Everyone is fascinated by the idea of "living fossils," which
isn’t a precise scientific term but is a popular way to refer to animals which
have survived unchanged while the evolving world passed them by. I looked at a lot of these in Rumors of Existence, my first book, and they still interest me.
The coelacanth is the most famous: others
include the tuatara, a lizardlike New Zealand reptile whose three eyes (the
third is degenerate but functional) watched the dinosaurs come and go.
The term "living fossil" is not reserved for
vertebrates. Among the myriad specimens
dredged up by the famous Galathea expedition in the early 1950s were ten
limpet-like shelled animals. They came
from sea-bottom mud over 3,000, t beneath the surface off the Pacific coast of
Costa Rica. What were they? No one was
sure. The new discoveries had pale
yellow shells with an oval shape, about four cm long and one cm high. A
large foot (colored pink and blue) was surrounded by five pairs of primitive
gills.
While the shell
and teeth said "mollusc," the gill regions showed a segmented
construction resembling annelid worms.
Was the new animal either of these, or was it something entirely
unique? It most resembled a model that
biologist Brooks Knight had created showing what the ancestor of today's
molluscs might have looked like. But
that hypothetical animal - no actual fossil had ever been found - was presumed to have died out 350 million
years ago. Neopilina galathea filled an important gap in the
evolutionary record. Taxonomically, the
little critter was literally placed in a class by itself. Since then, more Neopilina species have been
dredged from the depths. The scientific
detective work of finding more examples and determining their exact place in
the parade of evolution goes on.
Neopilina (Harvard)
The great pioneering
undersea vessel, the submersible Alvin (still working today!) pulled in one of its many notable discoveries in 1979. Near a hydrothermal vent in the eastern
Pacific, researchers on the sub collected a strange-looking stalked barnacle,
the stalk serving to allow these normally fixed creatures some degree of
mobility. It had never been seen before,
even as a fossil, but apparently belonged to a group which flourished before
the dawn of the Age of Reptiles.
The waters off New
Zealand produced a similar surprise in 1985.
Clinging to sunken logs a thousand meters below the surface was a round
animal barely over a centimeter wide.
Named the sea daisy, it appeared to be a distant relative of the
starfish, even though only vestiges of the classic five-pointed starfish design
were apparent. That was enough to put it
into same phylum, the echinoderms, but it proved very difficult to classify
this diminutive invertebrate more precisely. The sea daisy is spiny on top, and
its underside is covered by a flat membrane that biologist Michael Bright
compares to plastic wrap stretched over an upside-down saucer. The sea daisy, too, was assigned its own
class (now the infraclass Concenticloidea, in which it inhabits the order Peripodida. Two other speies have been added). When it was discovered there was just nothing like it,
except for fossils predating the dinosaurs.
Fossils from the
same period included the graptolites, tiny colonial creatures who built homes
of collagen secretions layered in strips like mummy bandages. At one end of each 2.5cm-long long communal
house, a peculiar sharp spike rose like a TV antenna. Graptolites were presumed to be related to
modern homebuilders called pterobranches, but pterobranch dwellings lacked the
characteristic spike.
After an apparent
absence of 300 million years, graptolites resurfaced. In 1992, French researchers sent a sampling
of seafloor specimens to Dr. Noel Dilly, a London ophthalmologist whose
"hobby" of studying pterobranches grew on him until he became one of
the leading experts on the animals.
Dilly's first reaction was, "Not another boring collection to hack
through." His second was, "I
don't believe this." He was looking
at characteristic graptolite dwellings, spikes and all.
The graptolite is
a reminder that not all animals evolve: some just find a comfortable ecological
niche and settle down for a long stay.
There an awful lot
of little creatures like this to be found.
In the mid-1980s, Frederick Grassle of Rutgers University led an effort
to collect over two hundred core samples of the Atlantic seafloor. When all the sediment had been sifted, the
somewhat flabbergasted scientists found they had collected 460 new
invertebrates.
The littlest animals offer many surprises, and
no one thinks the surprises are over.
SOME FUN READING
Batten, Roger L. 1984. "Neopilina, Neomphalus and Neritopsis:
Living Fossil Molluscs," in Eldredge, Niles, and Steven M. Stanley (eds).
Living Fossils. New York: Springer
Verlag.
Bright, Michael. 1987. The Living World. New York: St. Martin's Press.
Cromie, William J. 1966. The Living World of the Sea. Englewood
Cliffs, NJ: Prentice-Hall.
Huyghe, Patrick. 1993. "New Species Fever," Audubon,
March-April.
Kaharl, Victoria A. 1990. Water
Baby: the Story of Alvin. New York:
Oxford University Press.
Soule, Gardner (ed.) 1968. Under
the Sea. New York: Meredith Press.
Svitil, Kathy. 1993. "It's Alive, and It's a
Graptolite," Discover, July.
Taylor, Mike. 1993. "Home and Away," BBC Wildlife,
March.
Wilson, Edward O. 1992. The
Diversity of Life. Cambridge, MA:
Belknap Press.
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