Looking Back: Little Living Fossils

              

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.