948 THE STRUCTURE OF EVOLUTIONARY THEORY
contribution to sustaining deep-sea life "can be punctuated by downpours of
'phylodetritus' (i.e., detrital material composed primarily of relatively fresh
phytoplanktonic remains), during which the flux of labile particulate organic
carbon to the seafloor temporarily exceeds biological demand, yielding a carpet of’
food'" (p. 7).
Finally, and to add a third punctuational source of maximally different
character from the physical and microfloral cases discussed above, Smith argues
(p. 10) that "whale falls" produce occasional and (obviously) "huge local pulses" of
organic matter that may decay to produce distinctive "chemosynthetic habitats"
supporting faunal associations much like those documented at deep-sea vents. For
example, in 1987, his team discovered a 21-meter whale skeleton at a depth of
1240 m: "The bones were covered with mats of sulfur bacteria and clusters of
small mussels and limpets; nearby sediments harbored large vesicomyid clams" (p.
10)—for a total of 42 macro-faunal species, only nine of which also inhabited
surrounding sediments. Smith concludes that "sunken whales may provide
dispersal stepping stones for at least some of the species dependent on sulfide-
based chemosynthesis."
Strong circumstantial evidence indicates considerable temporal and spatial
influence for this source that most of us would surely have regarded as dubious, if
not risible, at apparent face value of relative importance. A fossilized
chemosynthetic community has been reported from a 35 million year old whale fall
on the Northeast Pacific ocean floor (p. 10). "Whale skeletons," Smith concludes
(p. 10), "may be the dominant source of chemosynthetic habitats over the vast
sediment plains constituting most of the ocean floor."
At the opposite end of a hierarchy in spatial and temporal scales,
punctuational models continue to gain in strength and acceptability for events that
impact entire biotas at regional or even planetary scales—with catastrophic mass
extinction as a "flagship" notion, spurred by nearly conclusive evidence for bolide
impact as the trigger of the K-T global dying 65 million years ago (see Chapter 12
for full treatment). An expansion of research away from the extinctions
themselves, and towards the subsequent recovery phases as well, has strongly
accentuated the episodic and punctuational character of this most comprehensive
signal in the history of life.
Even after the Alvarez's impact hypothesis forced paleontologists to
acknowledge the potentially catastrophic nature of at least some mass extinctions,
students of fossils usually assumed that subsequent recoveries of global faunas
must have been tolerably gradual. This expectation has not been fulfilled, and
episodes of recovery from maximum decimation at the extinction to full
reestablishment of previous levels of diversity occur more quickly, and in a much
shorter percentage of the "normal" time (until the next mass extinction), than
previously suspected. (Of course, no one expects that recoveries which require
successive events of branching can be nearly as rapid as truly catastrophic
extinctions, which can feature truly simultaneous killings—so the complete record
of an extinction-recovery cycle will surely remain asymmetric. But the recoveries
now seem to occur rapidly enough, in