[Paleopsych] SW: Dinosaurs and Grass
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Paleontology: Dinosaurs and Grass
http://scienceweek.com/2005/sw051223-4.htm
[And God said, Let the earth bring forth grass, the herb yielding seed,
and the fruit tree yielding fruit after his kind, whose seed is in itself,
upon the earth: and it was so. And the earth brought forth grass, and herb
yielding seed after his kind, and the three yielding fruit, whose seed was
in itself, after his kind: and God saw that it was good. And the evening
and the morning were the third day. --Genesis 1:11-13
[And God created great whales, and every living creature that moveth,
which the waters brought forth abundantly, after their kind, and every
winged fowl after his kind: and saw that it was good. And God blessed
them, saying, Be fruitful and multiply, and fill the waters in the seas,
and let fowl multiply in the earth. And the evening and morning were the
second day. --Genesis 1:21-23
[Paleontologists, however, hold that there were sea animals long before
there was grasss. In either case, the role of grass is generally
overlooked.]
The following points are made by D.R. Piperno and H-D. Sues (Science
2005 310:1126):
1) Grasses (family Poaceae or Gramineae), with about 10,000 extant
species, are among the largest and most ecologically dominant families
of flowering plants, and today provide staple foods for much of
humankind. Dinosaurs, the dominant mega-herbivores during most of the
Mesozoic Era (65 to 251 million years ago), are similarly one of the
largest and best known groups of organisms. However, the possible
coevolution of grasses and dinosaurs has never been studied. New
work[1] reports analysis of phytoliths -- microscopic pieces of silica
formed in plant cells -- in coprolites that the authors attribute to
titanosaurid sauropods that lived in central India about 65 to 71
million years ago. Their data indicate that those dinosaurs ate
grasses.
2) Part of the difficulty in studying the question of dinosaur-grass
coevolution results from the poor quality of the fossil record for
early grasses. The earliest unequivocal grass fossils date to the
Paleocene-Eocene boundary, about 56 million years ago [2,3], well
after the demise of nonavian dinosaurs at the end of the Cretaceous
Period. Pollen and macrofossils of Poaceae are uncommon in sedimentary
strata until the middle Miocene, about 11 to 16 million years ago,
when the family is thought to have undergone considerable evolutionary
diversification and ecological expansion [2]. Thus, dioramas in
museums have long depicted dinosaurs as grazing on conifers, cycads,
and ferns in landscapes without grasses. The work of Prasad et al [1]
is the first unambiguous evidence that the Poaceae originated and had
already diversified during the Cretaceous. The research shows that
phytoliths, which have become a major topic of study in Quaternary
research over the last 20 years [4,5], can provide a formidable means
for reconstructing vegetation and animal diets for much earlier time
periods when early angiosperms were diversifying. These results will
force reconsideration of many long-standing assumptions about grass
evolution, dinosaurian ecology, and early plant-herbivore
interactions.
3) Researchers have long known that grasses make distinctive kinds of
phytoliths in the epidermis of their leaves and leaflike coverings
that surround their flowers. More recent work has examined in greater
detail phytolith characteristics from a large set of grasses
comprising taxa representing the entire range of diversification
within the family, showing that discriminations at the subfamily,
tribe, and genus levels are often possible [1,4,5]. In addition,
publication of a well-resolved consensus phylogeny of the Poaceae by
the Grass Phylogeny Working Group (GPWG) considerably advances our
overall understanding of the evolutionary history of grasses and leads
to improved interpretations of the early grass fossil record. For
example, by mapping the phytolith characters that discriminate clades
and subfamilies of extant taxa onto this phylogenetic tree, we can
infer how phytolith morphology changed at the origin of major clades
and lineages.
References (abridged):
1. V. Prasad, C. A. E. Strömberg, H. Alimohammadian, A. Sahni, Science
310, 1177 (2005)
2. B. F. Jacobs, J. D. Kingston, L. L. Jacobs, Ann. Mo. Bot. Gard. 86,
590 (1999)
3. E. A. Kellogg, Plant Physiol. 125, 1198 (2001)
4. D. R. Piperno, Phytolith Analysis: An Archaeological and Geological
Perspective (Academic Press, San Diego, CA, 1988)
5. G. G. Fredlund, L. T. Tieszen, J. Biogeogr. 21, 321 (1994)
Science http://www.sciencemag.org
--------------------------------
Related Material:
DINOSAURS, DRAGONS, AND DWARFS: THE EVOLUTION OF MAXIMAL BODY SIZE
The following points are made by G.P. Burness et al (Proc. Nat. Acad.
Sci. 2001 98:14518):
1) The size and taxonomic affiliation of the largest locally present
species ("top species") of terrestrial vertebrate vary greatly among
faunas, raising many unsolved questions. Why are the top species on
continents bigger than those on even the largest islands, bigger in
turn than those on small islands? Why are the top mammals marsupials
on Australia but placentals on the other continents? Why is the
world's largest extant lizard (the Komodo dragon) native to a
modest-sized Indonesian island, of all unlikely places? Why is the top
herbivore larger than the top carnivore at most sites? Why were the
largest dinosaurs bigger than any modern terrestrial species?
