A Naturalist at Larg e 
The Elusive Eureka 
A puzzling fossil provides a lesson 
in the scientific method 
by Niles Eldredge 
The tendency of scientists to make 
pronouncements from remote Olym- 
pian heights lends credence to the 
charge that science is just another belief 
system, no different from any other. 
The current efforts of fundamentalist 
Christians to inject the biblical story of 
Creation into biology curricula across 
the nation exploit this very impression. 
Creationist cries of “equal time” appeal 
to people’s sense of fair play because 
science, like religion, often appears au- 
thoritarian. And it is true that science 
has become so complex that no one can 
hope to grasp it all. Most of us are con- 
tent to believe that the earth is round, 
even though we are personally unable 
to make the necessary observations that 
verify it. But before a scientific idea can 
earn this kind of routine acceptance, it 
must be subjected to the scrutiny of the 
“scientific method.” 
The scientific method is not some 
mysterious form of higher cerebration. 
Its principle is simply that our ideas 
about the universe — how it is con- 
structed, how it came to be — must con- 
form to our observations. Scientists test 
an idea by predicting what they should 
find if it is true, and their understand- 
ing of the natural world is refined in a 
constant interplay of ideas and observa- 
tions. This very human exercise is very 
different from the unquestioning accep- 
tance of “revealed truth.” A scientist is 
prepared to evaluate any new piece of 
evidence, ( even if it challenges an ortho- 
dox theory. Sometimes the theory will 
be discarded or modified; sometimes 
the evidence will be thrown out of 
court. But in either case something will 
have been gained. I have here a modest 
example of how the system works; per- 
haps it will give an idea of what scien- 
tists actually do for a living. 
No one (presumably) would dare 
write an article for a science magazine 
without being reasonably sure of his or 
her ground. So when I reported in the 
July 1980 issue of Natural History on 
distribution patterns of Devonian trilo- 
bites in the Southern Hemisphere, I was 
relying on 1 50 years of accumulated pa- 
leontological research, as well as ten 
years of my own contemplation of these 
fossils. The picture I presented was a 
neat one. Trilobites are extinct relatives 
of crabs and shrimps; those that I wrote 
about lived in what was evidently a 
cold-water habitat during the Devo- 
nian period, 395 to 345 million years 
ago. Their fossils are most numerous in 
Andean South America from southern 
Peru south through Bolivia and Argen- 
tina. They also occur in less exuberant 
numbers in the Amazon Basin, por- 
tions of southern Brazil and Uruguay, 
the Falkland Islands, and South Africa. 
The most reasonable explanation for 
this distribution is that there once was 
a great, partly submerged landmass, 
which geologists call Gondwana, that 
broke up to form South America, Ant- 
arctica, Africa, India, and Australia. 
The widely scattered localities that 
yield these cold-water Devonian fossils 
were once much closer together, and all 
were south of 60° south latitude. 
As the article for Natural History 
was in its final editorial phase, however, 
I received a letter from Alfonso Segura 
Paguaga, a Central American geologist 
living in Esparza, a rural area of Costa 
Rica near the Pacific coast city of Pun- 
tarenas. I was startled at his announce- 
ment of the discovery of a Devonian 
trilobite in Costa Rica. No rocks even 
remotely as old had ever been found 
there. Paguaga wanted me to examine 
the specimen, proposing to name it 
Phacops esparsocostariccensis. I hur- 
riedly wrote back that I would be de- 
lighted to work on the trilobite, the 
possibility of a bona fide discovery lurk- 
ing in the back of my mind. 
Paguaga arrived in my office with the 
trilobite, as well as fossils from four 
separate outcrops in the environs of 
Esparza. Eagerly unwrapping the trilo- 
bite first, I was flabbergasted. There 
before me lay a typical example of Me- 
tacryphaeus tuberculatus — a very com- 
mon trilobite, indeed, but one known 
only from southern Peru and Bolivia. 
“Were I not informed,” I wrote rather 
formally in my report to Paguaga, “that 
the specimen came from Costa Rica, I 
would without hesitation say it came 
from the region of Padilla, Bolivia.” 
Two things were puzzling about the 
fossil. The first was its age: as far as I 
knew, Venezuela and Colombia in 
South America, and Mexico in North 
America, were the closest places to 
Costa Rica that had ever produced a 
Devonian trilobite. Second, the speci- 
men was not the type of trilobite we 
would expect to find even in those 
areas. Trilobites from northern South 
America are of the same basic stripe as 
our own Northern Hemisphere Appa- 
lachian trilobites — and radically differ- 
ent from those, such as Metacryphaeus, 
that lived in the cold seas that bathed 
Gondwana. 
The geologic history of Central 
America and the Caribbean is extreme- 
ly complex and as yet only partly un- 
raveled. What we seemed to have was 
24 
