Showing posts with label distribution. Show all posts
Showing posts with label distribution. 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

    Tuesday, December 14, 2010

    Pseudoboa

    Pseudoboa neuweiidi, Trinidad, JCM
     There are six species of snakes in the genus Pseudoboa, all occur in South America, with one species, Pseudoboa neuweiidi, extending its range into the Lesser Antilles (Trinidad, Tobago, Grenada) and possibly into Panama. Pseudoboa coronata (the type species of the genus) and P. neuwiedii occur throughout the Amazon basin. Pseudoboa haasi and P. serrana are endemic to the southeastern and southern regions of the Atlantic rainforest respectively. Pseudoboa nigra occurs throughout the Caatinga, Cerrado, and Chaco biomes in Bolivia, Paraguay, and possibly Argentina. And, the most recently described species, Pseudoboa martinsi (Zaher et al. 2008), is from the Brazilian Amazon basin in the states of Pará, Amazonas, Roraima, and Rondônia. Recent molecular (Vidal et al. 2010) work places these snakes in the family Dipisididae, subfamily Xenodontinae, tribe Pseudoboini. The tribe also includes the genera Boiruna, Clelia, Drepanoides, Mussurana, Phimophis Oxyrhopus, Siphlophis, and Hydrodynastes. All of these are rear-fanged, venomous, and tend to feed on squamates and most of them show interesting ontogenetic color changes with the smaller individuals being more brightly colored or having more contrasting patterns, presumably the coloration and pattern are aposematic.  Hydrodynastes may be the exception to most of these generalizations. Additionally many of these snakes have a nape marking that is collar-like that may last into adulthood. 

    Recently, Orofino et al (2010) have reported on the natural history of Pseudoboa nigra using 147 museum specimens. That had been collected in Brazil’s Cerrado mostly from the states Sao Paulo and Mato Grosso. Females were found to be larger than males, the presence of eggs and neonates throughout the year suggests year round reproduction, and they suggest nigra has smaller clutches than other species in the genus. They found it feeds mostly on lizards, Ameiva were the most commonly found food. While the authors suggest that nigra is found in open habitats and other members of the genus are found in more forested environments, my experience with neuwiedii suggests it occurs in a variety of habitats including savanna, coastal, and semi-urbanized habitats that are open (Murphy 1997). While P. nigra may be uniform black in color, it can also have white, irregular blotches that could be considered aposematic coloration. Robert Mertens (1956) considered Pseudoboa a model for more palatable mimics in his proposed mimicry complex hypothesis that eventually became known as Mertens mimicry. To be sure the venom of Pseudoboa neuwiedii is quite toxic, being reported to kill cats, as well as conspecifics (Murphy, 1997). The fact that so many of these snakes are red or orange with a dark collar may mean they are all part of a mimicry complex that use the pattern to remind predators that the head is the business end of the snake. And, because the aposematic coloration is most often found in small individuals and usually not large adults, the warning message may be directed at smaller predators not capable of dealing with a larger snake.

    Literature
    Mertens R. 1956. Das Problem der Mimikry bei Korallenschlangen. Zool. Jahrb. Syst. 84:541–575.

    Morato, S. A. A., J. C. de Moura-Leite, A. L. C. Prudenete and R. S. Bernils. 1995. A new species of Pseudoboa Schneider, 1801 from southeastern Brazil (Serpentes: Colubridae: Xenodontinae: Pseudoboini). Biociências 3 (2):253-264.

    Murphy, J. C. 1997. Amphibians and Reptiles of Trinidad and Tobago. Malabar: Krieger Publishing.
    Orofino, R. de P., L. Pizzatto, and O, A. V. Marques. 2010. Reproductive biology and food habits of Pseudoboa nigra (Serpentes Dipsididae) from the Brazilian Cerrado.  Phylomedusa 9:53-61.

    Vidal, N., M. Dewynte,r and D. J. Gower. 2010. Dissecting the major American snake radiation: A molecular phylogeny of the Dipsadidae Bonaparte (Serpentes, Caenophidia). Comptes Rendus Biologies, 333:48-55

    Zaher, H.; Oliveira, M.E. & Franco, F.L. 2008. A new, brightly colored species of Pseudoboa Schneider, 1801 from the Amazon Basin (Serpentes, Xenodontinae). Zootaxa 1674: 27-37