56 Evolution and the Fossil Record
In the marine realm, there are many agents of destruction as well. Soft-bodied organ-
isms, like sea jellies and marine worms, almost never leave a fossil record. Even organisms
with hard parts, like mollusks and corals, are prone to destruction. Waves and currents wash
the shells back and forth and pound them into pieces, so only the most durable shells sur-
vive. Many shells are broken by predators, such as crabs and lobsters, which use their claws
or pincers to crack the shells or peel them open to get at the prey inside. Abandoned shells
are degraded by organisms that use them as anchors or places to attach. A whole group of
boring organisms, including sponges and algae, drill or dissolve holes in shells and reuse
their minerals, thus weakening the shells even further.
Once a bone or shell survives this grueling gantlet, there are still further hazards. After
burial, the shell or bone might be dissolved by water percolating through the sediments.
Many fossils are actually made of new minerals that have replaced the original minerals,
showing how little of the original material remains in the fossil record. As the potential fos-
sil gets buried deeper and deeper, the huge pressure on the pile of sediments above it may
distort the fossil or crush it entirely. Many deeply buried sedimentary rocks are actually
transformed by high heat and pressure into metamorphic rocks, and then all original fossil
traces vanish entirely.
If the shell or bone avoids or survives all these ordeals—dissolution, replacement, dis-
tortion, pressure, metamorphism—there are still more hazards ahead. Once the fossil-bear-
ing sediment is uplifted and exposed again, the fossil is prone to erosion. If it weathers out at
any time except when a paleontologist happens to wander by (which has only been happen-
ing at rare intervals in the past 200 years), then the fossil will be destroyed and lost forever.
There are only a few thousand paleontologists in the entire world who can only devote at
most a few weeks or months a year to collecting fossils, so most fossiliferous exposures go
unexamined and their fossils are lost. If you think hard about it, the odds that any given
organism will be fossilized and actually end up in our collections is minuscule. It is a miracle
that we have any fossil record at all.
There are other ways to estimate the quality of our fossil record (see Prothero 2013a:22).
At this moment, biologists know and have described and named about 1.5 million species
on earth (mostly insects), and some estimates say that the earth harbors at least 4 or 5 million
species in total. Yet there are at best only about 250,000 known species of fossil animals and
plants, or about 5 percent of the species living today. But today is only one time slice among
millions in the past 600 million years during which multicellular life has existed. If we total
up all those time slices, then the total number of species that are represented in the fossil
record is a tiny fraction of 1 percent.
Consequently, the fossil record of some groups that are entirely soft-bodied without hard
skeletons or shells (especially insects, worms, sea jellies, and the like) is so poor that most
paleontologists do not study them much and do not attempt to say much about their evolu-
tion. In certain groups with hard skeletons, however, the potential for preservation is much
higher. If we focus just on groups with excellent skeletons and a good chance for preserva-
tion (including microfossils, sponges, corals, mollusks, sea stars and sea urchins and their
relatives, trilobites, the “lamp shells” or brachiopods, and “moss animals” or bryozoans),
the fossil record is not nearly so incomplete. These groups have about 150,000 living species
but more than 180,000 fossil species. Depending on how you do the calculation, between
2 and 13 percent of all the species that have ever lived in these groups may be fossilized.
That’s still not great but much better than the fraction of 1 percent estimate we just discussed.