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Residential Life Goes
West • Chewing
on Evolution •
Atmospheric Ambiguities
Lobsters Play Biological Violins •
Remembering Wannamaker
When is a Platypus Not a Kangaroo?
• Solution
for Smokers •
In Brief
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Chewing on
Evolution
 esearchers
have discovered that a small mammal-like reptile that lived 260 million
years ago is the first known efficient land vertebrate chewer—able
to use a shearing chewing action to break down tough vegetation.
The finding, the scientists say, provides evidence that
the seemingly modest ability to orally process food efficiently allowed
animals to digest a wider range of vegetation, sparking the evolution
of a diversity of herbivores. This diversity enabled the evolution
of the modern terrestrial animal ecosystem, in which abundant herbivores
serve as food for a small number of carnivores. Before this evolution,
the scientists say, the ecosystem was quite different, with herbivores
being very rare and most vertebrates eating either invertebrates or
other vertebrates that fed on invertebrates.
In an article in a June issue of Nature, Duke graduate
student Natalia Rybczynski and Robert Reisz, professor of zoology
at the University of Toronto at Mississauga, report microscopic studies
of the teeth of the foot-long reptile Suminia, whose distant relatives
eventually evolved into mammals. Suminia predated the dinosaurs (which
branched from the vertebrate tree millions of years later), some of
whose plant-eating species evolved similar chewing mechanisms.
The scientists’ studies of the teeth of the gangly,
big-eyed, large-toothed Suminia found telltale horizontal scratches
whose structure revealed that the animal brought its posterior teeth
together and created an upward and backward shearing motion, called
a power stroke, to shred plant material efficiently.
“Chewing is particularly important because if an
animal can more efficiently chew its food, it can digest more quickly
and increase its rate of food intake,” Rybczynski says. “Such
increased intake could have supported an elevated metabolism, similar
to mammals.”
Rybczynski says herbivores without such chewing ability
tend to eat the more tender leaves, flowers, or buds of plants. Minimal
oral processing of vegetation is also associated with a slower digestion
rate. An example is the iguana, which, like other animals that do
not chew extensively, simply swallows vegetation and allows it to
digest for a long time. “So, Suminia is the best example we have
from such an early era of an animal that is adapted to high-fiber
herbivory. It was clearly more specialized to eat coarse, fibrous
food than anything else of the time.”
The scientists were prompted to study Suminia in part
because of anatomical evidence that a close relative, called dicynodonts—the
first successful terrestrial plant-eating vertebrate—was also
probably efficient at “oral processing” of food. Rybczynksi
notes that most dicynodonts could not be said to “chew,”
since that term applies only to animals with teeth.
“What is immediately striking about this animal is
that it has really large teeth and they occlude, or meet. This is
unlike iguanas, crocodiles, and most other non-mammalian vertebrates,
whose teeth don’t even touch. Since the teeth occluded, we knew
that Suminia had some sort of specialized chewing mechanism,”
Rybcynski says.
The scientists performed electron-microscopic studies
that revealed the details of striations resulting from shearing of
food. Besides the horizontal nature of those striations, the scientists
observed that one end of the worn surface showed a deeper “divot”
at the junction between the hard surface enamel and the softer dentine
than the other. The shape of this dentine surface indicated where
sand and other particles had been jammed against the enamel, proving
that the motion was backward.
The scientists obtained their specimens from the Paleontological
Institute of the National Academy of Sciences in Russia, which receives
support from the University of Toronto. Their study was supported
by the National Geographic Society and the National Sciences and Engineering
Research Council of Canada.
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