Monday, April 1, 2019

Surviving Extinction


SURVIVING EXTINCTION EPISODES

The pre-Cambrian asteroid bombardment may have been preceded by more spectacular episodes that delayed the evolution and entry of larger organisms into the perilous environment of earlier times.  Successful organisms would be dispersed in most directions and become established to spread the range of those species.  At the same time on the periphery of the range variation and natural selection resulted in new species when changes made them reproductively isolated from the ancestral ones.  Well established ones would outcompete and/or expand the range.

The deuterostomes may never have evolved without the drastic environment of the early asteroid bombardment.  Pogonophora are probably ancestral to all deuterostomes.  Protostomes probably include many groups whose separation from annelids ancestors was prior to, at the same time, or later than the separation from annelid ancestors of the first deuterostomes, the pogonophorans.

SURVIVAL MECHANISMS ON LAND AND SHALLOW SEAS

Many factors that contribute to survival are of value in reproduction and/or dispersal.

Lucky locations
Regions remote from impact may have had caverns, sediments, isolated aquatic habitats and other locations where survival of cysts, eggs, hibernating stages, or other mechanisms enabled survival.  Perhaps ice shelves in polar seas protected organisms beneath them.  Perhaps marine forms drifting down to death were sometimes lifted back into survival depths by a later impact at the end of bombardment.

Degrowth
Fat and some other tissues can be utilized to maintain life during periods of starvation.  An extreme example of degrowth is shown by some jellyfish that are capable of absorbing reproductive organs and regressing to earlier life stages as they grow smaller.  They may be able to do that in successive seasons as they drift from nutrient rich bays to open oceans with less food and back to nutrient rich locations; the cycle could be part of the annual cycle of productivity.

Absorption of dissolved nutrients
All phyla tested, except arthropods, have shown the ability of some species to absorb amino acids from dissolved amino acids in water at some stages of their lives.  The ability to take up dissolved organic matter from seawater is a particularly important method of nutrition for species having eggs with little yolk that hatch before feeding organs are well-developed.  The arthropods, with their chitinous exoskeleton, are not equipped to get nutrition via absorption of dissolved nutrients nor have useful degrowth; this may have been part of the reason trilobites became extinct at the end of the Paleozoic.

Resistant stages
Overwintering eggs of arthropods such as many insects, fairy shrimp, and perhaps many other animals have the ability to repopulate a habitant after adults die from winter freezing or ponds dry and refill.  Freshwater sponges produce asexual cysts (gemmules) that have similar use.  Such stages are often an important dispersal mechanism for organisms to reach new habitats via mud on water bird feet, or on mammal fur.  The stages may survive a trip though the digestive tract during dispersal.  Such mechanisms were probably partially selected by the value of providing a new generation into the areas of greatest damage after an extinction event before others lacking such a survival mechanism arrive and become competitors or predators.

THE ABYSS AS A REFUGE

The abyssal portion of the ocean is the most widespread area of the earth and the depth and stratification of the overlying sea make it likely that much of it would be relatively untouched by an asteroid or two sending catastrophic waves over land and shallow seas killing most organisms that did not have protected refuges (caves or burrows etc.).  Those not destroyed still had to find food and other requirements for survival.  The peak of bombardment was followed by a reduced supply of remnants of the fractured planet that occupied the zone between Mars and Jupiter.  One big one, like the last one the helped finish off the dinosaurs, produced many extinctions and opportunities for newly evolving species to take over roles of the extinct forms.

The pogonophorans would continue to expand range and send descendants into new areas.  Other groups would also be testing new environments, for example, the crinoids appear to have had adaptation to abyssal conditions and retained many of the adaptive features after repopulating the shallow seas after the worst of the bombardment was over.  Pogonophorans have been found in a few locations in sediments only a few hundred feet deep, but the many vertebrates and the connecting groups diverged from their common ancestors almost a billion years ago with origins at various depths and locations where some stability of the environment with hospitable conditions existed.

The specificity of environmental adaption for deep sea animals has been shown by the limited range of a few hundred meters isolating similar species to nearby depressions surrounded by other related species encircling the surrounding abyssal area.  Such environmental specificity should not be a surprise when you see how uniform the alpine limit of the tree line can be.

Some key features that enabled pogonophorans to survive in their abyssal habitat included:

A very slow metabolism compatible with survival on sediments receiving very little nutrient input in the form of amino acids from slowly decaying organic debris (perhaps supplemented by surface waters raining down organisms killed by the surface disturbance).

The depth of their tubes enabling survival from attacks by predators.

The slow reproductive rate and growth rate allowing reproduction hundreds of years after the worst of conditions.

Their hemoglobin and circulatory system enabling adequate oxygen to be stored and or transported to the posterior end where nutrients were absorbed in anaerobic sediments.

Planktonic larvae capable of dispersal over great distances and time to repopulate large devastated areas of sea sediments.  Those that did not progress to shallower seas are still much the same as the pre-Cambrian ancestral stage of the line ancestral to chordates.

Joseph G. Engemann   Emeritus Professor of Biology, Western Michigan University
 Kalamazoo, Michigan     April 1, 2019 (no fooling)


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