replenish what they lose. Reptiles (and birds) get rid of bodily waste in the form of uric acid in a solid or semisolid form that contains very little water.

On Pangaea, the mammals needed a water-rich area, so the availability of water played a decisive role in determining where they lived. “It’s interesting that something as basic as how the body deals with waste can restrict the movement of an entire group,” Whiteside said.

In water-limited areas, “the reptiles had a competitive advantage over mammals,” Whiteside said. She thinks the reptiles didn’t migrate into the equatorial regions because they already had found their niche.

The researchers compiled a climate record for Pangaea during the late Triassic period, from 234 million years ago to 209 million years ago, using samples collected from lakes and ancient rift basins stretching from modern-day Georgia to Nova Scotia. Pangaea was a hothouse then: Temperatures were about 20 degrees Celsius hotter in the summer, and atmospheric carbon dioxide was five to 20 times greater than today. Yet there were regional differences, including rainfall amounts.

The researchers base the rainfall gap on variations in the Earth’s precession, or the wobble on its axis, coupled with the eccentricity cycle, based on the Earth’s orbital position to the sun. Together, these Milankovitch cycles influence how much sunlight, or energy, reaches different areas of the planet. During the late Triassic, the equatorial regions received more sunlight, thus more energy to generate more frequent rainfall. The higher latitudes, with less total sunlight, experienced less rain.

The research is important because climate change projections shows areas that would receive less precipitation, which could put mammals there under stress. 

“There is evidence that climate change over the last 100 years has already changed the distribution of mammal species,” said Danielle Grogan, a graduate student in Whiteside’s research group. “Our study can help us predict negative climate effects on mammals in the future.”

Contributing authors include Grogan, Paul Olsen from Columbia University, and Dennis Kent from Rutgers. The National Science Foundation and the Richard Salomon Foundation funded the research.


  • Citation

  • Jessica H. Whiteside, Danielle S. Grogan, Paul E. Olsen, and Dennis V. Kent. 2011. Climatically driven biogeographic provinces of Late Triassic tropical Pangea. PNAS doi:10.1073/pnas.1102473108

    Saturday, March 5, 2011

    Are We In The 6th Mass Extinction?

    The following is a University of California press release.While the following study looks at mammals, it seems likely that similar situations exist with amphibians and reptiles.

    Tigers are one of Earth's most critically endangered species. Extinction of the majority of such species would indicate that the sixth mass extinction is in our near future. With the steep decline in populations of many animal species, from frogs and fish to tigers, some scientists have warned that Earth is on the brink of a mass extinction like those that occurred only five times before during the past 540 million years.

    Each of these ‘Big Five’ saw three-quarters or more of all animal species go extinct.

    In a study published in the March 3 issue of the journal Nature, University of California, Berkeley, paleobiologists assess where mammals and other species stand today in terms of possible extinction, compared with the past 540 million years, and they find cause for hope as well as alarm.

    “If you look only at the critically endangered mammals – those where the risk of extinction is at least 50 percent within three of their generations – and assume that their time will run out, and they will be extinct in 1,000 years, that puts us clearly outside any range of normal, and tells us that we are moving into the mass extinction realm,” said principal author Anthony D. Barnosky, UC Berkeley professor of integrative biology, a curator in the Museum of Paleontology and a research paleontologist in the Museum of Vertebrate Zoology.

    “If currently threatened species – those officially classed as critically endangered, endangered and vulnerable – actually went extinct, and that rate of extinction continued, the sixth mass extinction could arrive within as little as 3 to 22 centuries,” he said.

    Nevertheless, Barnosky added, it’s not too late to save these critically endangered mammals and other such species and stop short of the tipping point. That would require dealing with a perfect storm of threats, including habitat fragmentation, invasive species, disease and global warming,

    “So far, only 1 to 2 percent of all species have gone extinct in the groups we can look at clearly, so by those numbers, it looks like we are not far down the road to extinction. We still have a lot of Earth’s biota to save,” Barnosky said. “It’s very important to devote resources and legislation toward species conservation if we don’t want to be the species whose activity caused a mass extinction.”

    Coauthor Charles Marshall, UC Berkeley professor of integrative biology and director of the campus’s Museum of Paleontology, emphasized that the small number of recorded extinctions to date does not mean we are not in a crisis.

    “Just because the magnitude is low compared to the biggest mass extinctions we’ve seen in a half a billion years doesn’t mean to say that they aren’t significant,” he said. “Even though the magnitude is fairly low, present rates are higher than during most past mass extinctions.”

    “The modern global mass extinction is a largely unaddressed hazard of climate change and human activities,” said H. Richard Lane, program director in the National Science Foundation’s Division of Earth Sciences, which funded the research. “Its continued progression, as this paper shows, could result in unforeseen – and irreversible – negative consequences to the environment and to humanity.”

    The study originated in a graduate seminar Barnosky organized in 2009 to bring biologists and paleontologists together in an attempt to compare the extinction rate seen in the fossil record with today’s extinction record. These are “like comparing apples and oranges,” Barnosky said. For one thing, the fossil record goes back 3.5 billion years, while the historical record goes back only a few thousand years. In addition, the fossil record has many holes, making it is impossible to count every species that evolved and subsequently disappeared, which probably amounts to 99 percent of all species that have ever existed. A different set of data problems complicates counting modern extinctions.

    Dating of the fossil record also is not very precise, Marshall said.

    “If we find a mass extinction, we have great difficulty determining whether it was a bad weekend or it occurred over a decade or 10,000 years,” he said. “But without the fossil record, we really have no scale to measure the significance of the impact we are having.”

    To get around this limitation, Marshall said, “This paper, instead of calculating a single death rate, estimates the range of plausible rates for the mass extinctions from the fossil record and then compares these rates to where we are now.”

    Barnosky’s team chose mammals as a starting point because they are well studied today and are well represented in the fossil record going back some 65 million years. Biologists estimate that within the past 500 years, at least 80 mammal species have gone extinct out of a starting total of 5,570 species.

    The team’s estimate for the average extinction rate for mammals is less than two extinctions every million years, far lower than the current extinction rate for mammals.

    “It looks like modern extinction rates resemble mass extinction rates, even after setting a high bar for defining ‘mass extinction,’” Barnosky said.

    After looking at the list of threatened species maintained by the International Union for Conservation of Nature (IUCN), the team concluded that if all mammals now listed as “critically endangered,” “endangered” and “threatened” go extinct, whether that takes several hundred years or 1,000 years, Earth will be in a true mass extinction.

    “Obviously there are caveats,” Barnosky said. “What we know is based on observations from just a very few twigs plucked from the enormous number of branches that make up the tree of life.”

    He urges similar studies of groups other than mammals in order to confirm the findings, as well as action to combat the loss of animal and plant species.

    “Our findings highlight how essential it is to save critically endangered, endangered and vulnerable species,” Barnosky added. “With them, Earth’s biodiversity remains in pretty good shape compared to the long-term biodiversity baseline. If most of them die, even if their disappearance is stretched out over the next 1,000 years, the sixth mass extinction will have arrived.”

    Citation
    Barnosky, A. D., N. Matzke, S. Tomiya, G. O. U. Wogan, B. Swartz, T. B. Quental, C. Marshall, J. L. McGuire, E. L. Lindsey, K. C. Maguire, et al. Has the Earth’s sixth mass extinction already arrived? Nature, 471, 51-57. DOI: 10.1038/nature09678