Page 84 - Introduction to Paleobiology and The Fossil Record

TAPHONOMY AND THE QUALITY OF THE FOSSIL RECORD 71

bones or shells might be destroyed. Or if
the sediment is constantly being deposited Bias and adequacy
and reworked, for example in a river, any In his 1972 paper, David Raup argued persua-
skeletal remains may be worn and damaged sively that the fossil record is not only incom-
by physical movement. plete, but also that it is biased. This means
6 Diagenetic fi lters: after a rock has formed, that the distribution of fossils is not random
it may be buried beneath further accumu- with respect to time, but that it gets worse in
lating sediment. Over thousands or mil- older and older rocks. The evidence is two-
lions of years, the rock may be transformed fold: theoretical and observational. The theo-
by the passage of mineralizing waters, for retical evidence is persuasive. The last two or

example, and these may either enhance three of the filters just mentioned are time
the fossils, by replacing biological mole- related; the older the rocks, the more substan-
cules with mineral molecules, or they may tially they will have removed fossils from the
destroy the fossil. potential record. As times goes by, ancient
7 Metamorphic fi lters: over millions of fossiliferous deposits are ever more likely to
years, and the movements of tectonic have been metamorphosed, buried under
plates, the fossiliferous rock might be younger rocks, subducted into the mantle or
baked or subjected to high pressure. These eroded. The longer a fossil sits in the rock, the
kinds of metamorphic processes turn more likely one of these processes is to destroy
mudstones into shales, limestones into it. Further, paleontologists are familiar with
marbles. The fossils may survive these ter- this steady loss of information. If you try
rible indignities, or they may be to collect fossils from a Miocene lagoonal
destroyed. deposit, the shells are abundant and beauti-
8 Vertical movement fi lters: nearly all fossils fully preserved, and you can collect thousands
are in sedimentary rocks that have been in an hour or two. If you try to collect from
buried. Burial means the rock has been a fossiliferous deposit from the same environ-
covered by younger rock, and has gone ment in the Cambrian, fossils may be rare,
down to some depth. Tectonic movements they may be distorted by metamorphism, and
must subsequently raise the fossiliferous they may be hard to get out of the rock.
rock to the Earth’s surface, or the fossil Others have argued, however, that these
remains forever buried and unseen. biases apply only at certain levels of study.
9 Human fi lters: the fossil must fi nally be Clearly, in collecting individual shells, you fi ll
seen and collected by a human being. your rucksack faster at a Miocene locality
Doubtless, the majority of fossils that go than a Cambrian locality. You may also iden-
through the burial and uplift cycle are lost tify many more species based on those collec-
to erosion, washed away from the foot of tions. But, perhaps if you step back and
a sea cliff or blasted by sand-carrying consider families or genera, rather than species
winds in the desert. Someone has to see or specimens, and you consider the fossils
the fossil, collect it and take it home. Even from whole continents rather than just one
then, of course, the fossil has to be regis- quarry, the representation may be relatively
tered in a museum before it becomes part uniform. After all, you can recognize the pres-
of collective human paleontological ence of a species or genus from just a single
knowledge. Many that are collected specimen; it does not require a million
molder in someone’s bedroom before they specimens.
are thrown away with the garbage. In a study in 2000, Mike Benton and col-
leagues suggested that the temporal bias iden-
After all this, it’s a wonder any fossils survive tified by Raup might be an issue of scaling.

at all! Clearly Raup was right that fossils are steadily
The fact that the museums of the world lost from the record in older and older rocks.
contain so many millions of fossils is a testa- But could the record be adequate nonetheless
ment to the hard work of paleontologists of for coarser-scale studies? Benton and col-
all nations. But it also refl ects the enormity of leagues applied clade–stratigraphy measures
geological time and the sheer numbers of (Box 3.3) to a sample of 1000 published phy-
organisms that have ever existed. logenetic trees (see p. 129). These trees repre-