single-celled photosynthetic plankton
only live for a few days; months of darkness might easily destroy entire populations.
These photosynthetic cells form the base of oceanic food chains. If they die,
then the herbivorous plankton have nothing to eat and they perish; the carnivorous
plankton find no herbivores, the tiny invertebrates no plankton, the fish no invertebrates--up
and up to the fragile top of the ecological pyramid.
The rates of extinction
for genera in several groups of planktonic organisms are staggering (and imply
an even greater percentage of species deaths, for most genera contain several
species, and all must be killed to efface a genus, while surviving genera may
lose most of their species): 73 percent of coccolithophorids, 85 percent of radiolaria,
and a whopping 92 percent of foraminifera. But an exploration of the different
rules model must focus on winners and the reasons for their success. So let us
consider the primary anomaly, one cited by many authors as an argument against
the dust cloud hypothesis: the diatoms, perhaps the most prominent group of photosynthetic
plankton, sailed through the Cretaceous debacle with a generic loss of only 23
percent.
We might first take an experimental approach and ask if a few months
of darkness can cause not only the death of populations, but a differential
death--with some species surviving for predictable reasons, and others
dying because they lack the tools of success. A study just published by
Kathy Griffis and David J. Chapman supports this prerequisite for explanation
under the different rules model ("Survival of Phytoplankton under
Prolonged Darkness: Implications for the Cretaceous-Tertiary Boundary
Darkness Hypothesis," in Palaeogeography, Palaeoclimatology, Palaeoecology--called
P-cubed by those in the know--1988, pp. 305-14).
Griffis and Chapman obtained cultures of several photosynthesizing
planktonic species, including some closely related to forms that survived the
extinction and others belonging to groups that first appeared after the extinction
and might not have fared well in the preceding darkness. In a happy example of
relatively "small" science (you don't need million-dollar grants from
the National Science Foundation for all good work), they simulated the scenario
of darkness by wrapping 500-milliliter Erlenmeyer flasks, each holding a population
of one species, in aluminum foil. They controlled for other factors by providing
constant temperatures and adequate supplies of nutrients. Three species belonging
to groups that arose after the extinction were dead within a week. Two others,
closely related to forms that pulled through the great dying, survived eight to
ten weeks without light--a good estimate for the actual time of darkness in
several dust cloud models.
We must then ask if a factor of success, consistent with the different
rules model, can be identified. Jennifer A. Kitchell, David L. Clark,
and Andrew M. Gombos, Jr., have recently made such a strong argument for
the planktonic heroes of the Cretaceous debacle--the diatoms ("Biological
Selectivity of Extinction: A Link Between Background and Mass Extinction,"
in Palaios--not called P-to-the-first-power, even by those
in the know--1986, pp. 504-11). Kitchell and colleagues studied a
core of diatom-rich late Cretaceous sediments from the High Arctic (85.6°
north latitude). They noted that a stretch of the core contained finely
laminated sediments. The alternating layers consisted almost entirely
of diatom cells in different states: some layers made almost entirely
of cells in their phase of vegetative growth (up to 96.4 percent); others
of "resting spores" in a state of dormancy (up to 93.3 percent).
These alternations record nothing about the extinction to come, but must
represent an adaptive response to long seasonal fluctuations near the
poles: with nearly six unbroken months of darkness every year, a cell
that normally lives but a few weeks and subsists by photosynthesis must
evolve some mechanism of dormancy--a kind of hibernation--in order
to survive a long winter of worse than discontent for any organism dependent
upon sunlight. Many species of diatoms have evolved such a complex life
cycle; cells deprived of light or nutrients can shut down their metabolism,
form a resting spore by encystment, increase in density, and sink to lower
levels in the water column, awaiting the return of propitious times.
Removal of light is
not the only factor that elicits a transformation to resting spores. Diatoms build
their cell walls of silica, which they must extract from seawater. They therefore
thrive in areas of the ocean, called zones of upwelling, where deeper waters,
rich in nutrients (including the vital silica), rise to the surface. But periods
of upwelling are sporadic or seasonal. Diatoms must be flexible enough to take
advantage of these infrequent and uncertain bounties--able both to enter a
growth phase and produce a so-called diatom bloom when nutrients become available
and to hunker down as resting spores when their own growth depletes the temporary
building supply.Kitchell and colleagues discerned a common theme behind
this flexibility to exploit seasonal and sporadic sources of the two necessities
in a diatom's world--silica for construction, and light for growth and maintenance.
Diatoms developed the capacity to form resting spores in order to wait out predictably
fluctuating seasons of inhospitable environments. They evolved this key adaptation
for ordinary life in normal times, not in anticipation of relative success should
a comet strike the earth several million years in the future. Yet if months of
darkness triggered death by extraterrestrial impact, then diatoms were fortuitously
well placed to be survivors. Diatoms are not better than coccoliths or radiolaria;
they are not fiercer competitors on an oceanic surface jammed full of wedges.
They were just lucky enough to have on hand (if you will pardon an inappropriate
metaphor from the apex of the chain of being) a lucky physiological trick for
survival, evolved for other reasons in different times. (Interestingly, Griffis
and Chapman note only one common feature for the species that died within a week
of darkness in their experiments: "none. . .appeared to produce resting cysts
in response to the darkness.") Kitchell and colleagues end their paper with
a strong defense of the different rules model for diatom success: "These
data document an incidental, but causal, dependency between a biological character,
selected for in normal background times of geologic history, and evolutionary
survivorship during an exceptional time of crisis in earth history."
Life under the different rules model recalls the myth of Sisyphus, greedy
king of Corinth, who was punished in Hades by being forced to roll a heavy
stone up a steep hill; he groans and struggles, finally approaching the
summit, but the stone always slips and rolls back down to the bottom,
where Sisyphus must start all over again. Sisyphus, patiently and painfully
rolling the stone up the mountain, works like life under Darwin's metaphor
of the wedge--slow and steady progress by constant struggle, ad
astra per aspera. But this work of normal times is undone by moments
of catastrophe, and nothing ever happens in a larger sense.
This
comparison of life with the Sisyphus myth works up to a point, but then fails
in a crucial way. The undoing of the slow and patient work of the wedge does not
demote you all the way down to square one. Catastrophes of mass extinction do
not beat life back to an earlier starting point; rather, they deflect the stone
of cumulative organic change into some unexpected and unfailingly interesting
side channel. They create, by their imposition of different rules, a new regime
of oddly |