2) A useful starting point is the observation of Marquet and Taper
(1998), based on three data sets (Great Basin mountaintops, Sea of
Cortez islands, and the continents), that the size of a landmass's top
mammal increases with the landmass's area. To explain this pattern,
they noted that populations numbering less than some minimum number of
individuals are at high risk of extinction, but larger individuals
require more food and hence larger home ranges, thus only large
landmasses can support at least the necessary minimum number of
individuals of larger-bodied species. If this reasoning were correct,
one might expect body size of the top species also to depend on other
correlates of food requirements and population densities, such as
trophic level and metabolic rate. Hence the authors assembled a data
set consisting of the top terrestrial herbivores and carnivores on 25
oceanic islands and the 5 continents to test 3 quantitative
predictions:
a) Within a trophic level, body mass of the top species will increase
with land area, with a slope predictable from the slope of the
relation between body mass and home range area.
b) For a given land area, the top herbivore will be larger than the
top carnivore by a factor predictable from the greater amounts of food
available to herbivores than to carnivores.
c) Within a trophic level and for a given area of landmass, top
species that are ectotherms will be larger than ones that are
endotherms, by a factor predictable from ectotherms' lower food
requirements.
3) The authors point out that on reflection, one can think of other
factors likely to perturb these predictions, such as environmental
productivity, over-water dispersal, evolutionary times required for
body size changes, and changing landmass area with geological time.
Indeed, the database of the authors does suggest effects of these
other factors. The authors point out they propose their three
predictions not because they expect them always to be correct, but
because they expect them to describe broad patterns that must be
understood in order to be able to detect and interpret deviations from
those patterns.
Proc. Nat. Acad. Sci. http://www.pnas.org
--------------------------------
Related Material:
ECOLOGY: ON FOOD-WEB INTERACTIONS
The following points are made by A.R. Ives and B.J. Cardinale (Nature
2004 429:174):
1) Growing concern about how loss of biodiversity will affect
ecosystems has stimulated numerous studies(1-5). Although most studies
have assumed that species go extinct randomly, species often go
extinct in order of their sensitivity to a stress that intensifies
through time (such as climate change).
2) For two reasons, interactions among species make it difficult to
predict how ecological communities will respond to environmental
degradation. First, the sensitivity of an individual species to
environmental degradation depends not only on the direct impact of
degradation on that species, but also on the indirect effects on that
species caused by changes in densities of other species. For example,
environmental degradation may decrease the density of competitors
and/or predators of a species, thereby causing a compensatory increase
in the density of that species. Second, as species go extinct, links
within the food web are severed, changing the pathways through which
indirect effects operate. Changes in food-web structure depend on the
order in which species go extinct, making it difficult to extrapolate
from studies that assume extinctions are random to real communities
facing progressively intensifying stress from environmental
degradation.
3) To disentangle the effects of species interactions on the ability
of communities to tolerate environmental degradation, the authors used
mathematical simulations to compare how communities resist changes in
abundance as species go extinct randomly versus going extinct in order
of their sensitivity to an environmental stress. The authors
considered communities with three trophic topologies that span a range
of community types: tritrophic communities with plants, herbivores,
and predators; monotrophic communities comprising just competitors;
and communities with arbitrary topology containing prey and predators,
competitors, and mutualists. For each topology, the authors
constructed 1000 communities in which interaction strengths were
chosen from random distributions under the constraints of the
specified topology. The direct effects of the stressor on each species
were also selected at random, but constrained so that the stressor had
a negative effect on all species by decreasing their population growth
rates.
4) The authors demonstrate that the consequences of random and ordered
extinctions differ. Both depend on food-web interactions that create
compensation; that is, the increase of some species when their
competitors and/or predators decrease in density due to environmental
stress. Compensation makes communities as a whole more resistant to
stress by reducing changes in combined species densities. As
extinctions progress, the potential for compensation is depleted, and
communities become progressively less resistant. For ordered
extinctions, however, this depletion is offset and communities retain
their resistance, because the surviving species have greater average
resistance to the stress. Despite extinctions being ordered, changes
in the food web with successive extinctions make it difficult to
predict which species will show compensation in the future. This
unpredictability argues for "whole-ecosystem" approaches to
biodiversity conservation, as seemingly insignificant species may
become important after other species go extinct.
References (abridged):
1. Sala, O. E. et al. Global biodiversity scenarios for the year 2100.
Science 287, 1170-1174 (2000)
2. Chapin, F. S. I. et al. Consequences of changing biodiversity.
Nature 405, 234-242 (2000)
3. Ehrlich, P. & Ehrlich, A. Extinction (Random House, New York, 1981)
4. Terborgh, J. et al. Ecological meltdown in predator-free forest
fragments. Science 294, 1923-1926 (2001)
5. Naeem, S., Thompson, L. J., Lawler, S. P., Lawton, J. H. & Woodfin,
R. M. Declining biodiversity can alter the performance of ecosystems.
Nature 368, 734-737 (1994)
Nature http://www.nature.com/nature
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