Spineless Wonders of Evolution 193millions of years, you see them gradually lose their spongy outer layers and develop into
a small nucleus with four spongy arms (Lithocyclia aristotelis), then three arms (Lithocyclia
angusta), and finally two arms forming a spindle-like shape (Cannartus tubarius). The Cannar-
tus lineage then gradually develops a “waist” on the central sphere, then the arms get shorter
and thicker, and finally, they split into two lineages: Cannartus peterssoni-Ommatartus hughesi,
which evolves into a form with two arms with multiple spongy layers, and Ommatartus,
which develops shorter arms and a fatter central sphere. If you look at the two extremes
(a spongy sphere turning into a spindle-shaped shell with multiple caps), you could never
imagine that they are closely related—yet I have looked at the slides from those cores and
seen the gradual transition from one extreme to the other with my own eyes.
There are multiple studies on evolutionary patterns and transitions in other microfos-
sils, including the diatoms and coccolithophorids (see Lazarus [1983] and the articles in
Paleobiology, volume 9, number 4, fall 1983 for examples). For space reasons, however, we
will leave the extraordinary record of microfossils and look at the patterns in the more easily
studied macroinvertebrates.
The Story in the Seashells
Why don’t paleontologists bother to popularize the detailed lineages and species-to-
species transitions? Because it is thought to be unnecessary detail. . . . Paleontologists
clearly consider the occurrence of evolution to be a settled question, so obvious as
to be beyond rational dispute, so, they think, why waste valuable textbook space on
such tedious detail?
—Kathleen Hunt, FAQ, http://www.talkorigins.orgIf you find it difficult to relate to tiny fossils that can only be studied with expensive
microscopes, you can go right out to your local fossiliferous outcrop and study patterns
of evolution there. Take, for example, the famous cliffs along the shores of Chesapeake
Bay (fig. 3.2A and B) that are made of solid shells of mollusks from the Miocene (about
18–5 million years ago). Some of the most common and distinctive fossils are the large
scallops known as Chesapecten. One of these species, Chesapecten jeffersoni, is the state fos-
sil of Virginia. As numerous studies have shown (Ward and Blackwelder 1975; Miyazaki
and Mickevich 1982; Kelley 1983), the shells of these scallops change continually through
time. The earliest forms are known as Chesapecten coccymelus (fig. 8.8); they are abundant
in Zone 10 of the middle Miocene Calvert Formation. As you pass up from the Calvert
Formation to the Choptank Formation, there are specimens of Chesapecten nefrens, which
have shells that are longer from front to back than they are high from hinge to opening;
this trend continues through the series. As you move up through the series, the number of
ribs decreases as well, except in Chesapecten middlesexensis, which reverses the trend and
develops more ribs. Many more subtle differences in the shells can be detected (Ward and
Blackwelder 1975), so many that any competent paleontologist can tell the species apart
easily and decide what time in the Miocene or Pliocene is represented just by the scallops.
If you live in the United Kingdom, go out to the White Cliffs of Dover and nearby areas
and walk along the base of the cliffs. Weathering out of the soft chalky limestone (made
entirely of coccoliths, as we just mentioned) are hundreds of small heart urchin shells from