8
SPINELESS WONDERS
OF EVOLUTION
Invertebrate Transitions
There is a story, possibly apocryphal, of the distinguished British biologist J. B. S. Hal-
dane, who found himself in the company of a group of theologians. On being asked
what one could conclude as to the nature of the Creator from a study of his creation,
Haldane is said to have answered, “An inordinate fondness for beetles.”
—J. E. Hutchinson, “Homage to Santa Rosalia
or Why Are There So Many Kinds of Animals?”Most people are interested only in our own phylum Chordata, which includes verte-
brates. They don’t know or care about clams, snails, and “bugs” (or they consider them disgust-
ing and don’t want to know). Typically, people call most members of the phylum Arthropoda
(including not only insects but also spiders, scorpions, “pillbugs,” centipedes, millipedes,
lice, ticks, and many other unrelated groups) “bugs” and think of most marine invertebrates
that are worth eating as “shellfish.” Nevertheless, the invertebrates make up more than
99 percent of all living animals on earth. In fact, insects alone outnumber all other groups of
organisms in total diversity, and among insects, beetles are more diverse with more species
than any other group of animals. Despite the lack of public familiarity, invertebrates are not
only the most diverse animals alive today, but they also include the best-fossilized groups by
far. We will devote the remaining chapters of this book to looking at more familiar and more
popular examples from birds, mammals, and reptiles, but we cannot neglect the excellent
fossil record of transitions within the invertebrates. The invertebrates may not be as cute and
cuddly as mammals or birds, but they show us far more about evolution than the much less
complete fossil record of vertebrates.
The Incredible World of Microfossils
Unknown to Darwin, uninterrupted sedimentation does occur in the open ocean,
especially on aseismic ridges and plateaux. These areas experience a continuous rain
of particles to the sea bed, and are among the most geologically quiescent places on
Earth. A steady build-up of sediment is the result. . . . The sediments in question are
composed mainly of the shells of microscopic plankton such as foraminifera, radi-
olaria, diatoms and coccolithophorids. Large numbers of individuals can easily be
extracted. Their evolution can be followed through geological time, simply by com-
paring one closely-spaced sample with the next. This reveals morphologically iso-
lated and continuous lineages which it is reasonable to infer represent lines of genetic
descent. These lineages sometimes split from one another, and often evolve gradually
over vast periods of time, or become extinct. . . . Does the fossil record provide a true
and accurate record of first and last occurrences of species? Emphatically, the answer